GNSS Album: Images and Spectral Signatures of the New GNSS Signals

Co-authors Dennis Akos, Alan Chen, and Shermon Lo at Stanford GNSS Monitor Station

A lot of radio signals are traveling around out there in the ether these days — and an increasing number of them are carrying modernized GNSS navigation messages. Although invisible to the human eye and ear, these signals emanate distinctive spectral signatures of crucial importance to GNSS users and product designers: bandwidth, waveform, message code structure, spreading modulation, data rate, and so on. This article by an international team of radio and electronic engineers employs a variety of RF monitoring technologies to capture, portray, and characterize the new signals in space from GPS, GLONASS, Galileo, and Beidou.

Sherman Lo, Alan Chen, Per Enge and Grace Gao, Stanford
Dennis Akos, University of Colorado
Jean-Luc Issler, Lionel Ries, Thomas Grelier and Joel Dantepal, CNES

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Until now, civilian global satellite navigation systems (GNSS) receivers have had essentially only one signal, the GPS L1 C/A-code, reliably available for navigation. However, in the coming years, many more operational GNSS signals, systems, and frequencies will become available to civilian users.

Some of these signals represent new navigation systems, while others are modernizations of existing GNSSes. Although we cannot predict how these signals will be used when they become more prevalent, the current GNSS environment allows us to take a glimpse into the potentials and challenges of that future.

The new signals provide a wealth of possibilities for improving performance, such as accuracy and availability. Different signals and possibilities, however, also pose technical challenges. Research centers and universities around the world are working on ways to use and take advantage of this new GNSS resource. In order to do that, observations of the signal in space are necessary to develop an understanding of the real RF spectral dynamics.

The difficulty with such observations is that the power of spread spectrum RF transmissions, which comprise the GNSS signals, is well below the noise floor. Although correlation can be used to bring the signal out of the noise, the codes from some of the new signals are not yet publicly available. This article presents data and measurements performed by two organizations, Stanford University and CNES (Centre National d’Études Spatiales), which are independently studying the new signals.

Stanford University and CNES have had an active role in the development of systems such as SBAS (WAAS, EGNOS) and other GNSS for aviation. Stanford University pioneered research in GPS, pseudolite attitude determination, and differential GPS navigation for airborne and space users. CNES developed a family of spaceborne GNSS receivers, and had a key role in the GALILEO signal design. . .

Equipment
Stanford University has developed an on-demand capability for observing GNSS signals using the Stanford GNSS Monitor Station (SGMS). The SGMS has a 1.8-meter steerable parabolic dish antenna with an L-band feed and is pictured in the photo above.

When higher gain signals are desired, researchers can use a 150-foot (45.7 meters) parabolic reflector dish antenna (the “Stanford Dish” pictured on the opening page of this article). Located on the Stanford University Radio Science field, the antenna is operated by SRI International. Data is collected from either antenna using a vector signal analyzer.

GNSS measurements have been done in the Toulouse Space Center (CST) of CNES, in the Transmission Technique and Signal Processing Department. Measurements at CNES station are taken with a system receiving and processing GNSS satellites signals, developed in collaboration with the European Space Agency (ESA).

This system is composed of a tracking system (a 2.4-meter dish), a broadband digitizer (bitgrabber), and a high capacity recorder (datalogger). The system allows for postprocessing of signals. The CNES bitgrabber and Leeheim dishes were used together to collect data over a one-week period in early April 2006.

The monitoring earth station at Leeheim, operated by the Bundesnetzagentur, supported a data collection effort to study GNSS signals.

The station has two steerable parabolic reflector antennas with diameters of 12 (Antenna 1) and 7 meters (Antenna 4).

In addition to measuring GPS, GLONASS, and GALILEO signals, this location can also monitor one of the Chinese Beidou satellites. The CNES bitgrabber and Leeheim dishes were used together to collect data over a one-week period last summer.

Motivation
Examining the transmitted signal aids in understanding how to best utilize the signal. Even if the signal can be simulated under laboratory conditions, sooner or later we must measure and assess the actual transmitted signal in the field.

One early objective is assessing nominal signal performance. The ability to understand error modes and perform rapid diagnosis of signal anomalies requires an understanding of the directly observed signal operating normally. This has particular importance for navigation signals used for safety-of-life applications such as aviation.

A second objective is to understand the interference environment that the signal will have to operate in. A final motivating factor is that measuring the signals in space helps us to understand the actual transmitted GNSS bandwidth. This aids receiver designers to make the best compromise between maximum processed bandwidth and interference mitigation.

The abundance and diversity of signals will mean that hardware receivers cannot economically be designed to use all possible signals. Eventually, they must make a choice of what seems to be the optimal mix of signals for particular applications in the marketplace.

Ultimately, signal diversity favors software GNSS receiver designs, which could easily be adapted to process new signals with a new software version.

Conclusion
. . . Although we will soon have many more GNSS signals to choose from, economics will dictate that most consumer receivers use a subset of available signals. Hence, a receiver designer must choose with prudence. Perhaps only one, two, or three frequencies will be used, depending on the application.

The design of these new signals incorporate a variety of features. As such, some features are more suitable to a given application than others. Hence, we no longer have to make one signal fit all our applications. Rather we can choose the best signal for each use.

Furthermore, the frequency diversity offered by multiple GNSS signals also provides interference mitigation. This will make future GNSS receivers more robust. In using these new signals, receiver designers need to pay attention to the particular interference environment of the signal. For example, E5 and L5 users will need to mitigate pulse interference from DME. However, the challenges faced are small compared to potential benefits.

Few would have thought of all the uses and applications of original GPS signal when the system first came on line. The next generation of GNSS, too, will offer many possibilities beyond what we can imagine today.

(For the complete article, including photographs, figures, graphs, and
additional resources, please download the PDF version at the link
above. )

Manufacturers

Stanford University researchers use an 89600 vector signal analyzer, from Agilent Technologies, Palo Alto, California, USA, to collect signals received at either the SGMS or Stanford “Big Dish” antennas. An Agilent E4404B Spectrum Analyzer is used for collecting spectrum images. CNES uses two versions of the bitgrabber from SMP, Toulouse, France. The first version is a single channel broadband digitizer (0.5 – 2.2 GHz) sampling at 250 MHz on up to 10 bits. The new version now allows simultaneously processing of two or four GNSS bands and serves well for research on AltBOC E5, for instance. The ESA/CNES datalogger used to store the samples from the digitizer was developed by M3Systems, Lavernose, France. The CNES 2.4-meter dish has been delivered by Datatools, Strasbourg, France.

Author Profiles

Dennis M. Akos completed the Ph.D. degree in Electrical Engineering at Ohio University within the Avionics Engineering Center. He research interest include: global navigation satellite systems (GNSS), software defined radio (SDR), applied/digital signal processing, and radio frequency (RF) design. Currently he is an assistant professor with the Aerospace Engineering Science Department at University of Colorado at Boulder and holds a visiting professor appointment at Luleå University of Technology and a consulting professor appointment with Stanford University.

Alan Chen is a Ph.D. candidate in the Department of aeronautics and astronautics at Stanford University. He received an M.S. from that department and received his S.B. degree in aeronautics and astronautics from MIT. His current research interest involves UXOs, sensor fusions, autonomous helicopter, and GNSS signals.

Joel Dantepal obtained a European Diploma in wireless telecommunication from the University of Limoges and is currently laboratory manager at CNES in the Transmission Techniques and Signal Processing Department and previously in the Radionavigation Department since 1996. He is well experienced in the field of GNSS generators, receivers, simulators, pseudolites and bitgrabbers, among other RF hardware.

Per Enge is a professor of aeronautics and astronautics at Stanford University, where he is the Kleiner-Perkins, Mayfield, Sequoia Capital Professor in the School of Engineering. He directs the GPS Research Laboratory, which develops satellite navigation systems based on the Global Positioning System (GPS). He has been involved in the development of WAAS and LAAS for the FAA. Enge has received the Kepler, Thurlow, and Burka Awards from the Institute of Navigation. He received his PhD from the University of Illinois.

Grace Xingxin Gao is an Electrical Engineering Ph.D. candidate in the GPS Laboratory at Stanford University. She received a B.S. in mechanical engineering and her M.S. in electrical engineering from Tsinghua University, Beijing, China. Her current research interests include Galileo signal and code structures, GNSS receiver architectures, and GPS modernization.

Thomas Grelier has been a navigation engineer in the Transmission Techniques and Signal Processing Department at CNES since December 2004. He graduated from the French engineering school Supelec and received an M.S. in electrical and computer engineering from Georgia Tech. Galileo signal processing is his main area of research.

Jean-Luc Issler is head of the Transmission Techniques and Signal Processing Department of CNES, the French space agency, whose main tasks are signal processing, air interfaces and equipment in radionavigation, TT&C, propagation, formation flying, and spectrum survey. With the French ministries of transport and defense delegates, he represents France in the Galileo Signal Task Force of the European Commission. He is involved in the development of several spaceborne receivers in Europe. He received the “Astronautical Prize” from the “Association Aeronautique et Astronautique de France” for his involvement in the Galileo frequency choice and signal design.

Sherman Lo is a research associate at the Stanford University GPS Research Laboratory managing the assessment of Loran for civil aviation and also works on a variety of GNSS related issues. He received his Ph.D. in aeronautic and astronautics from Stanford University. He has received the Institute of Navigation (ION) Early Achievement Award and the International Loran Association (ILA) President’s Award.

Lionel Ries is a navigation engineer in the Transmission Techniques and Signal Processing Department at CNES since June 2000. He is responsible of research activities on GNSS2 signals, including BOC modulations and modernized GPS signals (L2C & L5). He is currently responsible for several pre-developments of GPS/GALILEO receivers. He graduated from the Ecole Polytechnique de Bruxelles, at Brussels Free University (Belgium), and then specialized in space telecommunications systems at Supaero in Toulouse.