Many parts of critical infrastructures rely on uninterrupted access to GNSS positioning, navigation and timing services, but, at the same time, threats to denial of GNSS services are increasing. Radio frequency interference can be unintentionally emitted by commercial high power transmitters, ultra-wideband radar, television, VHF, mobile satellite services and personal electronic devices. Moreover, malicious intentional interference is produced by jammers, whose rapid diffusion is becoming a severe threat to GNSS.

To ensure GNSS is protected, there is now a need to respond at an international level to ensure that there is: i) a common standard for real-world GNSS threat monitoring and reporting, and ii) a global standard for assessing the performance of GNSS receivers and applications under threat. GNSS threat-reporting standards would allow for compilation of real-world threats into a database that could be analyzed to develop GNSS receiver test standards that ensure new applications are validated against the latest threats. Both standards are missing across all civil application domains and are considered a barrier to the wider adoption and success of GNSS in the higher value markets.

This article discusses the STRIKE3 project that was specifically developed to address the issues outlined above.

STRIKE3 Overview

The STRIKE3 (Standardizsation of GNSS Threat reporting and Receiver testing through International Knowledge Exchange, Experimentation and Exploitation) project is a European initiative that addresses the need to monitor, detect and characterize GNSS threats to support the increasing use of GNSS within safety, security, governmental and regulated applications. STRIKE3 has deployed an international network of GNSS interference monitoring sites that monitor interference on a global scale and capture real-world threats for analysis and to ultimately test GNSS receiver resilience.

Using thousands of threats collected from their network over a three-year period, STRIKE3 has developed a baseline set of threats that can be used to assess performance of different GNSS receivers under a range of typical real-world interference/jamming threats. The resulting specification consists of five different threats: wide swept frequency with fast repeat rate, narrow band signal at L1 carrier frequency, triangular and triangular wave swept frequency and tick swept frequency. For details of how these five threats were selected, refer to the Additional Reading section at the end of the article.

Finally, the STRIKE3 project has begun using its test specification to test receiver performance in the presence of various threats. Below is a discussion of how this is done as well as some results for a specific type of interference.

Collectively, the above activities aim to improve mitigation and resilience of future GNSS receivers against interference threats.

Receiver Testing

The main objectives of the testing component of the STRIKE3 project are: first, to validate the proposed testing standards to demonstrate they are clearly defined, useful, and practical; and second, to assess performance of a variety of receivers against real-world threats detected by the STRIKE3 monitoring network. Using real-world threats detected at the monitoring sites enables interested stakeholders (e.g., certification bodies, applic

ation developers, receiver manufacturers, etc.) to better assess the risk to GNSS performance during operations and to develop appropriate countermeasures.

The remainder of this article presents some illustrative examples for multi-GNSS mass-market and professional grade receiver testing against a single interference type that is very commonly detected at STRIKE3 monitoring sites, namely a triangular chirp swept frequency signal as depicted in Figure 1.

The test platform used is shown in Figure 2. The clean GNSS signal is generated from a multi-constellation, multi-frequency Spectracom GSG-6 hardware simulator, whereas the threat signature is generated using a Keysight Vector Signal Generator (VSG) N5172B through the replay of raw I/Q (In-phase/Quad-phase) sample data. Raw I/Q data captured in the field for a real-world event is used as input to the VSG which then re-creates the detected threat by continuously replaying the data in a loop.

Both the GNSS signal simulator and the VSG are controlled via software in order to automate the testing process. The automation script is used to control these devices remotely and to limit human intervention. The script also provides synchronization between the two instruments in order to ensure repeatability of the tests and the reliability of the results.

The clean GNSS signal and the interference signal are combined using an RF combiner, and the interferencecontaminated GNSS signal is fed to the Receiver Under Test (RUT), which produces its own output metrics. For the validation of

baseline performance under nominal signal conditions, the VSG does not generate any interference signal. In this case, the input signal to the RUT is only the clean GNSS signal produced by the GNSS constellation simulator.

A laptop is used to record and analyze the performance of the receiver against the different threat signals. The analysis is performed using a MATLAB-based script that processes the NMEA output messages from the RUT.

For each receiver category — namely mass-market and professional grade — three different test methodologies are performed:

• Baseline – a clean GNSS signal in the absence of interference is fed to the RUT to validate its performance under nominal conditions. The total duration of this test is 60 minutes.
• Time To Re-compute Position (TTRP) solution – this test is used to measure the time taken for the RUT to recover after a strong interference event. In this test, the interference is switched on 14 minutes after the simulated scenario starts and it is applied for 90 seconds. The interference power is fixed to a value such that the receiver immediately loses its position solution. In this test case scenario the interference power corresponds to a Jamming-to-Signal (J/S) ratio of ~90 dB. The time taken between switching off the interference source and the first position fix is recorded as the TTRP. The profile of this test

  

  methodology, whose total duration is 30 minutes, is illustrated in Figure 3.

• Sensitivity – this test scenario is conducted by varying the power of the interfering signal. The interference is turned on 10 minutes after the simulation starts and it follows a two-peak ramp power profile. The initial interference power is such that J/S is ~5 dB, and then the interference power is increased by
  5 dB every 45 seconds until reaching a J/S of 65 dB. After the first peak has been reached, the interference power is decreased in a reverse manner. The power profile is then repeated a second time. The profile of this test methodology is illustrated in Figure 4.

In order to assess the performance of the RUT in the presence of interference, different metrics were selected. The following outputs from the GNSS receiver are recorded and analyzed for all the test methodologies:

• Number of tracked satellites
• Position fix indicator (a Boolean to indicate if a 3D position fix is available or not)
• Number of satellites used in fix
• Carrier-to-Noise density (C/N0) ratio
• East-North-Up position error

Moreover, depending on the test methodology, additional parameters are evaluated. For example, in the case of the TTRP test method, the time tak

en for the RUT to re-obtain a position fix after a strong interference event is measured. For the sensitivity test method, the Jamming-to-Signal ratio at which the position solution is no longer available and the availability of the position solution during the interference event are computed. Furthermore, position accuracy statistics are computed for the interval in which the interference is present when the receiver offers a valid position fix.

Currently, only GPS L1 and Galileo E1 signals are used for testing and the RUT is configured to operate in static stand-alone mode.

Table 1 provides an overview of the simulated scenario settings, including the receiver location, the start time, the duration, the GNSS signal power and the interference power levels for the different test methodologies.

When performing the tests, an elevation mask of 5° is applied for the Position, Velocity and Time (PVT) computation. The RUT’s default C/N0 mask is used in all cases. The RUT settings are summarized in Table 2.

Results

This section presents the results of the standardized tests of a mass-market and a professional grade receiver against one of the most frequently detected interference types at STRIKE3 monitoring sites. The spectrum and the spectrogram of such interference signal are sho

wn in Figure 1.

The accuracy and availability of the receiver’s position solution during the interference interval is analyzed in the sensitivity tests. As the interference power increases, the receiver performance continues to degrade and at some point the RUT loses the position fix. The East-North-Up (ENU) deviations of the position solution for the mass-market (top) and the professional grade receiver (bottom) are shown in Figure 5.

Both receivers offer inaccurate position solutions in the beginning, especially in the vertical component. This is due to the cold start and the resulting unavailability of ionospheric parameters, and to the convergence of the navigation filter.

It can be seen that the mass-market RUT prioritizes the availability of the position solution over its accuracy. In particular, during the interference interval, there are only a few epochs at which the receiver does not yield a solution, but this high yield comes with degraded positioning accuracy.

On the other hand, the professional grade RUT prioritizes the accuracy over the availability. It does not offer the position solution as often during the interference interval, but when it does the position errors are minor.

In order to have a better understanding of the interference impact on the RUT, a comparison with respect to the baseline test case is also carried out. Figure 6 shows the drop in the average C/N0 of the satellites used in position fix with respect to the baseline for the entire duration of the test. As expected, in the presence of interference, the signal quality worsens as the interference signal’s power increases. Given the wideband nature of the interfering signal, GPS and

Galileo are affected similarly.

The difference between the mass-market and the professional grade receivers’ behavior is also visible here. While the former continues to use very low quality signals in order to provide a position solution, even if inaccurate, for as long as possible, the professional grade RUT stops computing the solution when the signal quality decreases by about 20 dB.

A summary of the results is given in Table 3. The maximum horizontal and vertical errors are computed for the interval in which the interference is present when the receiver offers a valid position fix. As already discussed, the position fix availability during the interference interval for the mass-market receiver is high at the expense of position accuracy. On the other hand, the professional grade RUT preserves the position accuracy at the expense of solution availability: the maximum horizontal and vertical errors in the test case are only slightly larger than in the baseline case.

The J/S at which the position solution is no longer available, J/SPVT_lost, is also determined. It can be observed from Table 3 that the mass market RUT has much higher sensitivity as compared to professional grade RUT, when manufacturer’s default receiver settings are used. Finally, it can be observed that TTRP values are much better for mass-market RUT than professional grade RUT.

Conclusion

Given the increasing dependence on GNSS technology and its vulnerability to intentional and unintentional interference, it is important to understand the magnitude and evolution of the GNSS threat scene. The STRIKE3 project is addressing this need through the development of monitoring and reporting standards, the deployment of a worldwide monitoring network to test the reporting standards and to provide a database of real-world events, the development of receiver testing standards against threats, and an intensive testing activity against the detected real-world interferences in order to test the resilience of different multi-GNSS receivers.

Additional Reading

For more details on the European H2020 project ‘STRIKE3’, please refer to: STRIKE3 (2016) Standardizsation of GNSS Threat reporting and Receiver testing through International Knowledge Exchange, Experimentation and Exploitation [STRIKE3]. http://www.gnss-strike3.eu/.

For more details on STRIKE3 proposed GNSS threat reporting standards, please refer to: Thombre, S., Bhuiyan, M. Z. H., Eliardsson, P., Gabrielsson, B., Pattinson, M., Dumville, M., Fryganiotis, D., Hill, S., Manikundalam, V., Pölöskey, M., Lee, S., Ruotsalainen, L., Söderholm, S., Kuusniemi, H. (2017) “GNSS Threat Monitoring and Reporting: Past, Present, and a Proposed Future”, The Journal of Navigation 71(3):513-529.

For more details on draft standards for receiver testing against threats, please refer to: Pattinson, M., Sanguk, L., Bhuiyan, M. Z. H., Thombre, S., Manikundalam, V., Hill, S. (2017) “Draft Standards for Receiver Testing against Threats”, available online via: http://www.gnss-strike3.eu/.

Authors

Nunzia Giorgia Ferrara is a Research Scientist in the Department of Navigation and Positioning at the Finnish Geospatial Research Institute and a PhD candidate at Tampere University of Technology where she was a Marie Curie Fellow from 2014 to 2016. Her research focuses on multi-GNSS receiver design and interference detection and mitigation.

Dr. M. Zahidul H. Bhuiyan is working as a Research Manager at the Department of Navigation and Positioning in the Finnish Geospatial Research Institute. He is also serving as the head of the Satellite and Radio Navigation research group of the institute. His main research interests include various aspects of multi-GNSS receiver design, GNSS vulnerabilities, SBAS, differential GNSS, etc.

Amin Hashemi is a Research Scientist with the Navigation and Positioning department of the Finnish Geospatial Research Institute. His current focus is on

localizing GNSS interference sources.

Dr. Sarang Thombre is a Research Manager and Deputy Leader of the Satellite and Radio Navigation research group at the Department of Navigation and Positioning of FGI. He earned his Ph.D. degree in April 2014 from Tampere University of Technology, Finland. His research interests include GNSS receiver design and implementation, autonomous vehicle PNT techniques, and RF interference to GNSS.

Dr. Michael Pattinson is a Principal Navigation Engineer at NSL and jointly leads the Safety and Integrity business unit. His main activities include advanced position techniques (high accuracy and high integrity), as well as GNSS performance monitoring and anomaly investigation to enhance GNSS robustness and reliability.

The post How can we ensure GNSS receivers are robust to real-world interference threats? appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Unusually low power and instability in the radio signals received from the two satellites at the telemetry stations operated by the European Space Agency (ESA), the French National Space Agency (CNES) and the Swedish Space Corporation (SSC) showed that the satellites were not in their expected orbit. What followed, however, was a variety of procedures that led to the mission recovery and returned promise to the program.

Here, the authors explain how Galileo satellites 5 and 6, which were almost considered lost, put under test the ingenuity of the many engineers involved in the recovery of this mission demonstrating the technical excellence and collaborations with experts from various institutions, space agencies and industrial partners. The third launch of the Galileo constellation took place on August 22, 2014 by Soyuz ST-Fregat vehicle from Europe’s Spaceport in French Guiana, carrying Europe’s fifth and sixth Galileo satellites. These were the first series being built by OHB System AG, the company selected to complete the Galileo constellation. Due to a malfunction of Soyuz Fregat upper stage, the satellites were injected into a lower and elliptical orbit instead of the planned circular orbit and with only one out of the two solar array wings deployed on both spacecraft. As a result, the satellites were left in non-nominal orbit leading to exposure to the Van Allen radiation belts, non-nominal operation of the Earth Sensors and insufficient fuel to correct the orbits entirely. The prospects for the mission were gloom to say the least, however very quickly, experts across European institutions, space agencies and industrial partners joined forces to recover the satellites and to investigate possible operational scenarios. What followed was the history of one of the most remarkable successes in the recovery of a failed launch. This success has already allowed to put these satellites in use for search and rescue services and for an ambitious test of Einstein’s General Theory of Relativity. This article is the first in a series on the extraordinary journey of Galileo satellites 5 and 6 from launch to mission recovery and exploitation. Ultimately, the final stage will bring these satellites into service for navigation usage.

The Mishap

At the beginning everything seemed well, but then the Galileo’s Launch and Early Orbit Phase (LEOP) team at the European Space Operations Centre (ESOC) in Darmstadt, Germany, responsible for controlling the satellites, raised the alarm. Unusually low power and instability in the radio signals received from the two satellites at the telemetry stations operated by the European Space Agency (ESA), the French National Space Agency (CNES) and the Swedish Space Corporation (SSC) showed that the satellites were not in their expected orbit. The satellites were left in an elliptical orbit with the apogee at 25,900 kilometers above Earth and the perigee at 13,713 kilometers, with the orbit wrongly inclined by 49.77º with respect to the equator (Figure 1). The first priority was to ensure the satellites were safe and stable. With one solar wing deployed, there was enough power to ensure spacecraft viability and safety. The second urgent priority was to determine a sufficiently accurate orbit to ensure a proper pointing of the antenna in order to monitor and control the satellites. Within four hours of the first signal reception, the team determined the actual orbit, then generated new commands to repoint the ground antennas and established robust radio links. Over the following few days, with support from ESA’s Galileo project and satellite manufacturer OHB, the LEOP team developed, validated and rehearsed procedures to release the trapped solar wings, boosting the available power to its nominal value as they pointed toward the sun. Both satellites were left in a mode pointing to the sun, but they could not be used for navigation purposes nor be tested as the Earth Sensors used to point their navigation antennas stopped working around perigee because Earth’s disc filled their field of view. The Independent Inquiry Commission of Arianespace, European Commission (EC), ESA experts and Russia’s Space Agency Roscosmos concluded that the problem was due to a misfiring Fregat upper stage. Corrective measures were performed in the subsequent launches to prevent recurrence of the problem (Figure 2).

Mission Recovery

In parallel to the Independent Inquiry Commission, a multidisciplinary team of engineers at ESA initiated a number of mission recovery analysis in order to investigate possible operational scenarios which would allow to restore, as much as possible, the intended mission objectives and maximise the benefits for the program. Given the high eccentricity and low perigee of the injection orbit, the mission drivers for the recovery were to: • Reduce the L-band power dynamic range between apogee and perigee • Reduce Doppler as to facilitate L-band ground receivers to lock the signal • Increase visibility time for receivers • Ensure perigee high enough to allow Earth Sensors to be operational over the complete orbit • Reduce exposure to the Van Allen radiation belts to minimize satellite equipment degradation • Improve contribution to the global constellation performance Different scenarios were prepared by ESA, supported by experts from industry and national space agencies, and proposed to the owner of the satellites, the EC, which gave the “green” light to initiate the recovery actions. In September 2014, the control of Galileo satellites 5 and 6 was transferred from the LEOP team at ESOC in Darmstadt, Germany to the Galileo Control Centre (GCC) in Oberpfaffenhofen, Germany, operated by SpaceOpal. But the satellites’ incorrect orbits meant their navigation payloads could not be switched on for testing due to the non-nominal operation of the Earth Sensors. In addition, the lower orbits were exposing them to heightened levels of harmful radiation. Limited fuel meant the satellites’ originally assigned destination was out of reach, but their orbits could still be modified to make them suitable for navigation purposes. A recovery plan was devised between ESA’s Galileo team, ESOC flight dynamics specialists, along with personnel from SpaceOpal, OHB with the support of the French (CNES), Italian (ASI), German (DLR) and British (UKSA) space agencies experts. The scheme involved a multiple series of maneuvers, gradually raising the lowest point of the satellites’ orbits more than 3,500 kilometers while making them more circular and positioned 180º of each other. The fifth Galileo entered its corrected orbit at the end of November 2014, followed by the sixth Galileo in March 2015 with a total of 11 and 14 orbit raising maneuvers, respectively. The commands were issued from the Galileo Control Centre at Oberpfaffenhofen, Germany by SpaceOpal, guided by calculations from a combined ESA-CNES flight dynamics team. Commands were uploaded to the satellite via an extended network of ground stations, made up of Galileo stations and additional sites coordinated by CNES. Satellite manufacturer OHB also provided expertise throughout the recovery, helping to adapt the flight procedures. In the new orbit, the satellite’s radiation exposure has also been greatly reduced, ensuring reliable performance for the long term. The corrected orbit of the satellites is shown in Figure 3. The recovery strategy achieved a slightly elliptic orbit due to insufficient fuel in the satellites’ propulsion system to reach the nominal orbit (See Table 1). The major advantage in this slightly elliptic orbit with the two satellites separated by 180º is that the ground track repetition pattern is 37 orbital revolutions in 20 siderial days, with a sub-cycle of 10 days where the two satellites swap locations in the geometry. This is compatible to the nominal Galileo constellation having a repetition pattern of 17 revolutions in 10 siderial days (Figure 4). This allows a proper predictability and repeatability of the constellation and an inclusion of the satellites into the planning functions of the system. The revised, more circular and higher altitude orbit means the satellites’ Earth Sensors can be used continuously, keeping their main antennas oriented towards Earth and allowing the navigation payload to be switched on.

Testing the Signals for Navigation

With Galileo 5 and 6 having reached a more suitable orbit for navigation purposes, the In-Orbit Test (IOT) campaign could start and the navigation payloads were switched on. The IOT campaign was conducted from the ESA tracking station in Redu, Belgium, where the satellite broadcast navigation signal was monitored using the 20 meter L-band antenna to study the strength and shape of the navigation signals at high resolution with support from experts from OHB System AG – the satellite manufacturer in Bremen, Germany and Surrey Satellite Technology Ltd. (SSTL) – the payload manufacturer in Guildford, UK. First, the various payload elements, especially the Passive Hydrogen Maser (PHM) atomic clock, were warmed up, then the payload’s first signal in space (SIS) was transmitted. Galileo 5, the first Full Operational Capability (FOC) satellite transmitted its first navigation signal on November 29, 2014. Its navigation signal-in-space, transmitting in the three Galileo frequency bands (E1/E5/E6), was tracked by Galileo Test User Receivers deployed at various locations in Europe, namely Redu in Belgium, the European Space Research and Technology Centre (ESTEC), Noordwijk in the Netherlands, Weilheim in Germany and Rome in Italy. The quality of the signal was confirmed to be in line with expectations. Galileo 6 navigation payload IOT was successfully completed in March 2015. The successful results of the tests of the navigation signal were excellent news for Galileo, confirming the good design of the rest of the satellites which had yet to be launched. Furthermore, navigation message upload tests and Galileo-only position fix using Galileo 5 (GSAT0201) in combination with In-Orbit Validation (IOV) satellites Galileo 1, 2 and 3 (GSAT0101, 0102 and 0103) were successfully carried out in December 2014 demonstrating the excellent payload performance comparable to that expected in the nominal orbit and the potential of Galileo 5 to operate as part of the Galileo system (Figure 5). In preparation for the operational use of the satellites, ESA carried out an assessment of the operational benefits of Galileo 5 and 6 as part of the Galileo system and recommended some Ground Segment modifications required to support the future operational use of these satellites. The Ground Mission Segment (GMS) required modification to enable the processing and generation of navigation messages for Galileo 5 and 6 and uplink via both the S-band uplink by Telemetry Tracking and Control (TT&C) stations and via the C-band uplink by the Mission Up-Link stations (ULS). The Galileo Time and Geodetic Validation Facility (TGVF) – tasked with advanced Orbit Determination and Time Synchronisation (ODTS) processing and independent system performance monitoring, supported by a worldwide network of sensor stations – also required adaptations to allow the processing of Galileo satellites 5 and 6 in eccentric orbits to generate orbital data for the Search and Rescue (SAR) community. Based on the positive outcomes of this assessment, the EC authorized the initiation of the required modifications. Following the seamless upgrades of the Galileo core infrastructure from June to July 2016, Galileo 5 and 6 have been injected into the ground segment navigation processing and started the broadcast of navigation messages for testing purposes on August 5, 2016 (ref: Notice Advisory to Galileo Users – NAGU 2016029 and 2016030) with one S-band uplink per orbit (~14 hours). The almanacs for Galileo 5 and 6 are not broadcast since the orbital parameters, in particular semi-major axis and eccentricity, do not fit in the range of values foreseen for this field in the Open Service Signal-In-Space Interface Control Document (OS SIS ICD) given the eccentric nature of the satellites. The almanac data is a reduced-precision subset of the clock and ephemeris parameters of the active satellites in orbit. The Galileo almanac orbital parameters consist of, semi-major axis, eccentricity, inclination, longitude of the ascending node, argument of perigee and mean anomaly. The main hurdle in using Galileo 5 and 6 operationally was that their corrected orbits still fall outside of the “almanacs” broadcast within navigation messages to locate satellites. However, the satellite’s signal could still be received in open sky search. With the deployment of C-band dissemination capability in November 2016, the performance did significantly improve due to more frequent navigation messages uplinks from ground, resulting in a reduction of the Signal In Space Ranging Error (SISE). Figure 6 illustrates the instantaneous F/NAV SISE global average during the period January – March 2017. The effects of SISE with and without Galileo 5 (E18) and Galileo 6 (E14) were clearly shown before and after the Galileo ground segment optimization on March 18, 2017. The optimization led to the reduction in the Age of Data (AoD) – elapsed time since the generation of a navigation message data set by the ground segment and its final reception at user level – which resulted in improvements in the ranging accuracy. There is a particular sensitivity of Galileo satellites 5 and 6 SISE to the Age of Data, to a much larger extent than the satellites in nominal orbits, as shown in Figure 7. In nominal operational conditions, the Galileo ground segment is able to limit the maximum AoD to 100 minutes. However, due to ongoing deployment of the Galileo ground segment, improvement in the robustness of the navigation message uplink capability needs to be ensured. Furthermore, additional measures also will be taken on-board the satellites to protect the users with the automated SIS flag setting in case the navigation message is not uplinked from ground in a timely manner.

Searching to the Rescue

In addition to navigation services, Galileo also provides an SAR service, in the form of contribution to the International Cospas-Sarsat system, a satellite based Search and Rescue distress alert detection and information distribution system (Figure 8). Galileo satellites receive signals from Cospas-Sarsat-approved 406 megahertz emergency beacons, located anywhere on Earth’s surface and rebroadcast them in L-band to ground receiving terminals, called MEOLUTs (Medium Earth Orbit Local User Terminal), which detect and compute the distress beacon locations based on the measurements performed on the signals relayed by several SAR repeaters in visibility of the station, then report the beacon’s alert message and position to Cospas-Sarsat Operations. Galileo is one of the main contributors to the global MEOSAR alert service by providing a global space segment and the regional ground segment elements for detection/localization in Europe with three MEOLUT stations deployed in Spain, Norway and Cyprus. The management of the ground segment operations and service provision is carried out from the SAR/Galileo Service Centre located in Toulouse, France in the premises of the French National Space Agency (CNES). The SAR repeater on Galileo 5 (GSAT0201, Cospas-Sarsat designation: 418) was first switched on in December 2014. The SAR In-Orbit Testing (IOT) was performed at switch-on and the testing of the SAR repeater for Cospas-Sarsat (CS) commissioning was performed in November 2015. Followed by the testing of SAR repeater on Galileo 6 (GSAT0202, Cospas-Sarsat designation: 414) which was switched on in December 2015 and Cospas-Sarsat (CS) commissioning testing completed in March 2016. From August 2015 through February 2016 the Galileo Time and Geodetic Validation Facility (TGVF) located at ESTEC was adapted to allow the processing of Galileo satellites 5 and 6 in eccentric orbits. Orbit and clock predictions in suitable format were generated and made available on a server accessible by the European GNSS Service Centre (GSC) for subsequent dissemination to the SAR community and MEOLUTs. Adaptations of the SAR Ground Segment MEOLUT Tracking Coordination Facility (SGS MTCF) was also completed to allow the ingestion of orbital data retrieved from the GSC. The SAR repeaters on Galileo 5 and 6 were successfully tested, but not used until March 2016 due to the lack of orbital data in the navigation signal-in-space, which is essential for performing the SAR localization function. Their usage commenced when the orbital data generated by the TGVF was made available via the GSC in March 2016. This allowed MEOLUTs around the world to fetch the orbital data in order to track Galileo satellites, including Galileo 5 and 6. In the period from March 2016 until December 2016, Galileo 5 and 6 SAR repeaters have been used, together with other available Galileo SAR repeaters, for system performance validation activities and Key Performance Indicator (KPI) collection. Based on the positive results of the tests, Cospas-Sarsat considers the two repeaters commissioned and available for all users. On December 13, 2016, Cospas-Sarsat declared MEOSAR Early Operational Capability (EOC), based on commissioned MEOSAR repeaters, which included repeaters on Galileo 5 and 6 satellites and since then they are used operationally for MEOSAR by the French and US Mission Control Centers (FMCC and USMCC). With future deployment of the system, Galileo satellites will be capable of providing the Return Link Service to users with a feedback by sending an acknowledgement message to the users and informing them that the alert has been detected and eventually that rescue operations are under way.

Everything is Relative

Iterating with several European scientific institutions, it became apparent from the very beginning that the recovery orbits of Galileo satellites 5 and 6, still eccentric, were, in turn, offering a unique opportunity to conduct a test of Einstein’s General Theory of Relativity by measuring more accurately than ever before the way that gravity affects the passing of time. Indeed, Einstein’s Theory of General Relativity (GR) predicts that time flows differently for two clocks that have a relative speed and are placed in different gravitational potentials. It should therefore be possible to test General Relativity by comparing the frequencies of two atomic clocks, in a so-called gravitational redshift test. A gravitational redshift experiment tests the Local Position Invariance (LPI), which is one of the aspects of the Einstein Equivalence Principle which may be tested. As several alternative theories of gravitation predict violations of this effect – e.g. in attempts to unify GR and quantum theory – experimental constraints are of paramount importance. The recovery orbits of Galileo 5 and 6 and the specificities of the Galileo satellites made these especially suitable for a Gravitational redshift tests (Figure 9), noting that: 1. Galileo 5 and 6 remain in elliptic orbits, with each satellite climbing and falling some 8,500 kilometers twice per day, and providing a periodic modulation of the gravitational redshift at the orbital period (~ 13 hours). 2. Both satellites are equipped on-board with Passive Hydrogen Maser (PHM) atomic clocks, providing unique stability. 3. Nominal satellite life time after recovery remained long (~12 years), which allows the possibility to integrate test measurements during a long time. 4. Satellites are permanently monitored with the provision of high accuracy orbits. 5. Laser Retro Reflectors (LRR) are equipped on-board the satellites which allow independent orbit tracking by laser (of high interest to disentangle clock and orbit radial errors). 6. The realization of these tests does not interfere with the potential introduction of Galileo 5 and 6 for navigation service or the nominal Search and Rescue (SAR) service. Up to date, the most precise test of the gravitational redshift has been realized with the Vessot-Levine rocket experiment in 1976 (Figure 10), also named as the Gravity Probe A (GP-A) experiment, where the gravitational redshift was verified to a level of 1.4 x 10–4 accuracy. This gravitational redshift measurement had not been improved nor reproduced at this level since then. The analysis performed showed that proper tests with the two eccentric Galileo satellites could potentially provide a more accurate test of Einstein’s General Relativity Gravitational Redshift prediction. In view of the above considerations and following the recommendations of the ESA GNSS Science Advisory Committee (GSAC), two parallel studies were launched by ESA with SYRTE/Paris Observatory and ZARM/University of Bremen in 2015. The so called GREAT project (Galileo gravitational Redshift Experiment with eccentric sATellites) aimed at measuring the Gravitational redshift predicted by Einstein General Relativity Theory with the highest possible accuracy. These tests have been performed during a period of almost three years and are now concluding. On the occasion of the ESA 6th International Colloquium on Scientific and Fundamental Aspects of GNSS / Galileo held in Valencia, Spain, from October 25-27 2017, the two parallel consortia presented some preliminary results and the methodology followed, after having assessed more than 1,000 days of data from the two eccentric Galileo satellites and properly modelled systematic errors. A careful analysis of systematic effects was essential to calculate robust limits on the gravitational redshift. All systematic effects, potentially impacting the Gravitational redshift tests, were identified and analyzed. Effects acting on the frequency of the reference ground clock or on the radio link can be safely neglected. The activity focused then on the estimate of the effects acting directly on the frequency of the on-board clock, namely temperature and magnetic field variations, and on those systematic effects linked to orbit modelling errors. Concerning systematic errors associated to the orbit modelling, a one-year dedicated Satellite Laser Ranging measurements was performed during 2016 and 2017 in close cooperation with the International Laser Ranging Service (ILRS). These allowed disentangling in a good extent systematic errors coming from the orbit and impacting the clock determination from other systematics. A major support for this activity was also received from ESA’s Navigation Office at ESOC in Darmstadt, Germany, whose experts generated the accurate clock and orbit products for Galileo satellites 5 and 6 using precise satellite models, which allowed a very accurate modelling of the non-gravitational orbit perturbations, notably from solar radiation pressure (SRP). For the other systematic errors, potentially affecting the on-board clocks, conservative upper limits were derived based on the knowledge on the satellite environmental conditions and their known effect on the frequency sensitivity of the PHM atomic clocks. Preliminary results presented by SYRTE and ZARM at the ESA’ scientific colloquium were very positive. Both consortia did manage to confirm, in an independent way, the gravitational redshifts with accuracies slightly better than the Gravity Probe A reference. During Q1 2018, some of the orbit systematic errors have been further reduced following refinements of the orbit modelling algorithms performed by ESA’s Navigation Support Office. The new estimates are currently under final consolidation by the two consortia and it may be anticipated that results will be several times better than the GP-A Reference. Final consolidated results are planned to be submitted in a scientific journal later this year. The success of the Galileo General Relativity tests has triggered a number of additional Fundamental Physics general tests which could be performed exploiting the eccentric Galileo satellites 5 and 6 which are currently under assessment by the ESA Galileo Navigation Science Office with the support of ESA’s GNSS Science Advisory Committee. ESA also intends to make available the accurate clock and orbits processed for these two eccentric satellites to the public sometime this year for potential exploitation by the scientific community.

Conclusion

The recovery orbit of Galileo satellites 5 and 6 is compatible with the nominal Galileo constellation and some challenges due to the eccentric characteristics of the orbit have been overcome by the modifications in the ground segment. The quality of the navigation signals transmitted by these satellites have been tested and confirmed to be in line with expectations. However, all navigation signals remain flagged in “Test” mode pending the implementation of necessary measures to improve robustness of the ground segment for navigation message uplink and the deployment of on-board software for automated SIS flag setting which are required to include the satellites as part of the Galileo constellation for navigation and SAR applications. Although the SAR repeater on-board Galileo satellites 5 and 6 are already part of the SAR Initial Service contributing to the Cospas-Sarsat Program, the setting of SIS flags to “Healthy” would also have positive benefits to the SAR Service since it would allow the signals to be processed by the Galileo receivers installed in the SAR ground segment. This would make the Galileo SAR repeaters available to all MEOLUTs worldwide, without requiring those MEOLUTs to retrieve the orbital data from the server made available by GSA for this purpose. The most accurate measurements ever of Einstein’s predicted General Relativity gravitational shift were made possible thanks to Galileo, transforming an unfortunate difficult situation into an excellent scientific opportunity with further potential exploitation by the scientific community. The Galileo satellites 5 and 6 which were almost considered lost back in the summer of 2014 put under test the ingenuity of the many engineers involved in the recovery of this mission, thus demonstrating the technical excellence and collaborations with experts from various institutions, space agencies and industrial partners. Updates on the extraordinary journey of Galileo satellites 5 and 6 can be followed in future editions of Inside GNSS, featuring the final detailed results of Einstein’s General Theory of Relativity, the GREAT project and their ultimate quest towards the provision of navigation service.

Acknowledgements

The success of Galileo 5 and 6 mission recovery and exploitation would not have been possible without the excellent team effort and technical expertise across European institutions, space agencies and industrial partners. The authors would like to thank: • OHB satellite manufacturer and SSTL payload manufacturer • Airbus Defence and Space (ADS), the Ground Control Segment contractor • Thales Alenia Space, the Ground Mission Segment contractor and system engineering support • SpaceOpal, the Galileo Service Operator • French (CNES), Italian (ASI), German (DLR) and British (UKSA) space agencies experts for the support on the mission recovery plan • SYRTE, Observatoire de Paris, the GREAT consortium • ZARM, University of Bremen, the GREAT consortium • International Laser Ranging Service (ILRS) • The European Commission (EC) • The European GNSS Agency (GSA) Exploitation and Market Development Teams • The entire ESA Galileo Project Team, ESA Navigation Science Office and ESA Navigation Support Office Additional Resources 1. H. Côme et al. “GALILEO 5 and 6 LEOP or How to Handle and Recover Two of the Most Feared Failures Occurring Simultaneously,” SpaceOps Conference, 16 – 20 May 2016, Daejeon, Korea 2. A. Ayala et al. “Galileo Extended Slots Characterisation and Relation with the Nominal Constellation,” International Symposium on Space Flight Dynamics (ISSFD), 6 – 9 June 2017, Matsuyama, Japan 3. P. Delva et al. “Test of the gravitational redshift with stable clocks in eccentric orbits: application to Galileo satellites 5 and 6,” Journal on Classical and Quantum Gravity, Volume 32, No. 23 (2015) 4. R. F. C. Vessot and M. W. Levine. “A test of the equivalence principle using a space-borne clock.” Journal on General Relativity and Gravitation, Volume 10, No. 3 (1979), pp. 181 – 204 5. P. Delva et al. “An SLR campaign on Galileo satellites 5 and 6 for a test of the gravitational redshift – the GREAT experiment”, Proceedings of the ILRS Technical Workshop, 26 – 30 October 2015, Matera, Italy. 6. P. Delva et al, “Testing the gravitational redshift with eccentric Galileo satellites,” 6th International Colloquium of Scientific and Fundamental aspects of Galileo / GNSS, 25 – 27 October 2017, Valencia, Spain 7. S. Hermann et al., “Galileo Gravitational Redshift test with eccentric satellites”, 6th International Colloquium of Scientific and Fundamental aspects of Galileo / GNSS, 25 -27 October 2017, Valencia, Spain

Authors

Nityaporn Sirikan is the Galileo Signal In Space Service Provision System Engineer in the Directorate of Navigation based at the European Space Agency (ESA) – European Space Technology and Research Centre (ESTEC) in Noordwijk, The Netherlands. Hervé Côme was the ESA Service Manager for the Galileo LEOP service (Launch 3 – 8) and was the lead Flight Operations Director for Launch 3 LEOP (Galileo satellites 5 and 6). He is working in the Directorate of Operations based at the ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany. Igor Stojković is the Principal Search and Rescue (SAR) Engineer in the Directorate of Navigation based at ESA – ESTEC in Noordwijk, The Netherlands. Javier Ventura-Traveset is the Head of the Navigation Science Office and the Executive Secretary of ESA’s GNSS Science Advisory Group leading ESA’s GNSS scientific activities based at ESA’s European Space Astronomy Centre (ESAC) in Villanueva de la Cañada, Spain. Rafael Lucas is the Galileo Services Engineering Manager in the Directorate of Navigation based at ESA – ESTEC in Noordwijk, The Netherlands. Marco Falcone is the Galileo System Manager in the Directorate of Navigation based at ESA – ESTEC in Noordwijk, The Netherlands.

The post Galileo 5 and 6 Eccentric Satellites: Mission Recovery and Exploitation Part I appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

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Authors: Pauline Martin Thales Alenia Space Thibaud Calmettes Thales Alenia Space Yoan Gregoire CNES Mercedes Reche PILDO LABS Christophe Chatain ECAGROUP Michel Monnerat Thales Alenia Space

On June 1, 2009, the Air France AF447 flying from Paris to Rio experienced a stall that led to the crash of the aircraft in the Atlantic Ocean with 228 passengers and crew members onboard. As the aircraft was transmitting its location nominally only every 10 minutes, the location of the accident site could not be determined with a good accuracy. The search efforts to recover the wreck and the flight data recorder lasted almost 2 years, involving significant aeronautical, maritime and sub-maritime equipment, leaving open the threat that the still unexplained cause of the accident could be at the origin of a similar one in the meantime. Finally, the wreck and the victims were localized in spring 2011 thanks to a submarine robot. The total cost of the operations amounts to more than €34 million, or about $41 million US dollars. 

Later, on March 8, 2014, the Malaysia Airlines MH370 flying from Kuala Lumpur to Beijing disappeared with its 239 passengers and crew members. Four years later, the mystery remains despite the more than 33 months of research and the approximate amount of $200 million US dollars spent in the recovering operations by 26 countries. Contrary to the case of the AF447, the MH370 was not even transmitting its location every 10 minutes and all communication links with the aircraft were shut down.

These two tragedies have highlighted the limitations of the existing air navigation and distress systems: with extensive identification and localization delay of the aircraft in distress, the effectiveness of Search And Rescue (SAR) efforts and recovery operations is dramatically reduced. In both cases, no distress messages transmitted by the Cospas-Sarsat Emergency Locator Transmitters (ELT) distress beacons (mandatory on-board any commercial aircraft by International Civil Aviation Organization [ICAO] requirements) were received by the SAR ground segment. The current ELTs are designed to be triggered automatically further to a shock or upon activation by water (typically after a crash or a hard landing on ground or into the ocean) or manually to transmit a distress signal at 406 MHz for at least 24 hours. For flights AF447 and MH370, no alert messages transmitted by the ELT were received. The reasons are still unknown but several hypotheses can be formulated: the transmission system was damaged at the impact and transmission could not occur, or the wreck and the distress beacon sank very quickly before the beacon could transmit its first distress message. Without the alert messages, no information independent of the aircraft systems was available to determine the position of the crash for both the AF447 and the MH370 after all the aircraft equipment failed. To prevent this situation from occurring again in the future, the solution would be to be able to receive information about the location of the aircraft during the in-flight distress phase. Based on this, ICAO and the international aeronautical community carried on activities to reconsider the effectiveness of the existing SAR operations by improving the in-flight tracking of the aircrafts during the entire flight and under all circumstances, including normal flight and distress. The activities of the Ad-hoc Working Group on Aircraft Tracking led to the definition of a Global Aeronautical Distress and Safety System (GADSS) and additional amendments to the ICAO Annex 6 divided into 3 functions (Figure 1):

• Aircraft Tracking (AT): transmission of data of time and position or information to determine the position of the aircraft at least every 15 minutes during normal flight phase

• Autonomous Distress Tracking (ADT): transmission of time and position or information to determine position of the aircraft at least every minute during in-flight detected distress phase

• Post Flight Localization and Recovery (PFLR): localization of flight data recorders thanks to beacons associated to the FDR.

The ADT function requirement is the answer to the AF447 and the MH370 accidents and its necessity has been confirmed by other aeronautical accidents (like the Egyptair MS804): by initiating the tracking of the aircraft during in-flight distress phase and with the availability of transmitting accurate time and position data, the probability to receive a distress message is increased, as well as the accuracy of the location of the crash area. The evolution of the Cospas-Sarsat system, mandatory onboard all commercial aircraft, into an ADT system appeared as an appropriate solution and is further described in the following sections.

Cospas-Sarsat Solution to ADT Recommendations

a. The Cospas-Sarsat program

The International Cospas-Sarsat Program provides accurate, timely and reliable distress alert and location data to help SAR authorities assist persons in distress. The objective of the Cospas-Sarsat system is to reduce, as much as possible, delays in the provision of distress alerts to SAR services, and the time required locating a distress and providing assistance, which have a direct impact on the probability of survival of the person in distress at sea or on land. To achieve this objective, Cospas-Sarsat Participant governments and agencies implement, maintain, coordinate and operate a satellite system capable of detecting distress alert transmissions from radio-beacons that comply with Cospas-Sarsat specifications and performance standards, and of determining their position anywhere on the globe. The distress alert and location data is provided by Cospas-Sarsat Participants to the responsible SAR services. Cospas-Sarsat cooperates with the ICAO, the International Maritime Organization (IMO), the International Telecommunication Union and other international organizations to ensure the compatibility of the Cospas-Sarsat distress alerting services with the needs, the standards and the applicable recommendations of the international community. It is today the most important worldwide rescue system, having been used from 1982 to December 2016 to provide assistance in rescuing at least 43,807 people in 12,664 events.

From the beginning, Cospas-Sarsat has been integrated to aircraft safety and security systems with two types of beacons: ELT (Emergency Locator Transmitter), attached to the aircraft and mandatory in most of them; and PLB (Personal Locator Beacons), attached to the pilot, widely spread in general aviation. Aviation domain today represents 20% of the 2 million deployed beacons. In 2016, the Cospas-Sarsat system answered to 876 SAR events (177 distresses were related to the aviation domain); contributing to save 2,057 lives (355 lives involved in aeronautical SAR events). Thanks to the efficient communication link between the Cospas-Sarsat Mission Control Centre (MCCs), and the Rescue Coordination Centers (RCCs), the ground operational segment of Cospas-Sarsat in general, and the aeronautical branch in particular, has reached the rank of the most performing SAR system as Cospas-Sarsat has been involved in half of the distress events managed by RCCs around the world.

Over the last 34 years, with 30 operational MCC deployed in 30 countries all around the world, and more than 100 RCC/SPOC, Cospas-Sarsat consolidated its international SAR network and earned a rich experience in coordination of information and rescue means in all the events it treated. Every year, new countries join the program and contribute to the Cospas-Sarsat ground segment.

Today, the Cospas-Sarsat space segment is composed of five satellites in Low-Earth Orbit, eight satellites in Geostationary Orbit, and 37 satellites in Medium Earth Orbit, equipped with SAR payloads to receive and forward distress messages broadcasted by the Cospas-Sarsat SAR beacons. The Cospas-Sarsat system is free of charge for the user and entirely financed by the partners of the program.

Figure 2 illustrates the nominal operation chain of the Cospas-Sarsat system and the Return-Link use (violet arrows); the three space segments of Cospas-Sarsat are represented: the Low Earth Orbit SAR (LEOSAR) space segment, the Medium Earth Orbit SAR (MEOSAR) space segment and the Geostationary orbit SAR (GEOSAR) space segment:

1. Distress beacons are activated by aircraft (ELT and ELT(DT)), ships (EPIRB: Emergency Position Indicating Radio Beacon) or individual users (PLB: Personal Locator Beacon)

2. The SAR payloads aboard Cospas-Sarsat satellites (including Galileo satellites) relay the alert to LUT

3. The Local User Terminals (MEOLUT, LEOLUT and GEOLUT) detect, demodulate and localize the distress beacon signals

4. The LUT forwards the alert data (including the independent location computed by MEOLUT and LEOLUT) to the appropriate MCC (i.e. the MC in charge of the area where the beacon is localized)

5. The Mission Control Centres (MCC) distributes alert data to Rescue Coordination Centres (RCC)

6. RCC organizes and coordinates SAR services and operations

In addition, each Cospas-Sarsat beacon can be localized either thanks to the position encoded in the alert message transmitted or by the independent localization of the alert provided by the Cospas-Sarsat ground segment. This independent localization is based on the capacity to localize the beacon that transmitted the signal based on the reception characteristics of the signal by the Cospas-Sarsat ground segment. With the LEOSAR system, it relies on the Doppler effect but requires several bursts and is disturbed by the beacon motion, with the MEOSAR system relying on the Frequency of Arrival and Time of Arrival of the signal and basically consists in a triangulation just like what is done with GNSS. It eliminates the risks linked to GNSS, as the SAR operators dispose at any time of an estimation of the location of the beacon.

b. ELT(DT) used in the MEOSAR system

In parallel of the MEOSAR evolution, Cospas-Sarsat is also developing new standards of beacons:

• ELT(DT) (Emergency Locator Transmitter for Distress Tracking) a special type of beacon designed to be activated in flight as soon as a distress situation is detected

• SGB (Second Generation Beacons) specially optimized for MEOSAR system and offering an improved performance (detection and independent localization accuracy) compared to current beacons. SGB standard will cover all types of Cospas-Sarsat beacons.

ELT(DT): the Cospas-Sarsat ADT Solution

In order to answer ICAO requirement of Annex 6 for ADT, Cospas-Sarsat has been working on a new beacon standard named ELT(DT). Compared to other types of beacons used in Cospas-Sarsat (EPIRB, ELT, PLB), ELT(DT) has been designed for a particular use case: being activated in flight upon a distress situation. This use case involves additional constraints that were taken into account during the specification phase:

• Based on BEA (Bureau Enquête et Analyse: French Office for Aeronautical Accidents Investigation) studies, it appears that in many cases an aircraft will impact the surface within just a few minutes after the detection of the distress situation. At impact, the beacon and/or the external antenna on the fuselage may be destroyed or the aircraft may sink underwater. In both cases, it is no longer possible to receive signals from the beacon. It was then determined that the beacon has to transmit its signal as soon as a distress situation is detected and until the crash, with a repetition rate high enough to get a sufficient detection probability at ground stations and to make sure that the last burst is not too early before the crash:

■ Transmission of the first burst 5 seconds after reception of the trigger (manual or automatic)

■ A repetition rate of:

• 5 seconds during the first 120 seconds after activation

• 10 seconds after the first 120 seconds and before 300 seconds after activation

• 30 seconds after 300 seconds after activation

• A special transmitted message was designed to indicate at least:

■ The trigger which activated the beacon (automatic trigger or manual trigger)

■ The encoded location from an integrated GNSS receiver

Last but not least, one can note that the MEOSAR system is well suited to process ELT(DT) signals as it allows instantaneous detection and global coverage. MEO satellite visibility offers redundancy then allowing for a high detection rate of the signals. Moreover, if a signal is detected through a sufficient number of satellites, the ground station can compute an independent location associated with speed estimation. Finally, completed by an internationally well-known ground segment for the distribution of the alerts between countries and geographical areas, Cospas-Sarsat offers an efficient solution to ADT.

Another advantage of the Cospas-Sarsat solution is the possibility to combine the ELT(DT) function with a legacy ELT beacon which triggers on crash. Into a single device, it is then possible to answer to both ICAO requirements for ADT and for ELT with minimum evolutions on the aircraft. The specification for this combined solution is currently refined in standardization bodies, in particular in terms of use of homing signal and battery duration, but this would be a significant additional advantage of the solution. To comply with existing environmental and test conditions (lightning, radiofrequency, altitude, pressure, shocks, vibration, fire, transmission duration), ELT(DT) also have now to comply with recent aeronautical standards (DO 160, ED 162) and quality standards: REACH, ROHS, EMC and ESD including documentation, being understood that those requirements have to be fulfilled without any compromise on product availability and reliability, offering ELT(DT) with higher performance requirements.

The Cospas-Sarsat ADT solution has been defined and specified in aeronautical standardization bodies for Europe (EUROCAE) and North America (RTCA) through documents ED-62B and Doc10054, under consolidation and with an expected closure date by June 2018. Currently, it is the only ADT system for which such specification exists, which should ease the support to its adoption.

MEOSAR: Enhancing Location Performance

The MEOSAR system will be interoperable with the current 1.9 million deployed First Generation Beacons. While the LEOSAR and GEOSAR systems have been used for decades, the MEOSAR system just entered in early operations capability in December 2016. The evolution towards MEO orbits is supported by GNSS constellations (Galileo, GPS and GLONASS and future plans for BeiDou) equipped with payloads that relay the distress signals towards ground stations (MEOLUT). MEOSAR uses a different approach compared to LEOSAR. While LEOSAR relies on a multiple burst location method, exclusively based on accurate frequency measurements, MEOSAR relies on instantaneous spatial diversity (reception of the same burst by multiple satellites) and uses both time and frequency measurements (TOA: Time Of Arrival, FOA: Frequency Of Arrival) to compute a location. Then, compared to LEOSAR, the new location method used in MEOSAR allows:

• Localizing a beacon with a single burst

• Localizing a beacon that is fast moving

This transition will increase the quality of service of the Cospas-Sarsat service, with several key added values compared to LEOSAR and GEOSAR system:

• Worldwide real time detection 

• Instantaneous independent localization (localization without reliance on the possible GNSS receiver of the beacon, but based instead on the characteristics of reception of the beacon signal by the SAR system itself) with improved accuracy

• Opportunity to implement the Second Generation Beacon (SGB)

The MEOSAR system opens many new possibilities:

• The beacon may be activated and independently localized in-flight, even in single-burst and even when fast-moving and at high altitude.

• The beacon is continuously tracked after activation, during the complete descent phase, so that rescue teams may be continuously aware of the trajectory evolution. The last burst, just before crash, is also known and localized, which is crucial for the evaluation of its position.

In addition and in parallel of the MEOSAR evolution, the Galileo system introduces the Return Link Service which offers a new feature: the possibility to acknowledge to the user of a beacon the reception of the distress message transmitted and therefore inform them that the alert and its location are processed. This is called the Type-1 RLM. Although Type-1 RLM is the only use case that has been agreed yet by SAR authorities, the Return-Link capability opens several other possibilities to contribute to SAR operations improvement that will be analyzed within a EUROCAE Working Group 98 in 2018, such as:

• A dialog between the ground systems and the beacon, to adapt the transmission periods, the beacon message, and to create system monitoring solutions. A particular dialog case is the switch-off after cancellation, thanks to acknowledgement logic. This switch-off is very important to create autonomous cancellation capacity, as it avoids maintaining alerts if the situation has gone back to normal, and therefore reduces the used bandwidth and prevents from system overcapacity.

• The Return-Link may be used for remote activation. This is particularly useful if the aircraft disappears from other communication systems as the beacon is autonomous (works if everything else in the aircraft is off). Such remote activation could in particular have been used for MH370 flight disappearance.

Cospas-Sarsat operators have expressed their requirements in terms of SGB performance. These requirements are described in document Cospas-Sarsat G.008 “Operational Requirements for Cospas-Sarsat Second-Generation 406-MHz Beacons” that is accessible on the Cospas-Sarsat website. A detailed presentation of SGB specifications and performance can be found in Additional References, and the key improvements are recalled here:

• Better detection performance thanks to a stronger error correcting code

• Increased message content with flexible structure allowing the transmission of information through multiple burst

• Increased TOA measurement accuracy thanks to the use of a spread spectrum modulation, allowing more accurate locations

In particular, the improvement of TOA accuracy compared with FGB characteristics allows reaching the same location accuracy for static and moving beacons. In other words, while TOA and FOA measurements have to be used jointly to estimate the location of an FGB in order to get sufficient location accuracy, the use of TOA only is sufficient for SGB and typically improves the location accuracy by a factor of 10. Since TOA is not affected by the beacon speed, the SGB independent location, which mainly relies on a significantly improved TOA measurement, is also independent of the beacon speed, which is a great asset for independent location of ELT(DT). With accurate and reliable TOA-only position estimation, it becomes interesting to use FOA measurements for beacon instantaneous speed estimation.

The MEOSAR system is also improving the Cospas-Sarsat global performances thanks to the continuous increase of tracking capability on the ground stations (called MEOLUT), with the deployment of new stations in the world (12 commissioned as per today): the addition of dish-antennas to some of them, or deployment of phased-array antennas (see Figure 4). This last solution, deployed as the operational French MEOLUT since 2016, provides the capability to track in parallel all satellites in view instead of selecting some of them pointed by a few dish-antennas: as we are dealing in the end with all GNSS constellations, it will represent up to 30 satellites tracked in parallel.

The gains provided by a higher number of simultaneously tracked satellites are multiple:

• increased probability of detection of the beacon signal through at least one satellite

• higher chances to receive the beacon signal bursts through at least three satellites, enabling the computation of an independent location

• More TOA/FOA measurements, and then a better accuracy on the independent location.

The ground segment is then continuously improved to get real-time access to more satellites relaying alerts from larger areas, as part of the first and most important requirement of the MEOSAR system in Cospas-Sarsat. In the frame of ELT(DT), this objective of improving the performance is particularly important:

• The beacon may only transmit a few bursts before crash: the detection and location probability per burst shall be particularly high

• The beacon localization adds new unknowns: the altitude (usually considered on ground for other beacons, including current ELTs that are triggered by the crash) and/or the speed (in particular for FGB where it impacts the FOA and then the overall accuracy). Even if some default calculations can be implemented which provides quite satisfying results, the access to additional measurements through a fourth satellite is particularly interesting and significantly improves the independent location accuracy. The access to a fifth or sixth — or more — satellite allows to even refine the location performance accuracy.

c. The pre-operational MEOSAR-system used to localize an in-flight activated beacon: the MS804 accident

On May 19, 2016, the MS804 flying from Paris to Cairo crashed into the Mediterranean Sea with 66 people onboard. Unexpectedly, the distress beacon of the aircraft (a standard ELT) transmitted two test messages at 00:36:52 and 00:36:59. These two messages were received after radar contact was lost and a few minutes after the reception of the last ADS-B message, probably several seconds before the crash.

The two messages were received by several SAR satellites of the Cospas-Sarsat space segment:

• 2 GEO satellites: MSG-2 (tracked by the Greek and Turkish GEOLUTs) and MSG-3 (tracked by the French GEOLUT)

• 10 MEO satellites: GPS (PRN 1, 3, 9, 17 and 23), Galileo (PRN 8, 14, 20 and 26) and GLONASS K1-2, received by MEOLUTs in Cyprus, France, Norway, Russia, Spain, Turkey, and USA.

The first message was received with very low signal to noise ratio, therefore, only the second one could be used. The beacon model was not equipped with a GNSS receiver, thus no position was encoded in the alert message. Some MEOLUTs could compute an independent location based on TOA/FOA measurements using standard location algorithm (i.e. not taking into account the fast motion of the beacon) but these locations were highly inaccurate (more than 200 kilometers away from the crash site). An analysis conducted by CNES using location algorithms adapted to fast motion allowed to compute a better location, a few hours after the accident. This information was promptly transferred to search teams and was used to localize the wreck and quickly recover the flight recorders.

Although this accident did not involve the use of an ELT(DT), it demonstrates the ability of the MEOSAR system to provide reliable information to find the accident site in a timely manner.

GRICAS: Providing an Innovative Solution

From the beginning of the MEOSAR system, the European Union (EU) has been deeply involved in the deployment, promotion and enhancement of the system. On one hand, the EU contributes to the space segment of the MEOSAR system with SAR payloads onboard each and every satellite of the Galileo constellation and constituting the Galileo SAR service. On the other hand, EU, via its institutions like the European Commission (EC) or the European GNSS Agency (GSA), every year funds several research and development projects working on the development of the MEOSAR services and the design of new SAR services relying on the Galileo SAR: GRICAS, HELIOS, MAGNIFIC, SAT406M, GRIMASSE, SINSIN, iSSAR… Among those projects, the GRICAS project (Galileo SAR Return-Link Improvement for a better Civil Aviation Safety) is funded by the GSA under the European Union Horizon 2020 Research and Innovation program (grant agreement no. 687556). Initiated in February 2016, it was to be concluded in April 2018 and gathers a consortium of seven companies and administrations from France (CNES, Thales Alenia Space and ECA GROUP (with its brand ELTA), Italy (STMicroelectronics), Spain (PildoLabs, Aeroclub Barcelona Sabadell) and Africa (ASECNA the Agency for Aerial Navigation Safety in Africa and Madagascar). The main objective of the project is to demonstrate the compliance of the Cospas-Sarsat MEOSAR system to the requirements of the Autonomous Distress Tracking function of the GADSS developed by ICAO. In 2017, all the developed operational concepts have been demonstrated through a set of in-flight trials in Europe, testing in particular the first Cospas-Sarsat ELT(DT) (Emergency Locator Transmitter for Distress Tracking) developed for the project. The definition of the operational concept is based on the ED-237 MASPS published by EUROCAE Working Group 98 and the solution proposed is compliant with Cospas-Sarsat specification documents.

d. An operational concept that covers all distress situations taking into account consequences of the distress on the cockpit crew

GRICAS operational concept is based on three main distress scenarios:

• the automatic activation by avionics upon detection of one or several distress situations such as:

■ Criteria defined by EUROCAE as mandatory:

• unusual attitude

• unusual speed

• unusual altitude

• total loss of propulsion

■ Additional criteria identified as relevant by the GRICAS project:

• transponder codes (7500, 7600, 7700)

• fire

• depressurization

• the manual activation by crew

• the manual remote activation from ground through the Galileo Return-Link Service (not required by ICAO at this time).

Based on these scenarios, the solution design requires:

• a new type of Cospas-Sarsat aeronautical distress beacon called ELT(DT)

• an ELT continuously armed and tracking GNSS with its own internal GNSS receiver (GPS + Galileo):

■ The ELT is always ready to be triggered to transmit a distress message as well as the needed data to be localized, as soon as a distress situation is detected.

■ The bi-constellation function of the GNSS receiver also offers robustness to spoofing and jamming on GPS signal

■ beacon can receive an activation command sent via the Galileo Return-Link

• continuous power supply by the aircraft and a 24-hour autonomy at 406 megahertz thanks to the internal and rechargeable battery

• The shape, size and weight of this ELT(DT) are similar to the standard ELT’s  already commercialized.

Moreover, the designed solution offers a strong robustness to possible attempts in tempering the system with:

• robustness to GNSS spoofing: the beacon independent location is computed thanks to the beacon signal characteristics, independently from the message content or availability of GNSS onboard the aircraft, and specific spoofing detectors are implemented together with the beacon internal GNSS to ensure that such attempt triggers the SAR transmission. The beacon can then start transmitting in case of GNSS unavailability (jamming) or unreliability (spoofing), and will then be localized by Cospas-Sarsat MEOSAR independent localization, which will then become the only remaining solution to localize the beacon. To increase the gain of this capability, the beacon transmission implements the Second Generation Beacon waveform, which provides a more accurate in-flight independent localization 

• the acknowledgment from the ground under the responsibility of RCC to avoid fake cancellation of a manually triggered ELT, to avoid ill-intentioned cancellation.

In addition, human factors have become more and more important in recent years in the design of aeronautical systems, and the GRICAS project took these studies into account and implemented a manual acknowledgment from the cockpit crew to be requested in case of automatic activation by avionics, before the avionics send the cancellation command to the beacon. Thus, the pilots can confirm they are back in control of the flight in addition of the flight dynamics criteria characterizing a normal flight situation.

From the beacon point of view, the Cospas-Sarsat ELT(DT) designed for the GRICAS project to comply to the Autonomous Distress Tracking recommendations is identical to currently commercialized and operated ELT’s in terms of electronic components, mechanical and functional interfaces (with an additional interface for the automatic activation). However, this new ELT with Distress Tracking functionality benefits from a new design fitting with aeronautical ADT requirements:

• It implements the new modulation, called Second Generation, of the Cospas-Sarsat system specifically designed to maximize the performances of the MEOSAR system.

• It integrates an internal Galileo compatible GNSS receiver that is continuously turned on:

■ to provide a source of localization, independent from any other aircraft systems

■ to be able to receive any Galileo Return Link messages

■ to be able to identify spoofing or jamming attempts on the GNSS signal

• During normal flight conditions, the ELT(DT) is armed and monitors information

■ From a beacon activation logic that computes the automatic triggers based on detection of a distress situation

■ From the Galileo navigation signal to identify a RLM with its ID encoded and to process the RLM as appropriate (activation for example)

■ From the avionics: the position provided by the avionics GNSS receiver to encode it in the distress or cancellation message when the beacon is activated

• It starts transmitting within five seconds upon reception of triggering command (either manual, automatic or from a remote RLM activation command)

• It is powered by the aircraft main power bus and by an internal rechargeable battery. This double power supply source enables the ELT(DT) to continue to operate when there is a total loss of power onboard and for the whole remaining flight (up to 24 hours).

e. A fully independent solution for distress delivery in all situations

The operational concept and the solution design proposed by GRICAS ensures the ELT(DT) mission not be affected by a communication link inhibition with the avionics, a loss of communication with the aircraft’s GNSS receiver, or even a GNSS jamming or spoofing in the vicinity of the aircraft.

Finally, the ELT(DT) can be triggered in a completely independent process without any inhibition risk thanks to the manual remote activation from ground by Return-Link Message. This service could be provided to airlines, which will be able to request a remote activation (and deactivation) to the Return-Link Service Provider, knowing that the activation of the ELT(DT) will set off a distress at the RCC responsible for the area.

f. Flight trials

All the operational concepts developed within the GRICAS project have been demonstrated through a set of real-time in-flight trials:

• A first campaign, called “ELT(DT) field trial” performed in Sabadell, Spain, onboard a Cessna 182 in April 2017. It was meant to be an in-flight dry-run of the future flight trials and was dedicated to the adjustment of the test equipment. All the test scenarios were performed in-flight and the first in-flight independent localization using the MEOSAR system was realized, with results even better than expected.

• A second campaign, called “in-flight RLS field trial” performed in Sabadell, Spain, onboard a Cessna 182 in November 2017. Its objective was to test the Return-link Service (RLS) user cases defined in the GRICAS operational concept with the pre-operational RLS Provider (RLSP).

• A third campaign, called “Commercial aviation field trial” performed in Toulouse, France, onboard a Falcon 20 of SAFIRE (The French Service of Instrumented Aircraft for Environmental Research) early December 2017. Its objective was to test the GRICAS SGB ELT(DT) onboard an aircraft offering a flight domain similar to the one of an aircraft dedicated to commercial flights (as targeted in GADSS). The Falcon 20, with its 800 km/h cruise speed and 10 kilometer cruise altitude was a perfect test aircraft for this flight trial.

• A fourth flight trial, called the “Final Commercial Aviation field trial”, performed in February 2018 in Dakar, Senegal, onboard the ATR 42 test aircraft of ASECNA. It was a field trial mostly dedicated to dissemination activities around the Cospas-Sarsat compliance to ADT requirements and the improvement of SAR in Africa.

In addition, a helicopter field trial was performed in July 2017 in Graulhet, France, onboard an AS 350 B2 Eurocopter “Squirrel” helicopter. It aimed at initiating work on addressing the problem of ELT(DT) designed for rotorcraft.

For these campaigns, a dedicated platform was developed by PildoLabs in the frame of the project, following the standards and best practices used for laboratory equipment devices that are carried out in research aircrafts (see Figure 5). This demonstration platform contains as main equipment the Second Generation ELT(DT) developed by ELTA, a Remote Control Panel to monitor the beacon status and an EGNOS GNSS receiver to be used as the beacon’s position external reference. The platform was temporarily installed in the aircraft during the flight campaigns and connected to the SAR antenna, as it can be seen in Figures 6-8. The manufacturer also developed an emulator of the Beacon Activation Logic that sends the automatic activation command to the beacon when triggering criteria are met.

The flight tests performed during the project are unique in many aspects:

• It was the first time that an ELT(DT) prototype, representative of what a real ELT(DT) could be, flew onboard airplanes and a helicopter, and was automatically triggered in-flight.

• It was the first time an independent localization of a second generation Cospas-Sarsat beacon was computed during a transmission on-board a flying aircraft and helicopter.

• It was the first time that the Galileo SAR RLS was used to remotely activate an ELT(DT) aboard a flying aircraft.

• Finally, it was also the first time first and second generation beacons modulated distress messages transmitted onboard a flying aircraft were recorded (in the same test environment) to provide consistent data to compare the relative performances of both modulations for independent localization of fast-moving beacons.

Scenarios

The attention of the reader is drawn to scenario 2 (see Figure 9) which is completely automatic. Indeed, for this scenario the beacon was automatically triggered by a Beacon Activation Logic (BAL) based on GNSS information on the flight altitude of the test machine. For the sake of simplicity, the BAL used computed an automatic trigger based on an altitude threshold (instead of proximity with ground); the reader will anyway note the representativeness of the automatic activation process. After take-off, the test pilot pursues the ascent until overtaking a threshold altitude (agreed with the GRICAS engineers and encoded as the threshold altitude in the BAL). The BAL, collecting the position data provided by the GNSS receiver of the demonstrator (emulating the GNSS receiver of the avionics for the tests), identifies the unusual altitude, computes a trigger “Unusual altitude” and sends it to the ELT(DT) prototype. The ELT(DT) prototype receives the trigger and starts transmitting a distress signal with the relevant activation method encoded in the Rotating Field #1. The MEOLUT receives and processes the signal. It computes in real time a single and multi-burst independent location and delivers the alerts to the test ground engineer. Finally, after 5 to 20 minutes (depending on the test case), the pilot flies again under the threshold altitude, the BAL checks that the pilot is in control, then computes a cancellation trigger, the beacon starts transmitting a cancellation message. The cancellation messages were well received, processed and delivered to the test ground engineer by the MEOLUT.

Results

The independent localization accuracy of the ELT(DT) prototype based on Cospas-Sarsat second generation wave form is very good, with single-burst accuracy at 90% of 800 meters and at 95% of 850 meters. It is significantly better than the target performances for second generation beacon specified in Cospas-Sarsat documents: 5 kilometers @ 90% and 13 better than the performance obtained for an ELT based on first generation wave form.

In the case of the Final Commercial aircraft tests, the flights took place outside of the declared Coverage Area of the French MEOLUT (Toulouse, France) used for the tests (in Dakar, Senegal, 3,600 kilometers far from Toulouse, whereas the coverage area is a 3,000-kilometer-radius circle centered on Toulouse), i.e. outside the area where the performances of the MEOLUT are guaranteed. The performances obtained are very good considering the constraints on the tests.

The performance accuracy is a bit lower for the helicopter tests which can come from the interferences observed between the engine and the antenna used to transmit. Further tests will be performed in 2018 and 2019 onboard helicopters with adapted beacon and antenna solution.

The Beacon/MEOLUT Latency

During the field trial, the latency was inferior to 20 seconds between the activation of the beacon and the delivery of the demodulated, independently localized and interpreted signal to the operator on testing site emulating the MCC. This means that the Cospas-Sarsat MEOSAR based ADT solution can pretend to be a real-time flight distress tracking solution that would dramatically improve the efficiency of the SAR operators with a typical overall latency of one minute for the delivery of the distress message to the operators.

Conclusion

Today, it appears that the Cospas-Sarsat MEOSAR system, relying on payload deployed on GNSS constellations (GPS, Galileo, GLONASS), offers all the conditions to meet the new requirement of ICAO for  ADT system for Commercial Aviation, with a new generation of in-flight triggered beacons, identical to the current ELT in terms of aircraft integration, but capable of receiving triggers and cancellation events from the avionics, from the crew or from internal sensors, and of detecting and managing their inhibitions to maintain the capability to raise alerts and be localized in any situation. The redundancy of the localization information (GNSS position encoded in the message and independent MEOSAR localization), the gratuity of the service, the worldwide instantaneous coverage of the space segment and the worldwide coverage of the ground segments guarantee a high quality of service and performances that will contribute efficiently to improve air navigation safety.

Thanks to projects like GRICAS, SAR and aeronautical authorities have a common understanding on the fact that MEOSAR is a strong, reliable system to ensure commercial flight safety. The space segment is currently performing well and, while it is still under deployment, it has already demonstrated its capability to contribute to SAR operations for aeronautical events. The aircraft segment is migrating to benefit from this space segment and the existing SAR ground segment is providing strong results and migrating to the MEOSAR system. However, to fully embrace the MEOSAR evolution and as new operational needs appear, an improvement of the ground segment (MCC, RCC) is required and will allow the MEOSAR system and the Galileo SAR service to reach their maximal potentials. To this extent, EUROCAE will initiate mid-2018 activities to define a Minimum Aviation System Performance Specification for Aircraft Emergency Locator Transmitter Return Link Service including but not restricted to remote activation of ELT(DT), command to modify the content or transmission rate of the distress message and acknowledgment of reception of beacon messages.

In parallel to the on-going development of an ADT solution for commercial aviation, it is not foreseen to extend the ADT to general aviation. General aviation’s needs offer very specific challenges in terms of ADT:

• large variety of aircrafts requiring a better beacon autonomy and flexibility,

• many different user profiles and stakeholders requiring a common communication solution, 

• stronger cost constraints.

Future efforts will now be oriented towards such user sector, with new activities starting in the coming months (in particular with the GRIMASSE project) to make sure that the MEOSAR system ADT solution can answer to General Aviation needs and improve air navigation safety and security to help saving more lives.

Acknowledgment

The GRICAS project has received funding from the European GNSS Agency under the European Union’s Horizon 2020 research and innovation program under grant agreement No 687556.

Additional Resources:

1. cospas-sarsat.int, “Specification for Second-Generation Cospas-Sarsat 406-MHz Distress Beacons”, C/S T.018, which can be found on COSPAS-SARSAT website https://www.cospas-sarsat.int

Authors

Pauline Martin, technical manager of the H2020 GRICAS project, and Operational Concept Engineer in Data Collection in charge of navigation-based innovative services within the Navigation Domain of Thales Alenia Space. She is in charge of developing the new operational concepts for MEOSAR applications. She is also in charge of Thales Alenia Space attendance to COSPAS-SARSAT.

Thibaud Calmettes is technical manager for Data Collection and Scientific Application Programs Service within the Navigation Business segment of Thales Alenia Space France. After being the technical responsible for the on-board processing equipment during ARGOS 4 development, he is now in charge of various data collection systems, such as Satellite-AIS and VDES, and of developments around MEOSAR, including new generation beacons, signals, and MEOLUT processing. As manager for the scientific domain, he also works on the innovative use of GNSS receivers on-board satellites.

Yoan Gregoire is radionavigation and radiolocation engineer in the navigation/location signals and equipment department in CNES, the French Space Agency. His COSPAS-SARSAT activities cover second-generation beacons specifications development and performance evaluation. He is in charge of the development of a MEOSAR open reference chain developed by CNES and used for signal characterization and performance evaluation.

Mercedes Reche obtained her MSc in Telecommunication Engineering from the Universitat Politecnica de Catalunya in 2002. She started to work on Satellite Navigation in 2003 with the Centre Nationale d’Etudes Spatiales (CNES) in France. In October 2004 she joined PILDO LABS as aerospace engineer, giving technical support in activities related to GNSS Operational Validation (mainly EGNOS and GBAS) and acting as project manager of several international projects for Eurocontrol, the European Commission and the European GNSS Agency. Nowadays she is one of the managers of the company, in charge of the GNSS Monitoring department, and provides support to the business development activities.

Christophe Chatain  is in charge of developing and promoting MEOSAR ELT (DT) product line at ECA Group (ELTA Subsidiary). He received his Engineer diploma from Polytech Lille in 1986.  He is now involved in several research projects to define future features of COSPAS-SARSAT beacons for ECA Group.

Michel Monnerat, manager of the advanced projects within the Navigation Business segment of Thales Alenia Space France. After working on many radar programs within Alcatel Space, and being in charge of the onboard processing of the ARGOS/SARSAT payloads, he has been involved in the Galileo program since 1998, particularly for the signal design and performance aspects. He is now in charge of the advanced projects department dealing with the developments of Galileo and EGNOS ground stations, Satellite-Based data collect systems, GNSS regulation including standardisation and Spectrum management, as well as Engineering for innovative Location Solutions for land applications. The department is also in charge of Search and Rescue developments.

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High-tech standout Broadcom has sold over one billion GNSS chipsets worldwide, leveraging all major global satellite navigation constellations and the full range of GNSS features, including Galileo’s dual-frequency and innovative BOC modulation capabilities.

Broadcom is doing things that no other company in the world is doing. Positioned right at the leading edge of GNSS-for smartphone technologies, the company boasts an extensive portfolio of standalone GNSS receiver chips and combination GNSS receiver and sensor-hub, or location-hub, chips.

Already an industry leader in multiconstellation GNSS technologies, present in the latest, top-of-the-line handsets, Broadcom positively threw down the gauntlet last year with the inauguration of its new BCM47755 chipset, the first dual-frequency GNSS chipset for smartphones. The unveiling, which came in the fall of 2017, led European GNSS Agency (GSA) Director Carlo des Dorides to suggest we might see a fully functional dual-frequency smartphone as early as summer 2018.

Obviously Multi-Constellation

Broadcom has always been a firm believer in the cumulative value of each additional GNSS constellation. Speaking from his office in Madrid, Broadcom’s Associate Director for GNSS Product Marketing Manuel del Castillo told us, “We initially supported GPS and then progressively added all of the other major GNSS constellations – GLONASS, BeiDou and Galileo.

“We treat the different GNSS signals in an equivalent way, in terms of acquisition and tracking, and interchangeability in subsequent fixes. Our motivation has always been accuracy and yield improvements in challenging urban environments, where our customers have constantly pushed us to keep improving.”

The first Broadcom multi-constellation chip, adding GLONASS to GPS, came in 2011. “After that, in 2013, we added BeiDou,” del Castillo said. “And in 2014, two years before Galileo Initial Services were announced, Broadcom added Galileo.

“Our Galileo chip features a multipurpose, sensor-hub and sensor-fusion software for use in smartphones and tablets, as well as ‘system-on-chip’ architecture, so we can meet the challenge of always-on location with very low power. And of course our users benefit not only from the additional Galileo satellites, but from the new BOC modulation, which itself improves accuracy.”

In each case, del Castillo said, whether it was adding GLONASS, BeiDou or Galileo, similar processes were involved, including understanding the ICD, discussing implementation and testing initial prototypes, developing the B0 revision and carrying out receiver tests.

“In the case of Galileo, however,” he said, “there is a lot more support and clarity compared to previous constellations, in particular BeiDou. And Galileo is easier in terms of the RF part than GLONASS or BeiDou. On the other hand, Galileo’s baseband is more complex due to the longer codes, secondary codes and BOC modulation.”

Dual Frequency at Last

Last year, Broadcom launched the chip that changed everything. “The BCM47755 in includes support for dual frequency in both GPS L1 and L5, and Galileo E1 and E5,” said del Castillo.

Until now, mobile positioning and navigation devices have been powered by single-frequency GNSS receivers. The expanded availability of L1/E1 and L5/E5 frequencies, thanks especially to Europe’s Galileo constellation, now means reduced multipath and ionospheric interference, improving positioning in urban and other environments.

“We are working with a number of handset vendors, Samsung in particular, to bring multi-constellation and multi-frequency capabilities to your next smartphone,” said del Castillo. “The BCM47755 provides a high level of accuracy with minimal power consumption and footprint, and is capable of enabling an entirely new set of high-precision LBS applications.” These include lane-level vehicle navigation, advanced gaming apps, mobile augmented reality, car-hailing applications, driving assistance for cars, drone guidance, and many others that have yet to be imagined.

Del Castillo said the decision to go dual-frequency was an easy one to make. “Once we were sure that the cost increase was going to be tolerable and the technical benefits would be far greater than the added cost, we forged ahead quickly. Of course we were moving into unexplored territory for a mass-market chip. The development process was extensive and it involved a substantial chip revision.

“A major challenge was always going to be the ten-fold increase in complexity of L5/E5 signals, and the fine-tuning of the internal phase delays between L1/E1 signals and L5/ E5 signals. Also, there was the need for additional memory and computational resources, while reducing power consumption. And we had to do all of this with a minimal cost impact for our customers.”

“Bringing in Galileo and, most importantly, implementing the dualfrequency support has absolutely paid off,” del Castillo said. “The benefits for our customers start, of course, with the performance improvement, delivering reliable sub-meter accuracy even in difficult environments. But for Broadcom itself, supporting Galileo in E1 and in E5 means we are now clear leaders in the innovation race, and we are benefiting from joint actions with the GSA and working in partnership with ESA.”

Not Standing Still

Competitors beware; the gang at Broadcom shows no signs of resting on their laurels. In another example of innovative thinking, the company has recently partnered with Google to host some location-based applications within its GNSS chipsets. Activity recognition is one such application that uses smartphone sensors to determine what the user is doing, i.e. walking, biking, driving, etc. The application can run on the smartphone’s own applications processor, which uses quite a bit of power, or, now, it can be pushed down to a low-power processor in an onboard Broadcom GNSS chip.

Del Castillo said Broadcom is intent on staying smart, harnessing its multiconstellation and multi-frequency GNSS technologies to maintain and increase its advantage over all competitors. “We see further refinements coming in our E5 implementation, ultimately allowing us to deliver even higher accuracy for newer and even more exciting applications. As long as we keep moving the innovation bar higher and higher, we believe we can stay a step ahead of the competition, and the ultimate winner will be the GNSS user.”

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The goal of the new PNT effort, the Army said in a Feb. 27 special announcement, is to “provide position, velocity, and time data with the aid of non-GPS augmentation to the dismounted and mounted Soldier in GPS degraded or denied environments.”

The announcement on the Fed Biz Opps website was issued “for planning purposes” under solicitation number: Army_Program_Manager_Positioning_Navigation_and_Timing

To accomplish its goal the service’s PNT office said it is considering using the Consortium for Command, Control, and Communications in Cyberspace (C5) to pursue prototype A-PNT systems. C5 has an agreement that enables the Army to fund research using Other Transaction Authority (OTA), a mechanism created as a work-around for cumbersome procurement rules so that program managers can identify and fund a project in months instead of years. Department of Defense (DoD) managers can make OTA agreements valued at up to $50 million without additional approvals. Program managers can enter contracts for up to $250 million with a supporting determination from the department’s senior procurement executive according to a May 2017 article in the legal journal The Government Contractor.

“OTA is basically an alternative to the Federal Acquisition Regulation and it’s designed to make it easier for so-called non traditional defense contractors – think Silicon Valley – to make it easier for startup companies, cutting-edge, small technology companies that historically have had no interest in working with the government, Defense Department or otherwise,” said Charlie McBride, president of the Consortium Management Group, which is the management organization that owns and operates C5.

The way it works, he said, is that a program manager determines a need and comes to C5 with their requirements and a rough budget. C5 puts that information into the form of a request for white papers, which is then sent to all its members; they typically have several weeks to reply. Those organizations that are interested and can meet, or nearly meet, the requirements submit a white paper describing their approach. The government manager evaluates the papers, choosing the organization(s) he or she wants to work with. “There’s a process that takes over then that leads to an actual agreement between the government and C5 on behalf of our member,” said McBride. That last step, he said, takes 90 days. The project can also be expanded and even enter into production under OTA rules.

Organizations apply for membership online, McBride said. The C5 staff talks with applicants and check the applicant’s website, but approval is generally granted quickly “unless there’s something egregiously wrong.” Membership information is available at www.c5technologies.org/ though the organization was having website issues as SIGNALS went to press.

This full article can be read online at http://insidegnss.com/node/5811

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The Next Generation Operational Control System (OCX) being developed by Raytheon is essential for the Air Force to be able to fully utilize the capabilities of the GPS III and GPS IIIF satellites. The program is five to six years behind and, according to a March 2017 report from the Government Accountability Office, surged in cost from $3.6 billion in November 2012 to $5.5 billion in September 2016 – a 53.2 percent increase in fiscal year 2017 dollars. The December report by MITRE, obtained and described by Bloomberg News, put the cost at $6.1 billion.

The OCX contract was originally valued at slightly more than $1.5 billion with options, when it was awarded to Raytheon in 2010.

“That program is a perfect example of the problem we have in the Air Force,” Rogers (R-Alabama), chairman of the House Armed Services Committee Subcommittee on Strategic Forces, said at the forum in late February. “We’ve had that black hole for money for years and it’s still not able to be executed. And show me the person responsible for that. You can’t. It’s all those committees, they’re all pointing at each other. ‘Oh it’s not me; it’s them. It’s not me; it’s them.’ In the meantime we still don’t have the capability and we don’t see an end in sight, and this thing has been incredibly over budget.”

MITRE had reportedly suggested abandoning OCX in favor of upgrading the Lockheed Martin ground system currently being used. When asked if he wanted to see the Air Force make more changes to try to reform that program – or to have it try to stabilize and execute on the current program of record – Rogers indicated a need to push forward.

“I think we need to get it right,” he told a CSIS forum on the FY19 space budget. “I don’t think we should just stop with what it can do right now. But I don’t see that happening. I’ve been so disappointed in that program.”

Rogers shared the stage with Rep. Jim Cooper (D-Tennessee), his committee’s ranking member. The Strategic National Security Space: FY19 Budget Forum was held February 28 in Washington, D.C.

“Properly understood,” said Cooper, “the Constitution makes Congress a board of directors. We shouldn’t micromanage. We should look at the big policy decisions and then let capable services implement them. But this is a situation that really is a nightmare. To have satellite capability and no ground communications for over a decade and really no hope in sight – and with no accountability. This would never stand for a second in the corporate world. And the corporate world is not perfect but at least there tends to be accountability.”

Cooper, who did not suggest during the forum that OCX should be changed or dropped, agrees with Rogers that there should be a separate Space Corps as a way to elevate and speed military space programs and increase accountability.

This full article can be read online at http://insidegnss.com/node/5812

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1. Pipeline Inspecting Blimp

South San Francisco, California

Mothership Aeronautics is using autonomous solar-powered blimps to disrupt long-distance aerial data collection. The TerraSoar Aerial Intelligence Airship features a powerful combination of lighter-than-air lifting and high efficiency solar cells that enable higher endurance and longer-range flights than possible with drones, while also operating heavy payloads like LiDAR.

The name, TerraSoar, stems from a family of prehistoric flying reptiles. The Pterosaurs were the first vertebrates known to take to the clouds just as the TerraSoar is the first blimp-type drone to hit the drone inspection market and Mothership Aeronautics’ first Aerial Intelligence Craft.

2. Remote Control Fireboats

<em?>Vancouver, British Columbia, Canada

To address the evolving safety and security needs of modern ports, Vancouver-based naval architects and marine engineers Robert Allan Ltd., and international marine technology specialist Kongsberg Maritime are collaborating on the development of a radically new remotely-operated fireboat that will allow first responders to attack dangerous port fires more aggressively and safer than ever before.

The Kongsberg Maritime control & communications system will feature a robust high-bandwidth, low latency wireless link to a semi-portable RALamander operator console that can be located on a manned fireboat, or other vessel of opportunity such as a tug boat or pilot boat. In common with other KONGSBERG autonomous control systems, the architecture of RALamander’s control system will leave the door open to a range of autonomy levels, which are configurable or future-upgradable to suit the operator or port’s evolving needs.

3. Cereal-Box Satellite

Aalborg, Denmark

The European Space Agency’s first mission of the year, launched in February, was the GomX-4B, the ESA’s most advanced technology-tester yet, featuring a hyperspectral camera and tiny thrusters to maneuver thousands of kilometers from its near-twin to try out their radio link.

These CubeSats are built around standard 10×10 centimeter units by GomSpace in Denmark. As “six-unit” CubeSats they are as big as cereal boxes – but double the size of their predecessor GomX-3, released from the International Space Station in 2015.

“ESA is harnessing CubeSats as a fast, cheap method of testing promising European technologies in orbit,” comments Roger Walker, heading ESA’s technology CubeSat efforts. “Unlike GomX-3, GomX-4B will change its orbit using cold-gas thrusters, opening up the prospect of rapidly deploying future constellations and maintaining their separations, and flying nanosatellites in formations to perform new types of measurements from space.”

Image credits
1. Mothership blimp from the company: Mothership Aeronautics
2. Fireboat from Robert Allan Ltd.
3. Cereal-box satellite from the European Space Agency

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An end-to-end simulation is presented using the Positioning and Integrity Performance Evaluator (PIPE). The simulation includes side by side comparison of a vehicle path in the presence of spoofing as evaluated by the authors’ Generalized Pseudo Bayesian 1 (GPB1) algorithm and a snapshot least squares algorithm. Every field application has its own operational conditions and resulting requirements with respect to accuracy, availability, and continuity for systems that provide position, velocity, and time (PVT) measurements. For example, Global Satellite Navigation Systems (GNSS) found application for certain approach procedures in commercial aviation due to the advent of Satellite Based Augmentation Systems (SBAS) that provide additional integrity information to GNSS receivers. GNSS reception environments for aircraft are nominally clear-sky conditions and the combination of GNSS, SBAS, and advanced processing techniques like Receiver Autonomous Integrity Monitoring (RAIM) yields the required compliance for probabilities of detecting malfunctions and times-to-alert.

Providing absolute, three-dimensional positions on Earth together with well-proven and cost-efficient receiver technologies predestines GNSS equipment to enter the emerging markets of land-based users such as autonomous vehicles and the railway industry. However, since the operational conditions of these applications differ dramatically from those present in the aviation world, PVT measurements with integrity based on GNSS as previously developed cannot be translated directly to land-based users.

This article presents the scope and an application example of the Integrity for Navigation Land Users (INLU) research study as part of the European Space Agency’s Technology Research Program (see Additional Resources, J. Wendel et alia (2016a)) that develops techniques to provide PVT solutions to land-based users within defined integrity bounds.

The environments in which landbased users move impose reception limitations on GNSS receivers. In cities, the GNSS signals are frequently blocked and reflected by buildings, structures, and other traffic, for example. The same is true for railway applications. Here, GNSS blockages arise from features ranging from shunting yards over railway stations to rides through cuttings and tunnels.

To be able to create new satellite navigation technologies that provide PVT information with integrity and solutions for high-precision positioning even under difficult conditions, a well-suited end-to-end simulation tool is required. End-to-end means that realistic signals from complete satellite constellations can be generated which take, for example, transmitter and receiver antenna characteristics, atmospheric and local wave propagation mechanisms, receiver dynamics, and other distorting effects into account.

Often such end-to-end simulations are performed using a radio frequency constellation simulator (RFCS) and hardware GNSS receivers. A number of commercial off-the-shelf solutions are available for each of these items. However, for the development of said algorithms, it is more efficient to be able to use pure software or hybrid hardware/ software solutions. This allows for a much higher frequency for the development-integration-test cycles of a project.

To respond to INLU’s requirements and to meet today’s needs of GNSS endto- end simulations, Airbus Defense and Space has been developing the Positioning and Integrity Performance Evaluator (PIPE) as a research activity since 2012. Beyond INLU, PIPE found applications for the development of novel tracking (F. M. Schubert et alia (2014a); F. M. Schubert et alia (2015)) and channel model research (I. Gulie et alia; F. M Schubert et alia (2016)), to name a few examples.

The article’s organization follows the high-level procedure of getting results from an end-to-end simulation run using PIPE. We first describe the scenario definition in terms of user trajectory, satellite constellation, and propagation channel conditions. Next, the configuration of the simulated receiver is reported with methods for acquisition and tracking of GNSS signals and computation of the PVT solutions. An example is then given that reports a filter bank approach developed during the INLU project. It is used for mitigating the influence a spoofer has on the receiver’s performance.

Scenario Generation

Figure 1 reports the general structure of end-to-end GNSS simulations that can be performed using PIPE. PIPE consists of three groups of programs: tools for digital signal processing (DSP), a GNSS scenario and constellation simulator, and a GNSS receiver. PIPE’s DSP provides classical signal chain simulations consisting of a signal source, one or multiple signal processors, and a signal sink. Among other programs, PIPE includes GNSS signal generators, filters, up- and down-converters, and interference signal generators.

PIPE’s scenario and constellation simulator programs are able to generate user trajectories, satellite positions for given times and orbits, and the propagation channel response based on multipath components. As this group of scenario-related programs differs from the DPS programs, they are called SNIPE tools — short for Scenarios for Navigation and Integrity Performance Evaluation. INLU also requires the possibility to process real-world signals. Signal sources for the PIPE receiver can be sampled signals as sensed by antennas, the PIPE software GNSS signal generator, or samples recorded from an RFCS’s output.

PIPE accommodates INLU’s diverse requirements by a modular approach of signal chain simulations: Certain software elements of the chain can be replaced by hardware components. Additionally, interfaces to front-end sampling and replay devices are available.

The following subsections describe the creation of the user’s trajectory, the simulation of a satellite constellation as well as the processing of the response stemming from a propagation channel model.

Trajectory Generation

The first step in the scenario generation process is the creation of the user’s trajectory. This trajectory serves as input for the generation of GNSS observations, as well as further sensor data like odometers, inertial sensors, baro-altimeters, and magnetometers. The challenge hereby is to produce consistent dynamics data. When the accelerations and angular rates provided by the trajectory generator are used for the generation of inertial sensor data assuming an ideal inertial measurement unit, the output of an ideally initialized strapdown algorithm must match the original trajectory exactly, even after longer simulation times. This is achieved in the trajectory generator first by producing desired positions and attitudes over time. Then, a strapdown algorithm together with a flight control-like algorithm is applied. From the small offsets between desired positions and attitudes and the strapdown state, accelerations and angular rates are generated, which are provided to the strapdown algorithm in the next epoch and drive these offsets to zero.

Constellation Simulation

From the trajectory generation, the GNSS antenna positions and velocities also can be calculated at equidistant points in time. For each of these points, the constellation simulation needs to calculate the corresponding satellite positions and velocities based on RINEX files. The trajectory time scale defines the times of reception of the satellite signals. In order to calculate the satellite positions and velocities, the times of transmission at the satellite need to be determined. This is achieved by approximating the satellite orbit in a time interval of T=0.1 seconds by a straight line, which is accurate to the sub-millimeter level. Denoting the time of reception with t0, the satellite position pS at the time of transmission t0–t can be expressed as

It must now be considered that the ECEF frame is rotating. The ephemeris describes the satellite position in the ECEF frame at the point in time for which a satellite position is calculated. In order to express the satellite position at t0–T in the ECEF frame that is valid at t0, the rotation of the ECEF frame in the time interval T needs to be considered:

In the following, pS(t0–T) always refers to the satellite position at t0–T, expressed in the ECEF frame at t0. In order to determine the time of flight of the satellite signal t, the equation relating the range between satellite position at time of transmission, pS(t0–t), and antenna position at time of reception, pA(t0), is used:

Here, c denotes the speed of light. Inserting the linear approximation of the satellite orbit and squaring the equation leads to

This is a quadratic equation that can be solved for the time of flight, t. Consequently, the satellite position at time of transmission, pS(t0–T), in coordinates of the ECEF frame at time of reception, t0, is obtained. The carrier phase, code phase, and Doppler measurements obtained at t0 can then be generated using appropriate error models.

Propagation Channel Models

The reception conditions for land-based users are impacted to a large extent by multipath propagation caused by objects and structures in the receiver’s vicinity. Moreover, in urban areas, signals are often blocked and diffracted by buildings and trees. State-of-the-art multipath propagation models reproduce these effects and generate channel impulse responses (CIR) at a given rate dependent on the receiver’s dynamics. CIRs contain components that reflect the complex amplitude and delay of line-of-sight as well as multipath components.

PIPE’s interface to a channel response generated by a multipath propagation model is given by the Channel Data Exchange format (CDX) (see CDX – Channel Data Exchange Library in Additional Resources). The PIPE GNSS signal generator can read the channel impulse response for every time step from a CDX file and generate the corresponding multipath components with their respective delays and complex amplitudes in the output signal. This allows for the usage of various channel models for INLU. Within the project, the model recommended by ITU-R P.681 for urban environments was used. Additionally, this model was extended for railway applications during a research activity (I. Gulie et alia). Figure 2 shows the visualization of a common railway scenery.

The INLU project also requires the generation of RF signals by hardware constellation simulators using the mentioned channel models. As these models produce more multipath components than a common hardware constellation simulator can re-produce, a component count reduction step is required. In INLU, a method based on F. M. Schubert et alia (2014b) is applied.

Land User Receiver Prototype

The PIPE GNSS Receiver used to run the INLU simulations is not only able to process samples, bit-true mode in INLU terms, it has also a semi-analytic operation mode that calculates correlation results based on pre-computed autocorrelation functions of GNSS signals. The semi-analytic mode runs multiple times faster than the bit-true mode and allows for simulation of long runs. The scenario input data as well as the receiver’s implementations for items such as tracking loops and PVT computation are identical for both modes.

Tracking Algorithms

In addition to standard tracking methods like Early-Minus-Late and Bump Jumping, the PIPE receiver offers a number of state-of the-art tracking techniques, such as the Double Delta Correlator, Kalman filter-based methods, Double Estimator, and Maximum Likelihood-based methods, such as Multipath Estimating DLL and Vision Correlator. All these methods are scrutinized during the INLU project. For the following example, the Astrium Correlator (AC) is applied which offers robustness against locks to false side peaks while tracking binary offset carrier (BOC) signals (F. M. Schubert et alia (2014a)). For signal tracking, the AC uses a BOC’s signal subcarrier to exploit the higher accuracy that can be achieved when tracking such signals to the legacy binary phase shift keying (BPSK) signals. At the same time, the AC checks if it tracks the BOC signal’s central peak via the observation of the BPSK envelope of the signal. If a lock to a side peak is detected, the tracker is commanded to switch its tracking point toward the correct main peak.

Integrity Algorithms ARAIM Tailored to the Railway Environment

The main objective of INLU was the development of integrity algorithms tailored to the land user environment. The following integrity concept for railway users was developed as an INLU scenario.

Providing integrity for train position information is a major technical challenge due to the small integrity risk that is tolerated in railway applications. In the current implementation of the European Rail Traffic Management System (ERTMS), the train position is propagated using odometry, and corrected when a balise group is reached. A balise is a transponder that provides an absolute location reference to the on-board unit of the train, allowing the train to locate itself within a movement authority. GNSS positioning is being considered in the context of the ERTMS evolution for the realization of a virtual balise concept, where GNSS is used for the detection of virtual balises. The approach of virtualizing the balise transmission system aims to reduce the cost of trackside infrastructure associated with the installation and maintenance of physical balises, while minimizing changes to the existing system and maximizing interoperability. Consequentially, requirements on the integrity of position information provided by the GNSS receiver are very stringent.

The integrity concept within INLU can be seen as a step towards a realization of the virtual balise function. A block diagram of this concept is shown in Figure 3. The integrity algorithm processes pseudorange measurements from a GNSS receiver, measurements from odometry, and exploits information contained in a map database. It consists of two major building blocks, a module for pseudorange measurement rejection, and the integrity module that calculates the projection level.

The measurement rejection module consists of a Kalman filter that integrates the odometer measurements with the track database and the pseudoranges from the GNSS receiver. The a priori position available from this filter is used to calculate the Mahalanobis distance of each pseudorange, and upon excess of a pre-defined threshold for the Mahalano-bis distance, the respective pseudorange is rejected.

The integrity module calculating the projection levels uses the pseudoranges that passed the Mahalanobis distance check in the measurement rejection module as well as information from the track database. In consequence, the position solution provided by the integrity module is independent of the odometry, in the sense that the odometer readings do not enter in the position calculation. For projection level calculation, a solution separation approach is used. In solution separation RAIM, the spread of subset position solutions with respect to the full set position solution is assessed. Subset position solutions are obtained by excluding subsets of satellites from the position calculation. Thus, the number of satellites that are excluded must reflect the number of simultaneous faults that need to be considered. The probability that a higher number of simultaneous faults occurs is very low, but might still be included in the integrity risk budgeting.

The major difference between a conventional solution separation RAIM outlined above and the proposed integrity concept is that from the pseudoranges which have passed the measurement rejection module, a three dimensional position solution is not calculated, but rather a GNSSbased odometer distance results. Using this GNSS-based odometer distance, a three-dimensional position solution can then be obtained from the track database. Consequently, for the calculation of protection levels, the spread of GNSS-based odometer distances is assessed, not the spread of position solutions. Obviously, calculating a GNSSbased odometer distance instead of a three dimensional position reduces the number of unknowns from four to two, which means that with two pseudoranges only, a solution can be obtained. More details of this integrity algorithm are given by J. Wendel et alia (2016b).

Filter Bank for GNSS and INS Integrity

Within the INLU project, integrate navigation systems were also addressed, in which the software GNSS receiver is combined with inertial sensors in loose, tight, and ultra-tight coupling architectures. Such architectures do not allow for the calculation of protection levels using ARAIM algorithms because these require that full set and subset position solutions are calculated using snapshot least squares.

A variety of techniques can be found in literature which aim at the provision of integrity for integrated navigation systems. Batch processing approaches re-formulate the GNSS/INS data fusion as a least squares problem, which then allows us to apply RAIM or ARAIM techniques (M. Joerger and B. Pervan). Another option is to use a filter bank. Each elemental filter in the filter bank is robust with respect to a specific fault.

In the simplest case, an elemental filter does not process the measurements of a specific satellite. In case the measurements of this satellite are faulty, this filter is not affected. This also avoids the need for a pseudorange fault model which is an advantage because such a model is in general rarely available. Examples of filter bank approaches can be found in (M. Brenner; J. Diesel and S. Luu), with the basic concept illustrated in Figure 4 with three filters only. Obviously,
a real GNSS/INS filter bank contains many more filters. For the single fault case, the number of elemental filters matches the number of satellites from which measurements are available, with possibly an additional elemental filter assuming no faults are added. Each of the elemental filters propagates its state and covariance matrix forward in time using the measurements provided by an inertial measurement unit (IMU).

When pseudorange and delta range measurements become available, they are processed by each elemental filter except for those the elemental filter assumes to be faulty. Hereby, the model probabilities are also updated. For each elemental filter, the model probabilities represent the likelihood that the assumptions of the filter are correct, i.e., that the satellites that the elemental filter assumes to be faulty actually are faulty. This update is based on the ability of the filter to predict measurements. The better the elemental filter pseudorange and delta range predictions match the actually available measurements, the higher the model probability.

After the measurement processing, a mixing step is executed. In this mixing step, each elemental filter is reinitialized with a new state estimate and covariance matrix, which are calculated from the model probabilities, state estimates, and covariance matrices of all elemental filters. It is important to note that for the most widely used filter bank, i.e., the Interacting Multiple Model (IMM) filter bank, all elemental filters are initialized differently. Therefore, state and covariance of each elemental filter must be propagated separately, even if all elemental filters assume the same system model.

The main drawback of such a filter bank is the huge computational load. In most integrated navigation systems, most of the computational cost is spent in the propagation step. The reason for this is that the state and covariance matrices of the navigation filter must be propagated with a reasonable update rate in order to cope with the vehicle’s dynamics. For example, in a GNSS/INS system, several propagation steps are performed (for example, every 5 milliseconds when the inertial sensors provide measurements) before one measurement step takes place, i.e., every second when a typical GNSS receiver provides measurements. With the values given in this example, 200 propagation steps are performed before one measurement step is performed.

Within the INLU project, a GNSS/INS integrity algorithm based on a Generalized Pseudo-Bayesian 1 (GPB1) filter bank was developed. The only difference between an IMM and a GPB1 filter bank is the mixing step. For the GPB1 filter bank, the mixing step initializes all elemental filters identically. As all the elemental filters have the same system model — namely the error propagation equations of inertial navigation plus additional states to estimate the inertial sensor biases — the use of a GPB1 filter bank allows INLU to perform propagation steps with one elemental filter only, instead of with each elemental filter of the filter bank. Then, when GNSS measurements become available, all elemental filters are initialized with the propagated state and covariance of this first elemental filter before each elemental filter processes the measurements. This approach avoids, to a large extent, the increase in processing complexity typically connected to filter bank integrity algorithms.

Application Example: Spoofing Scenario

The previously introduced technique is demonstrated with a scenario employing a spoofer using a hardware constellation simulator. The scenario simulates a short drive of a land vehicle. The multipath environment was generated according to the ITU-R P.681 channel model; nominal Galileo and GPS constellations were assumed. Additionally, an ideal spoofing of one Galileo and one GPS Open Service signal was simulated. From a certain point in time onwards, a ramp on the pseudoranges of the respective satellites was generated, starting from zero. Using this RFCS scenario, baseband samples were recorded and then post-processed with the PIPE Receiver. Additionally, artificial inertial sensor data simulating a medium-grade MEMS was generated.

The results of the post processing of the recorded baseband samples are shown in Figure 5. The trajectory starts in the left upper corner and follows the road that is visible in the Google Earth picture. Two different PVT solvers were used: a snapshot least squares solver that produced the position fixes indicated by the red markers, and the GNSS/INS filter bank approach described in the previous section, indicated by the blue markers. Obviously, the snapshot least squares position fixes, for which no attempt to counter spoofing was made, walks off the road. In comparison, the GPB1-based GNSS/INS integrity algorithm proves to be robust with respect to the simulated spoofing threat.

Conclusion

To further grow the application of GNSS receivers for landbased applications, the integrity of the computed PVT solutions must be ensured for this user community in a comparable fashion as is done for the aviation industry. In doing so, the integrity risks of the respective applications like vehicles in cities and railways have to be accounted for. The list of nominal and non-nominal threats is comprehensive, ranging from false locks during tracking over unintentional interference and intentional counterfeiting of GNSS signals, i.e., spoofing.

During the INLU project, various state-of-the-art and promising candidates of future receiver techniques are studied to understand how they can contribute to robust PVT results for land-based users. This comprises not only tracking and RAIM algorithms but also integration of GNSS with inertial measurements using, for example, filter banks.

A GNSS end-to-end simulator needs to have a high versatility to be able to accommodate the manifold requirements that such a project imposes on it. Key factors for the achieved flexibility of the PIPE and SNIPE tools are:

• partitioning into units that serve single tasks with clear inter-module and input data interfaces,
• abstraction layers that allow for the addition of tracking and PVT computation methods to the receiver with minimum effort and the ability to operate these in bit-true and semianalytic modes.

The effectiveness of the chosen approach was demonstrated in a scenario where a traditional GNSS receiver was misled by a spoofer while the implemented filter bank approach corrected for the error transparently.

A separate article will report on the entirety of INLU’s results, i.e., the conclusions drawn from results gained from a number of stochastic scenarios.

The post Navigation Integrity for Land Users Robust Positioning in Challenging Environments appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Among the hot topics in Munich was the role of GNSS in the burgeoning field of automated and unmanned systems. Michael Baus, Program Director at Robert Bosch GmbH, described his company’s fresh approach to precise point positioning (PPP) for autonomous road transport.

“Our new vehicle motion and positioning sensor (VMPS) will hit the road in 2020 and will be one of the first GNSS inertial positioning systems using correction data for highly automated driving,” he said.

“The VMPS will output a safe position, velocity, attitude and time, using a multi-frequency, multiconstellation GNSS system and our sophisticated fusion and integrity algorithms. The system will use wheel spin sensors as well as automotive-grade, safe inertial sensors to bridge GNSS outages.”

Yes, that’s right, he said “safe” two times. And then he said it again, this time with a “not”: “You all know GNSS alone cannot be safe; we cannot control the infrastructure. So, we are using a correction service consisting of a dense network of reference stations spaced about 250 kilometers apart, and we’re starting in Europe, North America, going to China, Japan, South Korea and the rest of the automotive countries.” Processing centers, he explained, will calculate orbit and atmosphere corrections.

Data is transmitted via geostationary satellites as well as over cloud connection to the vehicle, Baus explained, and the VMPS uses it to calculate the output signals and the all-important integrity information.

“We are working in partnership with Trimble for safe and precise positioning, and with u-blox for automotive safe positioning, and, together with Mitsubishi Electronics and Geo++, we are a stakeholder of the Sapcorda joint venture, whose target is worldwide safe and precise correction data.”

SBAS for All?

Another way to better and surer precision is through space-based augmentation systems (SBAS), such as the GPS Wide Area Augmentation System (WAAS) in the United States or EGNOS in Europe. These regional satellite systems provide GNSS correction data, giving increased accuracy and integrity for key “safety-of-life” applications, including civil aviation.

“Regions like the U.S., Europe, Russia or China may be able to afford to build their own SBAS systems,” said Miguel Romay, Executive Director of GNSS Aerospace at Madrid-based GMV, “but what if you are in a plane flying from Washington to Rio de Janeiro? In the U.S. you have SBAS, but in Brazil you don’t.”

Romay was in Munich to talk about GMV’s brand new “magic” user terminal, which uses SBAS capabilities onboard an Inmarsat GEO satellite already in orbit to provide GNSS correction over a wide area. “We thought there was a possibility to develop a company that provides SBAS services to different countries,” he said. “So, they don’t have to develop their own space-based system.

“We are cooperating with Lockheed Martin in the U.S., and we have a project in the Australia/New Zealand region where we are already transmitting through Inmarsat.” Specifically, GMV is providing the processing facilities in charge of the augmentation system, Lockheed Martin is doing GEO satellite signal uplink, and Inmarsat is responsible for the SBAS payload on the 4F1 satellite.

“Having multiple constellations allows us to move in the direction of a global, not regional, SBAS,” Romay said. “We have an operational test bed. All the infrastructure was completed in October of last year, and since then we have been testing it in different fields; maritime, aviation, mining.”

Romay showed us a “demo” receiver, the key components of which, he said, can actually fit inside a smartphone. “I believe this is the first dual-frequency, multi-constellation SBAS receiver, giving PPP with integrity. Using this in Australia, you can receive the signal through the SBAS satellite, and if you are outside Australia you use the internet to receive the SBAS and PPP corrections.”

GMV intends to offer two different products: a lower-cost, single-frequency, PPP receiver, equipped with the u-blox M8 chip and delivering 40-50 centimeter accuracy with integrity; and a higher-performance, NovAtelequipped receiver using GPS, Galileo and GLONASS and capable of achieving around 5 centimeter accuracy with integrity.

“Many people at this conference are talking about PPP with integrity,” Romay said, “so, here it is, we have a solution that we are ready to bring to market, and it can be integrated with other technologies and sensors.”

Galileo CS Reverberations

Things do appear to be moving on the PPP front. For its part, the European Commission (EC) continued in Munich to defend its recent decision to provide for free a high-accuracy GNSS service, originally conceived as one component of a Galileo fee-based commercial service (CS).

On the industrial side, where a number of companies have already developed and are delivering high-precision positioning for a fee, some wondered out loud whether the Commission’s move will undercut their own business. Of these, President of Hexagon Positioning Intelligence Michael Ritter was among the more outspoken. He expressed some confusion as to what “high-accuracy” actually means in the context of the new Galileo service.

“We’ve heard different definitions of what that is – 20 centimeter accuracy? Higher? What convergence time?” he asked. “Because what gives you 20 centimeter in one minute gives you one centimeter in five minutes, so there’s a lot of ambiguous vocabulary being used right at the moment.

Ritter pointed out that existing markets of survey, mapping, agriculture and offshore are already well served by existing European PPP and RTK correction networks and services, and that Hexagon Positioning Intelligence has been providing correction services for three decades. While industry is already working to solve the next challenges of GNSS correction services, he said, the provision of free services will remove or severely diminish the revenue source that industry relies on to reinvest in research and innovation for the autonomous future.

About the American companies in the room, he said, “Most of their employees working on PPP are actually in Europe as well. We all need that money to feed our R&D chain and that ’s why of course we are opposed to that free service.

“The reality is the only way we can get to the kind of functional safety and integrity we need, that kind of authentication, that kind of accuracy in a short time, which involves not a few but thousands and thousands of reference stations, we need to finance that, or we all sit back and wait 15 years for the government to do it. I’d prefer the EC and GSA would use these funds to speed up the Galileo authentication service.”

Commission Come-Back

In response, European Commission Galileo Commercial Service Manager Ignacio Fernandez-Hernandez said, “We believe that in the long term high accuracy is becoming cheaper and will eventually be free.” Hence, the Commission’s desire to get in front of this inevitable wave.

“Some are asking how Galileo high accuracy will affect the industry. But what we are proposing is not comparable to an end-to-end guaranteed service. What we intend to offer stays at the signal level. This relates only to providing better satellite information, better atmospheric information, which is what satnav providers have been doing for the last decade.”

Fernandez reminded participants of the steady trend towards increased accuracy even among open GNSS services available to the general public, and then he said, “You, the industry, are building partnerships and you are evolving your business models. With
Galileo high accuracy, maybe some existing services may not be as relevant as before, but there will be other new services.

“Our perception is that companies are ready to integrate high accuracy with other services, so there is still room
for innovation.”

Ritter returned to the question of what exactly Galileo intends to offer: “I don’t think the GSA [European GNSS Agency] has made any statement on what the convergence time is going to be, so 20 centimeters in 30 seconds? 20 centimeters in a much faster time? Between Trimble, Fugro, Deere and Hexagon and others, we invest a lot of money into fast convergence and high accuracy, so it’s actually an answer I would like to hear.”

“Just to be accurate, convergence is not a service that will be provided by Galileo,” Fernandez said. “Convergence depends on the user algorithms and the information that these algorithms process, so we are providing part of this information, but we are not investing in developing convergence algorithms or developing end-user solutions. These will 100% remain the business of companies providing high accuracy services.”

As for the stated 20 centimeter accuracy level, Fernandez revealed that that figure actually comes from a “higher level”, which we take to mean a more political level.

“We are talking about a very highlevel text,” he said, “which is a Commission ‘Decision’. We have this benchmark of 20 centimeters that expresses the willingness to relax accuracy compared to the service as it was originally defined as a payable service. This is in order to interfere less with existing markets and serve some end-consumer applications. But we are just at the start of the process of defining the service. The statistical characterization, the user environments, the baseline algorithms, all that is still to come.”

Authentic GNSS

While some participants were making a splash, others were offering cash. “If you have any brilliant ideas we have some funding for you,” said GSA Market Development Officer Reinhard Blasi. Then, having peaked everyone’s interest, he proceeded to discuss a number of other items of interest to the GSA, and to everyone else of course, such as market perspectives on authentication and the trend towards authentication as a priority for safetycritical applications.

For the Galileo program, authentication comes in or will come in three different forms. First, there is Open Service Navigation Message Authentication (OSNMA), which will provide a basic level of authentication and some anti-spoofing protection. Blasi said, “This signal will be disseminated over the E1 frequency and is available for singlefrequency users. It is aimed at consumer users and offered for free, already prototyped and under testing.”

A much higher level of authentication will be provided by the robust and secure, limited-access Galileo Public Regulated Service (PRS).

In between those two will be the “new” Galileo Commercial Service, comprising a data authentication signal that will be access-controlled and provided for a fee, based on the spreading code encryption of E6C plus some ancillary data in E6B/ E1B, including OSNMA.

Interesting to note is that even just within the services provided by one GNSS constellation, Galileo, there will be multiple authentication options available, with the choice falling to end users of determining their own vulnerabilities and security needs. But how, someone asked, will users know which level of protection they really need?

Authentic Corrections

Hexagon Positioning Intelligence Vice President of Innovation Sandy Kennedy said she sees military-like GNSS requirements, both in terms of accuracy and security, moving into commercial areas. “There is an increased risk to all established GNSS applications, and with new applications such as autonomous vehicles and verified location for financial transactions coming on quickly, pseudorange-based positioning is not sufficient. Authentic correction sources are now as important as authentic GNSS signals.”

We simply cannot defend against all kinds of attacks, Kennedy said, and those attacks will evolve. So GNSS providers will need to be vigilant, proactive and quickly reactive as new information presents.

To end-users, she said, “So, do you want a sports car or anarmored car? A person who decides to buy a sports car values certain things; they want the engine, they want the speed, the maneuverability. Somebody who has an armored car, it’s still a vehicle, but they value the security. They’ve probably given something up in terms of driving performance, but it’s because safety is their priority.”

It’s the same with GNSS receivers, she suggested. “Which of the parameters are most important for a particular user and why do they choose one or the other? We’ve heard about having a very quick time to first fix, when you drive out of a parking garage and want a solution instantly. Well you can get that but you’re going to have to wait some time before that’s an authenticated or verified solution – or you might not be able to run your RTK (real-time kinematic) solution at faster than 20 hertz, or we might not be able to track every single satellite in view, and so on.”

Are users ultimately going to have to choose a level of vulnerability they are willing to accept? “It really depends on your perception of risk,” Kennedy said. “What we are working on is what level of protection can be provided by default in the background without causing any compromises to performance. How much CPU (central processing unit) can we be using to run a bunch of interconstellation and intersignal checks and that sort of thing.

“Right now what we are seeing is customers that can articulate very clear needs in terms of accuracy levels in this many seconds, but we don’t have that kind of specific definition by customers in terms of interference detection.”

And why is that?, someone interjects. “I think people just don’t know.” Bingo. So there’s an education process that still needs to take place. Kennedy borrows from an old Canadian TV comedy sketch: “GNSS users, you need to know what you need to be afraid of!”

Money, Honey

But really, what about that funding? Between 2014 and 2020, the GSA’s Reinhard “The Candyman” Blasi said, altogether 111.5 million Euro (approximately $138 million) will have been invested by the GSA, at the behest of the EC. This is mainly for Horizon 2020 Fundamental Elements projects, aimed at developing the likes of EGNOS and Galileo-enabled chipsets, receivers and user terminals.

“We want to build on innovative services and differentiators,” he said, “while addressing real user needs and maximizing the benefits for EU citizens.”

One of the key stated aims of all H2020 funding has been to increase industrial competitiveness within the EU, which means the program prioritizes EU participation, but parties from third countries can get funding under certain conditions.

Upcoming 2018 and 2019 grants, not yet awarded, will go to projects aimed at developing Galileo-based timing receivers, CS user terminals and enhanced GNSS user terminals. So, there you have it. Any brilliant ideas? Look for Reinhard Blasi.

The post Munich Hot Licks: Satellite Navigation 2018 appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

It is important, therefore, to ensure satellite orbit calculations are as accurate as possible. As discussed in this article, Earth rotation plays a key role in this regard but surprisingly few references on orbit calculation actually mention its affect explicitly or how to compensate for it. Don’t fret, however, the correction is certainly applied or positioning accuracy would be much worse than is currently attained.

Reference Frames

Earth rotation is important because of the choice of reference system in which orbital calculations are performed. In particular, GNSS orbits — either from the broadcast orbital models or precise post-mission estimation — are parameterized in an
Earth-Centered Earth-Fixed (ECEF) coordinate frame such as the WGS84 reference frame used for GPS.

A common definition of an ECEF frame is one whose z-axis is the rotational axis of the Earth (pointing north), whose x-axis is in the equatorial plane and includes the median passing through Greenwich, and the y-axis completes the frame (typically in a right-handed sense). By definition,such a frame rotates with the Earth and is thus time-varying in inertial space with a period of 24 hours.

In the context of satellite position computations, this means that satellite locations can be computed at any given time, in an ECEF coordinate frame that is valid at that same time.

An easy way to visualize this point is to consider an ideal geostationary satellite whose position relative to the Earth does not change over time — orbital parameters or orbital files would always yield the same coordinates for the satellite.

Effect of Earth Rotation

So where does Earth rotation enter the picture? Well, precisely from the fact that the time at which a satellite transmits a signal, and the time a receiver receives that signal differs. Between the time of transmission (tt) and the time of reception (tr) — roughly 70 milliseconds (give or take few milliseconds) for medium-Earth orbiting (MEO) satellites — the Earth has rotated by ωe . (tr – tt), where ωe is the rotation rate of the Earth.

To illustrate the effect of this, we return to our idealized geostationary satellite. We further consider a user located directly below the satellite. Figure 1 shows this situation looking down on the north pole. To simplify later discussions, we consider this figure to apply at the time of signal transmission.

Since the orbital radius of a geostationary satellite is known (approximately 42,164 kilometers) and the radius of the Earth is known (approximately 6,371 kilometers) the separation of the user and satellite at any given instant is constant and can be easily computed.

Now consider Figure 2, which shows the same figure but also includes the location of the user and satellite at time of signal reception. Because of Earth rotation, the signal travels the path denoted by the blue line, which is obviously longer than the instantaneous separation of the satellite and user. This is the path in inertial space (ignoring the Earth’s orbit around the sun for simplicity).

The problem, however, is that because orbits are parameterized in an ECEF frame, the computed position of the satellite will still be directly above the user. This leads to a situation where the true signal path and the computed signal path differ. Unless accounted for, this difference will manifest as a ranging error in the receiver’s position engine, which computes the difference of the measured and predicted signal paths (i.e., ranges). The magnitude of the position error depends on the number and distribution of satellites, as well as user latitude. As an example, in Calgary, Canada, ignoring Earth rotation results in a shift in the estimated user position of about 20 meters, primarily in the east/west direction.

Before moving on, although we used the example of a geostationary satellite, the exact same effect applies to non-geostationary orbits as well. The main difference is that the satellite positions in Figures 1 and 2 would not necessarily be directly above the user, and the distance between the user and satellites, projected into the equatorial plane (which is shown in Figures 1 and 2), will vary with time as satellites move along their orbits. The good news is that regardless of the orbit, the method of compensation is the same.

Simple Solution

To remove the discrepancy between the measured and computed signal paths, we need to compute the ECEF position of the satellite at the time of transition in the ECEF frame at the time of signal reception. Fortunately, this is easily accomplished by realizing that the two coordinate frames are related by a rotation about the z-axis.

Mathematically, we can write

where is a position vector at the subscripted time (or frame), and R3 (ωe . (tr – tt)) is the rotation matrix about the z-axis by the angle subtended by the Earth rotated during signal propagation.

Applying the transformation in (1) yields the position of the yellow satellite in Figure 2, which allows for the proper computation of the (orange) user position.

The astute reader might be wondering how the propagation time is computed. This can be found by iterating to a solution: first, assume an initial distance between the user and satellite (e.g., 70 milliseconds); then compute the satellite position using this assumed distance (for Earth rotation compensation); use the approximate user position to re-compute the range to the satellite; and finally use this range to compute the satellite position.

The accuracy of the user position in the iteration is not typically a problem. The reason is because, even with a position error of 10 kilometers, the worstcase propagation time error would be 33.3 μs (i.e., 10 km / 3e8 m/s). Multiplying this by Earth rotation rate (~7.3e-5 rad/s) yields an angular error of about 2.4 nanoradians. Even over an orbital radius of 26,000 kilometers (assuming a MEO orbit), the orbital error is less than a decimeter. Then, of course, after the first epoch, the position error is typically several orders of magnitude smaller making the effect of user position error negligible.

Summary

This article has shown why Earth rotation needs to be accounted for when computing satellite coordinates for GNSS applications. The compensation is simple but crucial steps for obtaining the highest possible positioning accuracies.

The post How does Earth’s rotation affect GNSS orbit computations? appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.