This regular Inside GNSS column explores the technical and scientific themes that underpin Global Navigation Satellite System programs and applications.
It features analyses and discussions by engineers and researchers on topics ranging from GNSS interoperability to preferred technologies for GNSS receivers to antispoofing tools for GNSS signals.
The column is coordinated by Prof. Dr.-Ing Günter Hein, head of Galileo Operations and Evolution Department of the European Space Agency.
Previously, he was a full professor and director of the Institute of Geodesy and Navigation at the University FAF Munich. He is one of the CBOC inventors. He is also member of the European Commission’s Galileo Signal Task Force and founder of the annual Munich Satellite Navigation Summit in Germany.
If you have questions or comments, or would like to contribute to “Working Papers,” contact Dr. Hein.
Detrimental effects of RF interference costs the GNSS industry millions of dollars each year. Among the various strategies adopted to address the problem, interference localization is becoming a priority because it provides authorities with the location of the interference sources and time of the interference events.
Radio Frequency Interference causes the satellite industry to lose millions of dollars per year due to detrimental effects, ranging from a degradation in the quality of service to the complete loss of service. As a consequence, it is becoming critical important to design space systems that are able to localize the source of interference, allowing actions that can prevent future repetitions of similar behaviors. This is the first of a series of articles on the issue of interference localization.
Widespread adoption of GNSS has turned into an incentive for malicious actions that exploit its vulnerabilities, by either disrupting or precisely modifying the PNT computation. Authenticating the GNSS signal at both the ranging and data levels is one way to detect and/or mitigate such threats.
Researchers describe the result of investigations into three narrowband interference mitigation methods based on time and frequency processing and compare them in terms of signal acquisition and tracking performance.
The use of satellite-based augmentation system (SBAS) corrections in maritime receivers is currently unregulated. This article discusses mid-term and long-term strategies for development of a dedicated standard that combines SBAS with differential GNSS services to provide a resilient positioning, navigation, and timing capability for maritime applications, including e-Navigation.
Multipath, interference, and ionospheric effects represent some of the leading sources of error affecting GNSS signals. This article proposes a combined architecture for future GNSS receivers that addresses all three.
This article describes development and testing of a novel time-keeping system that could provide increased time-synchronization performance for future upgrades of both the onboard and ground segments of SBAS systems.
GNSS signals experts present the code shift keying (CSK) modulation principle and highlight some of its specific features related to demodulation performance.
Recent increases in computational power can be used to build more efficient GNSS software receivers. Experimental benchmarks show how well the currently available technology can be exploited for these purposes. A personal computer (maximum number of channels greater than 1,000) and an embedded board (maximum number of channels less than 100) are tested under various scenarios, with promising results for future applications.
This article introduces an advanced tracking algorithm that exploits a new conception of the subcarrier to fully benefit from the structure of BOC-modulated signals.
Building on previous research efforts, the authors present a novel technique to improve ionospheric modeling using various configurations of GPS and Galileo signals in the E5 band.
The problem of GNSS signal authentication is drawing ever more attention to the potential risk of “spoofing” attacks and the need to develop methods for verifying the authenticity of GNSS signals.GNSS Authentication for Modernized Signals
Cospas-Sarsat, a 32-year-old emergency reporting system, is replacing its low Earth orbit satellite component with a GNSS-based architecture to improve its worldwide service. This article describes changes in the space segment and the new signals and user beacons that will be part of this modernization.
A team of researchers describes the development of a custom system for monitoring and collecting data on ionospheric scintillation and reports on its performance at facilities in the equatorial and arctic regions.
The low power and spread spectrum nature of GNSS signals make their detection and acquisition a key, but challenging aspect of receiver processing designs. A team of researchers investigated the performance of four new GNSS signals and the legacy GPS L1 C/A code, comparing their probability of detection at a specific level of received signal strength. Factors of particular interest included the bit sign transition, acquisition bin size, and uncompensated code Doppler.
Vector acquisition collects all of the signals in a particular place, sorts out the weak ones, and identifies the best ones for navigation and positioning. This discussion describes a new methodology and implementation algorithm that uses multiple GNSS signals more effectively in signal-challenged environments.
The rise of inexpensive yet powerful GNSS jammers is driving numerous R&D efforts to find ways to thwart the use of such devices. Researchers at the European Commission Joint Research Center describe their use of an infinite impulse response adaptive notch filter to mitigate the effects of jamming.
In order to improve the acquisition of GNSS signals and accelerate a receiver’s time to first fix, an international team of engineers propose the use of a new aiding signal, describe its design, and evaluate its performance.
Can low-cost, single-frequency receivers provide real-time RTK-level performance for road users?
Indoor positioning remains both elusive and an active area of research. New services such as GPS L5 and Galileo E5 may provide a quantum leap in accuracy over earlier signal designs. Here, two engineers describe a GNSS reflectometry system to measure the correlation function of various GNSS signals received indoors. The test results indicate that L5 and E5 will improve the indoor positioning capabilities of receivers.
Four Galileo satellites are in orbit, EGNOS Safety-of-Life Service is available, and other constellations are being upgraded to dual-frequency capability: now is a good time to investigate the benefits for integrity from the increased number of ranging sources. The authors compare two future integrity concepts that may benefit most: an upgraded dual-frequency satellite-based augmentation system and an advanced receiver autonomous integrity monitoring (RAIM) algorithm with optimized continuity and integrity allocation.
It’s an exciting idea: Use vehicular ad-hoc networks to locate in-car jammers. Theoretical and experimental results confirm that localization based on this approach has an error below 40 meters — that’s sufficient accuracy for determining the origin of an interference source.
Peformance analysis of GNSS signal properties and components is well defined in the technical literature. Terms such as code tracking noise, multipath error envelopes, and S-curve bias are widely used by scientists. However, the performance of GNSS data messages has yet to be fully assessed and compared. This article proposes well-defined “figures of merit” that can be used to better evaluate GNSS system performance.
Multiple signals on multiple frequencies provide the opportunity to develop new techniques for applying reflectometry. This article describes an E1/E5a/E5b ground-based altimetry application that has been tested with Galileo signals, including the use of AltBOC code measurements.
With Navipedia, the European Space Agency has introduced a common online entry point for GNSS know-how.
Single-frequency positioning can undoubtedly be improved with the deployment of new GNSS systems and the accompanying availability of new signals. Among various innovations, the Galileo E5 broadband signal deserves special attention. This article demonstrates the expected performance of E5 for selected land applications and precise orbit determination of low Earth orbiting satellites.
This second part of a discussion of peer-to-peer cooperative positioning revisits the topic of sharing critical information across clusters of GNSS users. This article focuses on users within GNSS-challenged environments equipped with both a GNSS receiver and a terrestrial ranging system and shows that sharing information can enhance the availability of position solutions to the network as a whole.
Difference correlators represent effective means to remove signal dynamics from correlator values and to dramatically increase the coherent integration time, thus, also increasing the carrier phase tracking sensitivity. In tests described here, a difference correlator software algorithm shows promising results both indoors and in a forested area.
Cooperative positioning among clusters of GNSS users allows for faster computation of positions with higher reliability. Its performance is similar to that of assisted GNSS, but without any infrastructure requirement. Peer-to-peer cooperative positioning could be a promising addition to fixed augmentation systems of the future.
Two or more modernized GNSS signals transmitted on the same carrier produce varying amplitudes that reduce the power amplifier efficiency and result in the need for aligning the group signal amplitude. Here, two Russian signals experts introduce a new symmetrized signals class that enables significant reductions in the loss factor created during this amplitude alignment. The authors also propose optimal com¬binations of three and four signals when exploiting multiple GNSS systems and offer an improved design for GLONASS L3 and L5 signals.
E911 and many other location-based services and applications currently require accurate urban and indoor positioning — challenging environments for GNSS. This article suggests that signals-of-opportunity based on the orthogonal frequency division multiplexing modulation offer a way to enhance positioning in these locales. The authors used digital video broadcast-terrestrial signals to develop and test a pseudorange estimation method employing real signals, with results that confirm the theory.
Unlike most families that become stronger the more they have in common, pseudorandom noise families grow more robust as their differences increase. This column introduces some new offspring from the familiar elements of GNSS and other radionavigation system codes.
Can a GNSS receiver capable of tracking carrier phase signals estimate a platform’s orientation precisely? This column introduces a new ambiguity-attitude estimator, in which ambiguity resolution and attitude estimation are coupled and resolved in an integral manner.
In GNSS, signal interference and jamming is difficult to solve because of low signal power on the ground and the large distance the satellite signals must travel. However, a mathematical tool — the Karhunen-Loève Transform — can help detect interfering signals with power levels even lower than those usually received on earth from GNSS satellites today.
Tracking the carrier phase of GNSS signals has evolved into a widespread practice for achieving rapid and very accurate positioning. A key to this process is implementing a robust method for determining the number of carrier waves between a GNSS satellite and receiver, including any fractional wavelength, in a given signal transmission — so-called integer ambiguity resolution. Researchers have developed a variety of approaches for calculating the number of integers, but reliable means for testing and accepting the results of such calculations — a crucial factor for ensuring the integrity of such measurements — are not as well developed. This column introduces the principle of integer aperturte estimation and show how it can accomplish this goal.
As interest in GNSS for safety critical applications gains momentum, interference concerns abound. Pulsed interference and continuous wave signals can degrade signal reception. The authors present emerging concepts for detecting and mitigating interference by means of digital processing techniques applied in the receiver.
A team of European researchers continue their assessment of the potential use of S-band spectrum for transmitting GNSS satellite signals, addressing the issue of RF compatibility with nearby mobile satellite services as well as the possible use of an orthogonal frequency division multiplexing (OFDM) signal modulation.
Frequency allocations suitable for GNSS services are getting crowded. System providers face an ever tougher job as they try to bring on new signals and services while maintaining RF compatibility and, in some cases, spectral separation. This two-part column explores the possibility, advantages, and disadvantages of using allocations in a portion of the S-band spectrum.
Germany's publicly-funded UniTaS IV
project investigates satellite navigation applications for aviation. In
the second part of this series, the authors investigate signal
authentication for safety-of-life and also describe the GATE Galileo
The publicly-funded German UniTaS IV project investigates problems in the application of satellite navigation for aviation. The authors studied adaptive beamforming antennas, a GNSS landing system that incorporates inertial sensors with a ground-based augmentation system (GBAS), multi-constellation RAIM (receiver autonomous integrity monitoring), and jamming, spoofing, and authentication of signals. Here are their results. (Part I of II)
What leads to delays in a receiver’s initial position fix and how do you estimate the TTFF for different GNSS signals and receiver start conditions? Includes detailed simulation results for GPS and Galileo signals.
Correct navigation information is key to flight landing procedures, maritime harbor maneuvers and other safety-critical applications. The existing protection level of SBAS + GPS and the alarm limit computation within the Galileo baseline integrity concept will soon give us two approaches to use. Although they both deliver integrity data, each one uses information from the other in unforeseen ways. The authors examine GPS and Galileo integrity methods and propose an algorithm for a reliable combined integrity approach. They also discuss practical issues of implementation and computation of associated protection levels.
Data bit transitions, oscillator jitter, and user dynamics prevent coherent integration time in a GNSS receiver for more than a few dozen milliseconds. But increasing this to several seconds would help solve three problems in a degraded signal environment: multipath, cross-correlation false locks, and the squaring loss. The authors introduce a highly-sensitive prototype that may solve these issues.
More and more mobile devices integrate GNSS —and this increases the pressure to improve positioning capabilities indoors, in urban canyons and other difficult environments. Working Papers examines a novel use of peer-to-peer transfer of position information that combines GNSS and dead reckoning technologies with Bluetooth communications to assist positioning indoors.
This month’s column continues the discussion of prospective use of C-band frequencies for GNSS systems, addressing optimal design of signals, navigation message structure, and user equipment.
The first of a two-part series that will examine the potential for transmitting GNSS signals in the C-band portion of the radio frequency spectrum. The series will cover C-band services and the effects on a GNSS constellation, space and ground segment, and C-band signal design and user equipment.
The idea of flying two or more satellites in controlled formations with
complementary sensors on board to execute scientific research missions
will receive a lot of practical attention in the near future. In some cases, GNSS signals will be used to meet the
high-precision positioning and orientation requirements for this
application. Other, higher-orbit missions will employ RF-based
techniques similar to those used to make GNSS carrier phase
measurements. The second and final part of the series describes the technologies and techniques used in this scientific
Formation flying can create large spaceborne instruments by using several smaller satellites in close formation — to the considerable benefit of many scientific missions planned in the near future. However, the concept requires very accurate relative positioning and orientation of the spacecraft, which can be accomplished at lower altitudes using GNSS techniques and at higher altitudes by employing a similar approach to relative positioning using RF measurement techniques.
At its core, the performance of a modern GNSS system depends on the quality of its timing. Galileo’s GIOVE-B satellite is flying the first space-qualified passive hydrogen maser, and active hydrogen masers are part of the ground control segment that will generate Galileo system time. This column discusses the overall timing operation of the current Galileo architecture and points to the possibility of an even more accurate time source for GNSS systems in the future: optical frequency standards.
Tracking low-power spread spectrum GNSS signals inside buildings is complicated by the effects of various architectures and building materials on signals passing through them. In this final installment of a three-part series, the authors identify, measure, and model some of the key variables affecting the indoor performance of the actual Galileo signal in space.
Everyone wants reliable positioning inside buildings, and it presents a major challenge for researchers and product designers. In this second of a two-part series, the authors identify and measure some of the key variables affecting signals indoors and outline a transmission model for their behavior.
The ability to receive low-power GNSS signals inside buildings wins the gold medal for product designers and manufacturers who can achieve it. The authors share results of a new test that creates an original model of signal behavior in indoor spaces of different shapes and material. First of two parts.
Imagine yourself in the middle of a battlefield with only one truly compelling objective: to maneuver yourself from one point to another and execute your mission — with the reward of your own survival. One eye on the threat, one eye on the horizon! Tension, perhaps a deep fear, seizes you as you confront mortal danger. This is your last shot! Wouldn’t you rather make it while seated behind a desk at a mission control station far from the raging conflict, directing an aerial vehicle without a human on board? A powered, aerial vehicle that can do more for than you could personally on the battlefield yourself?
Over the past 10 years, GNSS reference networks have simplified and extended the range of high-precision positioning over longer distances with the aid of differential corrections. However, a number of operational and environmental factors continue to limit the full realization of these techniques’ potential. Access to and use of additional satellite signals from multiple GNSS systems could help address these limitations. This column examines the potential benefit of GPS + Galileo positioning under a variety of ionospheric conditions.
Securing GNSS systems against unauthorized use and false signals (spoofing) is a matter of growing concern for GNSS operators and users. In this column, the second and final part of a series, the authors explore a variety of methods for user and signal authentication and discuss their application in GNSS.
Concerns about the authenticity and security of GNSS signals are usually associated with military applications. On the one hand, military GNSS users need to detect and avoid spoofing — the generation of false ranging signals by an adversary to mislead a foe — while on the other preventing the exploitation of one’s own GNSS broadcasts by an enemy. This two-part column explores the realm of cryptographic tools that can be used to ensure that only authorized users have access to GNSS services and that the positioning information they use — and report — is the real thing.
Radio frequency is a scarce commodity. No one’s making any more of it. L-band, the home for most GNSS signals, is particularly crowded. Some suggest that C-band, where Europe has filed for an allocation, could be an upscale solution. But will it cause more problems than it solves?
For reasons of political sovereignty, technological competition, policy differences, operational control, and perhaps just plain old national prestige, the planet Earth may have four complete global navigation satellite systems within five or six years. Let’s assume that happens. Are users and manufacturers destined to work through a labyrinth of competing technical specifications and management regimes in order to take advantage of the rich GNSS signal resource coming into existence? Or can we shape a better world of GNSS interoperability and cooperation?
It's not too early to begin thinking about what a multisystem GNSS might look like and mean for users, receiver manufacturers, and service providers.
Measuring variations in the Earth’s gravity field has practical implications for commercial exploration for natural resources as well as advancing geophysical knowledge. The value of gravimetric methodology relates directly to the precision of spatial resolution derived from the measurement instruments. For years, researchers have used the complementary technologies of GNSS positioning and inertial sensors to refine their methods. Today, new data-processing algorithms and the advent of Europe’s Galileo system promise new advances in these techniques.
High-accuracy users of public GNSS reference archives don’t always have access to data that matches the high sampling frequencies needed for real-time kinematic techniques. This column proposes a method for interpolating GNSS observation residuals so as to produce data that can be used in kinematic applications.
The advent of Europe’s Galileo system and introduction of new GPS signals stimulated a re-examination of the subject of codes, buttressed by advances in electronics that allowed new approaches to implementing codes in a GNSS receiver. This column explores the growing categories of codes, their production, and the qualities that make them suitable for use in GNSS systems.
The ability to replace some hardware components in a GNSS receiver with software-based signal-processing techniques has already produced benefits for prototyping new equipment and analyzing signal quality and performance. Now some developers are attempting to extend the flexibility and cost-benefits of software defined radios to commercial end-user products, including mobile devices incorporating GNSS functionality. This column takes a look at the history of GNSS software receivers, the opportunities and practical engineering challenges that they pose for manufacturers, and the state of the art and related applications of them.
This article introduces the multiplexed binary offset carrier (MBOC) spreading modulation recently recommended by the GPS-Galileo Working Group on Interoperability and Compatibility for adoption by Europe’s Galileo program for its Open Service (OS) signal at L1 frequency, and also by the United States for its modernized GPS L1 Civil (L1C) signal. The article provides information on the history, motivation, and construction of MBOC signals. It then shows various performance characteristics and summarizes their status in Galileo and GPS signal design.
Over the years, researchers have advanced the use of GPS receivers to measure water vapor content in the troposphere and model its effects on signal propagation. However, these techniques typically employ stationary GPS receivers. This column describes a method for measuring water vapor by GPS receivers on moving platforms and determining the associated atmospheric effects.
Since introduction of the first GPS receivers more than a quarter century ago, GNSS equipment has changed profoundly – from racks of computers and 25-pound “manpacks” into tiny integrated circuit chipsets suitable for inclusion in mobile phones and other portable devices. But the evolution of GNSS form factors is far from ended. Indeed, the appearance of new GPS and GLONASS signals and the arrival of Galileo has injected new vitality into design of GNSS products. This installment of Working Papers traces the trajectory – past, present, and future – of that technological evolution.
Part of the rationale for building additional GNSS systems, in addition to the motive of political sovereignty, is the argument that a single system is not able to meet all the requirements for use in challenging application environments such as large cities and mountainous terrain. In the end, our answer to the question of GNSS compatibility and interoperability also answers the question of whether GNSSes are complementary or competitive and mutually exclusive systems.