Global Navigation Satellite Systems Engineering, Policy, and Design
NOAA’s National Geodetic Survey (NGS) has recently added 43 GPS tracking sites into the Continuously Operating Reference Station (CORS) network overseen by the federal agency.
By Glen Gibbons
During the last 10 years, the GNSS panorama has changed rapidly, bringing new pressure on product designers and system developers to adopt new approaches to their efforts. Let’s take a look at some of these changes.
In the consumer mass market, low-cost GPS terminals and car navigation systems are widely available, and their integration on “smart-phones” is mature.
During the last 10 years, the GNSS panorama has changed rapidly, bringing new pressure on product designers and system developers to adopt new approaches to their efforts. Let’s take a look at some of these changes.
In the consumer mass market, low-cost GPS terminals and car navigation systems are widely available, and their integration on “smart-phones” is mature.
For commercial and institutional applications, however, some issues are still on the table. In particular, institutional users are asking for GNSS solutions that offer high configurability, reliability, precision, service guarantees, authentication, security, and anti-jamming — and, by definition, the availability of open and standard solutions.
High-accuracy terminals or other GNSS equipment with demanding requirements are often based on proprietary and expensive solutions. At the same time, in some countries high-accuracy GNSS augmentation services are the exclusive initiatives of a single private provider or local public agency.
Meanwhile, standards and regulations are changing for almost every application. Therefore, in the near future a user terminal should be expected to support numerous standards and protocols. Furthermore, today’s technologically skilled GNSS user is always asking for more advanced solutions in terms of accuracy — just as evolving demands for mobile communications required ever more bandwidth.
Systems and services are converging. Information and communication technologies (ICT) are ever more frequently providing integrated hardware/software solutions for navigation and communications. Integration of GNSS with inertial sensors, communication systems (e.g., WiFi) and assisted (AGNSS) solutions are, of course, essential for providing higher continuity of service in a range of indoor situations.
In the near future, guaranteed and augmented solutions (in terms of precision and integrity) will be necessary not only for geodetic and surveying applications, but also for mass market applications such as advanced driver assistance systems (e.g., automatic lane keeping).
Backward compatibility with respect to legacy systems also has to be assured for a real exploitation of institutional market. An example of such an application is introduction of GNSS/RFID technology for implementing well-consolidated customs operational procedures and regulations. However, that application will require a complete and transparent integration of new freight “location” reporting systems within existing tracking systems.
Introducing a new “black box” system for this purpose (despite its efficiency), leading to a complete revolution for institutional freight tracking, will not be accepted by the institutional customer. Similar considerations appear in considering the application of innovative high-accuracy technologies in cadastral surveying operations.
The availability of new GNSS constellations and multi-frequency solutions (e.g., TCAR, three carrier ambiguity resolution) will change the scenario within a few years, leading to the need for quick front-end and firmware upgrades in GNSS receivers. New long-range real-time kinematic (RTK) solutions are also coming, based on innovative ionospheric modelling.
In this technological and business environment, embedded systems offer a near-future path to providing tightly integrated navigation and communication solutions with low-cost, accurate, portable, and highly reconfigurable receivers.
A concurrent technological advance, GNSS software receivers (referred to hereinafter as SDR, for software defined radio), will allow developers to provide code/phase solutions and the processing of signals from satellite-based augmentation systems (SBAS) and regional GNSS reference networks in an open environment.
Furthermore, through a suitably flexible front-end, adapting SDR solutions to emerging navigation constellations will be easily implemented. This approach seems like an ideal solution for institutional applications.
A side effect of the development of such solutions may be the ability to implement low-cost GNSS SDR-based augmentation networks, overcoming the usual problems related to high installation costs and the need for continual firmware upgrades.
This article will describe the activities of Sogei, in cooperation with the University of Tor Vergata in Rome, to develop an open SDR GNSS receiver for such applications.
Sogei R&D Activities
Sogei is an Italian company owned by the Ministry of Economy and Finance of Italy charged with developing ICT solutions for national institutions. Among other responsibilities, Sogei manages the Italian system for updating cadastral maps as well as the information system for Italian Customs.
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Platform Architecture and Results
Development of Sogei’s GNSS SDR platform has been carried out using a popular simulation, modelling, and design software suite for rapid prototyping. Furthermore, we avoided any reliance on proprietary libraries and integrated our own C-code modules within the platform. Following this approach, our development platform is based on a bi-processor workstation, equipped with Windows OS, commercial software packages as well as free C compilers. Concerning hardware implementation and high performances, we are testing our code on an advanced DSP/FPGA SDR development platform.
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Design, Developments Hints
Developing a GNSS SDR platform implies continuous design choices in order to find the best trade-off between performance and computational efficiency.
Concerning the acquisition phase, the bottleneck is, of course, the fast Fourier transform (FFT) calculation for implementing the parallel search. To achieve this goal, we developed and integrated an efficient FFT and inverse FFT algorithm into the platform.
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GNSS SDR Results
The SDR platform performs nominally during the signal acquisition phase. After GNSS signal-in-space data acquisition and processing, the platform is able to carry out track signals from GPS, EGNOS, WAAS, and GIOVE-A. In the bottom graphs of the figure, note the two lateral autocorrelation peaks for GIOVE-A, as foreseen by the theoretical signal specifications for the binary offset carrier (BOC) signal modulation. Recently, we also performed GIOVE-B acquisition using our platform.
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Future Perspectives
Worldwide, the implementation of a real-time GNSS SDR receiver is typically based on the integration of the code in a FPGA or DSP platform, and the subsequent implementation in dedicated chips (ASICs). Creating a truly open and reconfigurable solution for institutional applications, however, depends in such case on the use of such kind of devices. The integration of SDR within a commercial PDA or laptop is currently limited by the usual problems of power consumption and computational load problems.
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Conclusions
GNSS civil and institutional applications will be of great relevance for future GNSS applications developments.
GNSS software receiver technologies offer the possibility to implement open and reconfigurable navigation and communication systems to be embedded in a PDA or Laptop. As a side benefit, low-cost terminals will be available for the user. Sogei R&D activities, in collaboration with the University of Rome “Tor Vergata”, allowed the possibility to implement an initial GNSS SDR platform able to work in a multiple constellation environment.
Meanwhile, the development of a network-RTK solution in the center of Italy, enabled users to achieve sub-decimeter accuracy with a reduced TTFA using complete standard protocols and interfaces. Modernized GPS and Galileo will allow the implementation of instantaneous and reliable high-accuracy services. RTK capabilities and TCAR processing within an open GNSS SDR environment could therefore provide a solution for future surveying and high-accuracy transport applications.
Acknowledgments
All the R&D activities described in the present paper have been developed within Sogei. A particular reference is here given to M. Torrisi and Andrea Properzi, working respectively on SDR development and GNSS network interfaces within the framework of the Masterspazio initiative of the University of Rome “Tor Vergata”.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
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Spain’s leading IT company, Indra, has begun a €1.5 million, 18-month project for the European Space Agency (ESA) to study the feasibility and definition of the European Geostationary Overlay Service (EGNOS) looking ahead towards a future multiconstellation regional system (MRS).
By Glen Gibbons
After nearly a year of silence, Magellan has returned to the OEM GNSS space with the MB 500 GPS/GLONASS/SBAS dual-frequency board — reportedly the first of a series of new products by the long-standing brand.
By Glen Gibbons
Trimble has announced its new GNSS reference receiver — the Trimble NetR8 — for precise scientific and network infrastructure applications. The NetR8 reference receiver has 76 channels and supports GPS L1, L2, L2C and L5 signals as well as GLONASS L1/L2 signals.
Four additional channels are dedicated to tracking space-based augmentation systems
(SBAS), including Wide Area Augmentation System (WAAS) in North
America, European Geostationary Navigation Overlay System (EGNOS) in
Europe, Multi-functional Satellite Augmentation System (MSAS) in Japan,
Omnistar services and others.
From the perspective of consumers, the yearlong rise in commodity prices — from oil and natural gas to corn and wheat — has clouded the economic outlook. But for producers, including many GNSS manufacturers and service providers, those clouds have silver linings.
Recent financial reports from companies active in agricultural and natural resource markets bear this out. GNSS products used to guide and control equipment are in heavy demand as are real-time differential correction services, particularly those using global satellite-based systems.
By Glen Gibbons
The GPS Southern Africa Conference and Exhibition – the first of its kind in Africa – takes place from 20 August to 22 August 2008 at the Indaba Hotel, Fourways, Johannesburg.
The conference will highlight the many new applications of GPS technology across the board and the penetration of GPS in transport, safety and security, mining, government, and mining.
By Inside GNSS
GNSS Solutions will lead 29 tutorials on six tracks at the Marriot Riverfront Hotel in Savannah, Georgia, USA before the ION GNSS 2008 conference this September . A discount is offered for ION participants.
Subjects include:
By Inside GNSS
Multiple reference station RTK (real-time kinematic) is a complex, yet natural extension of single reference station RTK. Single reference station RTK actively and dynamically measures GNSS measurement errors, most notably satellite orbit, troposphere, and ionosphere errors.
Multiple reference station RTK (real-time kinematic) is a complex, yet natural extension of single reference station RTK. Single reference station RTK actively and dynamically measures GNSS measurement errors, most notably satellite orbit, troposphere, and ionosphere errors.
These measurement errors are characterized by their spatial correlation. To this end, in single reference station RTK, the errors are assumed to be constant everywhere around the reference station. In reality however, because the errors are not constant, the quality of these error estimates degrade as a function of distance and can reach an unacceptable level for ambiguity resolution after tens of kilometers.
One approach to ensure an acceptable level of measurement error over a wide geographic region is to deploy many reference stations, each operating independently. Once this infrastructure is in place, users select the reference station that will provide them with the greatest reduction of measurement errors and apply the corresponding corrections in the traditional single reference station RTK approach.
Unfortunately, the decision of which reference station to use can be problematic especially when the user is located between reference stations with nearly equally spacing. The estimated measurement errors at each of the reference stations may be different but the user is forced to discretely choose one or the other.
The solution to this problem is multiple reference station RTK. Instead of choosing the solution from one reference station or another, the multiple reference station solution allows users to combine the estimated measurement errors at each of the reference stations and smoothly transition from the errors at one reference station to another.
The multiple reference station solution is not only better because of the ease of use when transitioning between reference stations but also because the smooth combined solution is more likely to represent the user-observed measurement errors. This provides an even further reduction of user measurement errors, relative to the single reference station case.
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The main advantage of multiple reference station RTK stems from the improved user performance. However, the improvement in performance can also be analyzed in an opposite manner, namely, as a way to increase the spacing between reference stations while still achieving the same level of performance. The performance improvement depends on many factors, including the variability of the measurement errors in the region and the ability to successfully resolve network ambiguities.
Multiple reference station RTK is more robust against station outages because a network solution can still be calculated even if individual reference station data is missing. However, due to the current trend of sparse network station spacing, the absence of any individual reference station would likely cause pockets within the network with less than desirable performance. Even under these conditions, the network solution is still more likely to provide a solution better than that from a single reference station.
This improvement comes at a cost of increased complexity and infrastructure. The data from all of the network reference stations must be collected in a central location for processing and then redistributed to network users. The cost of maintaining a processing center and data communication links for each reference station may be significant, depending on the number of reference stations and the country and region in which the network is located.
(For the rest of Paul Alves’ answer to this question, including figures and graphs, please download the complete article using the pdf link above.)
GNSS Solutions is a regular column featuring questions and answers about technical aspects of GNSS. Readers are invited to send their questions to the columnist, Prof. Mark Petovello, Department of Geomatics Engineering, University of Calgary, who will find experts to answer them.
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Recent leadership appointments at Topcon Positioning Systems (TPS) reflect the company’s efforts to expand its focus from being a vendor of equipment for surveying, civil engineering, and construction to a broad-spectrum provider of positioning solutions drawing heavily on GNSS-based technologies.
By Glen Gibbons
Europe’s Galileo program has supported a number of activities intended to promote innovations in receiver design, such as prototype Galileo user equipment, reference receivers, and so on.
Europe’s Galileo program has supported a number of activities intended to promote innovations in receiver design, such as prototype Galileo user equipment, reference receivers, and so on.
One such activity is a project named ARTUS (Advanced Receiver Terminal for User Services), 50 percent of which is financed by funds allocated by the Galileo Joint Undertaking (GJU). A consortium of four companies is leading the ARTUS project (see "Acknowledgments" below)
ARTUS supports the development of receiver technologies to aid the research and development activities for Galileo “professional” receivers. These efforts are designed to facilitate the availability of Galileo professional receiver prototypes and antennas at an early stage.
ARTUS provides Galileo/GPS navigation capability. All three Galileo frequencies (L1, E6 and E5a/E5b) are supported as well as the GPS L1, L2 and L5 (L5=E5a) frequencies.
The receiver supports any BPSK (GPS-C/A, Galileo E5a and E5b (sideband tracking), AltBOC (E5ab), Galileo L1-B/C (BOC(1,1)) as well as BOCc(15,2.5) (E1-A / E6-A); GIOVE-A transmits BPSK (E5a/E5b/E6) and BOC(1,1) (E1).
Although the receiver can track the modulations foreseen for the PRS, it cannot generate the corresponding codes. One can, however, do performance measurements using periodic substitute codes.
Although not initially planned, the consortium has decided to also implement the GPS L2 band for commercial reasons. The unit performs the measurements and processes the raw data to provide an RTK solution.
The Artus design will also form the basis for a breadboard development of the next generation RIMS receivers. This development will be conducted in the frame of an ESA contract lead by IFEN with NemeriX and Euro Telematik as subcontractors.
This article describes the design and operation of the second-generation ARTUS receiver with a particular focus on innovations in four key areas: antenna, RF front-end, digital baseband processing, and navigation software.
Although originally intended to focus primary on tracking Galileo and GPS signals, the flexible design of ARTUS also allows it to receive and track signals from the Russian GLONASS system and China’s Beidou.
After discussing the receiver design and operation, we will briefly describe some of the results of testing using combinations of laboratory GNSS signal simulators, signals-in-space, and simulated signals generated in the German Galileo Test Bed (GATE). . .
Conclusion
The ARTUS GNSS receiver described in this article offers a rich flexibility for various configurations of signals on different RF bands. The high performance antenna in conjunction with a flexible RF front-end design offers excellent performance on all currently available GNSS signal bands, including the upcoming Galileo system.
With the availability of up to 120 channels, the receiver is well equipped for future navigation systems; however, it can also be configured in a version with only 20 or 40 channels for tracking the currently available GPS (L1 and L2) alone.
The modular concept, applied even for the firmware of the baseband processor FPGAs, allows easy adaptation of the algorithms developed for the ARTUS receiver or fast implementation of new algorithms. And if the IP protocol is used, any user interface can easily connect — even remotely — to the receiver — whether for navigation or monitoring purposes.
The ARTUS project is now in its qualification phase. Further developments aim for the commercialization of the receiver.
Acknowledgments
ARTUS was developed in the framework of a GJU 50 percent–funded project, contract GJU/05/2414/CTR/ARTUS. These activities have been taken over by the European GNSS Supervisory Authority (GSA). This support is gratefully acknowledged. IFEN served as the principal contractor for ARTUS.
The consortium members involved in the ARTUS receiver development are ifEN (overall system design and baseband processing), NemeriX (analog RF-front-end), Roke Manor Research (antenna and RF splitter), Leica Geosystems and inPosition (RTK software). In essence the ARTUS design is based on previous receiver developments carried out by IFEN in the frame of the German Galileo Test Bed (GATE).
GATE is being developed on behalf of the DLR (German Aerospace Center, Bonn-Oberkassel) under contract number FKZ 50 NA 0604 with funding by the BMWi (German Federal Ministry of Economics and Technology). DLR kindly gave its permission to publish the preliminary test results.
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NovAtel’s Waypoint Products Group offers the GrafNav/GrafNet Version 8.10 software, a high-precision GNSS post-processing package that supports raw data from most available GNSS receivers. Using data from both a roving station and as many as eight base stations, centimeter-level positions can be computed, according to the company. For applications in which base station setup is difficult or not desired, precise point positioning (PPP) is offered, which uses downloadable GPS clock and orbital corrections to compute solutions accurate to between 5 and 40 centimeters.
By Inside GNSS
GPS Wing Commander David W. Madden will keynote ESRI’s Survey & Engineering GIS Summit in San Diego during the plenary session on Saturday, August 2. Col. Madden is responsible for the multinational, multiservice development of all GPS space, satellite, and ground segments.
By Inside GNSS
