GIOVE-B on the Air

Understanding Galileo's New Signals

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Europe’s second Galileo In-Orbit Validation Element (GIOVE-B) test satellite was launched on April 27, 2008, at 22:16 UTC and began transmissions on May 7.

On the same day, we observed signals in the L1, E5a, and E5b bands, with the L1 spectrum showing multiplexed binary offset carrier (MBOC) modulation. We then identified the generators for the PRN codes in each band, using the approach described in our previous Institute of Navigation (ION) conference papers listed in the Additional Resources section at the end of this article. We revealed these codes to be 13- or 14-stage Gold codes, different from the memory codes in the Galileo Interface Control Document (ICD).

Our results were validated by acquiring the transmitted signals with these codes. In the following discussion, we elaborate on our data collection apparatus, the observed signal spectra, and the revealed PRN code generators of GIOVE-B.

GIOVE-B Transmissions
In order to obtain a positive signal-to-noise ratio to view the individual spread spectrum chips, we used highly directive antennas: the 1.8-meter parabolic Stanford GNSS Monitor Station (SGMS) in California and the 20-meter parabolic antenna at Table Mountain, Colorado.

As in previous observations described in a May/June 2006 article in Inside GNSS, both antennas were connected to an vector signal analyzer that enabled the capture of extended data records of multiple seconds of 36 MHz bandwidth at the various frequency bands of interest. We used data from the SGMS to determine the codes associated with the GIOVE-B L1 transmission, while data collected with the larger-aperture antenna was used to study the GIOVE-B E5 code generation.

The 20-meter parabolic antenna shown in the photo above is located at Table Mountain in Colorado and owned by the Institute for Telecommunication Sciences (ITS). The institute is the research and engineering branch of the National Telecommunications and Information Administration (NTIA). Until recently, the facility had sat dormant but has now undergone a renovation bringing it to operational status, thanks in part to a joint effort involving ITS, the volunteer Deep Space Exploration Society <http://deep-space.org/index.shtml>, and the University of Colorado.

Precise tracking files were generated based on the publicly available Two Line Orbital Elements (TLEs) obtained from Dr. T.S. Kelso’s Celestrack webpage <www.celestrak.com/NORAD/elements>, which provided sufficient accuracy to track GIOVE-B. (Note: The page is part of Center for Space Standards and Innovation /Analytical Graphics Inc.)

Observations were taken on May 7 during a pass over Boulder, Colorado. During this time, we observed the expected spectral signatures on the L1 and E5 frequencies; however, no signal was observed on the allocated E6 frequency.

The middle part of the L1 spectrum displays the MBOC modulation for the Galileo Open Service (OS) signals, while the two side lobes 15 MHz from the center frequency show BOC(15, 2.5) modulation for Galileo’s Public Regulated Service (PRS) signals. The E5 spectrum indicates AltBOC(15, 10) modulation.

We assume the asymmetry of the L1 or E5 spectrum is due to early stage components (filters and amplifiers) in the RF chain and not directly representative of the satellite signal. The issue probably arises from the filter connected to the antenna, not the filter in the software receiver. In other words, the asymmetry is from capturing the data, not processing the data.

Time domain data was collected independently at E5a and E5b for PRN code determination at those frequencies. The data were also used for L1 PRN code validation. . .

Conclusion
Our early analysis of GIOVE-B produces two observations. We confirm that it transmits on L1, E5a, and E5b bands with MBOC modulation in L1, and we find the PRN generators in all bands to be 13- or 14-stage Gold codes, not the memory codes specified in the Galileo ICD.

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

Manufacturers

In our analysis, we used the 89600 Vector Signal Analyzer from Agilent Technologies, Palo Alto, California, USA.

Author Profiles

Grace Xingxin Gao is a Ph.D. candidate under the guidance of Professor Per Enge in the Electrical Engineering Department at Stanford University. She received a B.S. degree in mechanical engineering and an M.S. in electrical engineering from Tsinghua University, Beijing, China. Her current research interests include Galileo and Compass signal and code structures, interference mitigation, GNSS receiver architectures, and GPS modernization.

Dennis M. Akos completed the Ph.D. degree in Electrical Engineering at Ohio University within the Avionics Engineering Center. His research interests include: GNSS systems, 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 the University of Colorado at Boulder and holds a visiting professor appointment at Luleå University of Technology, Sweden, and a consulting professor appointment with Stanford University.

Todd Walter is a senior research engineer in the Department of Aeronautics and Astronautics at Stanford University. Walter received his Ph.D. from Stanford and is currently developing GPS Wide Area Augmentation System (WAAS) integrity algorithms and analyzing the availability of the WAAS signal. He is a fellow of the Institute of Navigation.

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. He has been involved in the development of the Federal Aviation Administration’s GPS Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS). Enge has received the Kepler, Thurlow, and Burka awards from the Institute of Navigation. He received his Ph.D. from the University of Illinois. He is a member of the National Academy of Engineering and a Fellow of the IEEE and the Institution of Navigation

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