Compass MEO Satellite Signals

Initial Observations and Analysis

Much remains to be learned about Compass, the GNSS being developed by China. Filings with the International Telecommunications Union indicate that Compass will broadcast signals in four frequency bands, some of which overlap other GNSS signals. Engineers at CNES, the French Space Agency, are monitoring Compass broadcasts and provide an initial anaylsis of the system's signal design.

Thomas Grelier, Joel Dantepal, Antoine Delatour, Alain Ghion, and Lionel Ries
(Centre National D'Études Spatiales)

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On April 13 2007, the People’s Republic of China launched the first middle earth orbiting (MEO) satellite in its Compass GNSS system, 21,550 kilometers (or about 13,200) miles above the Earth. The spacecraft began transmitting signals on three frequencies within a few days, much more quickly than operational satellites in other GNSSes.

Engineers at the Centre National d’Études Spatiales (CNES, the French Space Agency) have been monitoring the newest Compass (Beidou) satellite. On April 23, they made a series of observations to characterize the signals, all of which will overlay GPS and Galileo signals’ spectra. This article presents the results of those observations and a subsequent demodulation analysis of the Compass pseudorandom noise (PRN) codes.

Collection of signals has enabled us to determine Compass signal structure in E2, E6, and E5b bands. All three frequencies are QPSK modulated. The in-quadrature codes were characterized: primary codes are 1-ms long and are modulated by a 20-ms secondary code.

The E1 signal structure could not be determined as this signal has not yet been observed at CNES. However, the CNES dish is frequently pointed at Compass satellites so as to detect any broadcast. Observing both E2 and E1 signals simultaneously would be very instructive as it would allow us to determine if signals are generated independently or if a specific modulation is used to generate both signals.

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Manufacturers

The digitizing equipment used by CNES was developed by SMP (Systèmes Midi-Pyrénées) of Toulouse, France. It allows the sampling of two to four GNSS bands simultaneously. The datalogger used to store the samples was developed by M3Systems, also of Toulouse. It offers a recording rate of 250 MB/s and a total capacity of several hundred gigabytes.

Author Profiles

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 (USA). Galileo signal processing is one of two main areas of research. He analyzed GIOVE-A, GPS IIR-M, Beidou-1 S-bnad, and Modernized GLONASS signals. Grelier has also developed various Galileo #5 ALTBOC tracking techniques and analyzed their theoretical performances. His other main activity concerns the signal processing development and testing of RF satellite formation flying hardware, using pulsed GPS C/A codes transmitted in S-band.

Joel Dantepal obtained a European Diploma in wireless telecommunication from the University of Limoges, France. He 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, L1, L2, L5, E6, and E4 simulators, pseudolites and bitgrabbers, among other RF hardware. He recently performed intensive tests of the CNES GNSS “Juzzle” software receiver, developed by Géraldine Artaud.

Antoine DeLatour has been a navigation engineer in the CNES Transmission Techniques and Signal Processing Department since 2003. He is involved in the Galileo program in which he supports the European Space Agency, the European Commission, and the European GNSS Supervisory Authority. DeLatour is involved in the design of the Galileo signals, in the use of GNSS for space applications, in the GPS/Galileo radio frequency compatibility assessment and in the development of a GNSS RF signal simulator. In particular, he studied in depth the CBOC GALILEO signal definition and the PRS signal as well. DeLatour proposed new signal tracking techniques for generic receivers and spaceborne receivers. He was graduated from the Ecole Supérieure d’Electricité ( Sup’Elec) in Paris and obtained a master’s degree from the University of Stuttgart.

Alain Ghion has worked in the CNES Transmission Techniques and Signal Processing Department since September 2005 as associate manager of the transmission and radionavigation laboratory. He is in charge of most of the ISO-9001 and security management aspects of this laboratory. Ghion is also responsible for some experimental activities involving the parabolic antenna system that tracks and measures GNSS spectra (such as GIOVE-A, GLONASS-M, GPS IIR-M, and Beidou-2/Compass).

Lionel Ries has been a navigation engineer in the CNES Transmission Techniques and Signal Processing Department since June 2000. He is responsible for research activities on GNSS2 receivers and signals, including BOC, ALTBOC, and MBOC modulations. Ries is one of the CBOC inventors and participated in developing innovations in the GALILEO CBOC multiplexing technique as well. He is involved in the Galileo program in which he supports the European Space Agency, the European Commission, and the European GNSS Supervisory Authority. He is involved in the predevelopment of several hardware GNSS receivers, for ground or spaceborne applications. Ries also supervised the development of the CNES GNSS “Juzzle” software receiver. He graduated from the Ecole Polytechnique de Bruxelles, at Brussels Free University (Belgium) and received an M.S. degree from the Ecole Nationale Supérieure de l’Aéronautique et de l’Espace (SUPAERO) in Toulouse (France).