IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 3, NO. 4, AUGUST 2009 599 Compass-M1 Broadcast Codes in E2, E5b, and E6 Frequency Bands Grace Xingxin Gao, Alan Chen, Sherman Lo, David De Lorenzo, Todd Walter, and Per Enge, Fellow, IEEE Abstract—With the launch of the Compass-M1 satellite on 14 April 2007, China is set to become the latest entrant into global navigation satellite systems (GNSS). Understanding the interoper- ability and integration of the Chinese Compass with the current GNSS, namely the U.S. Global Positioning System (GPS), the Eu- ropean Galileo, and the Russian GLONASS, requires knowing and understanding its signal structures—specifically its pseudorandom noise (PRN) codes and code structures. Moreover, the knowledge of the code is a prerequisite for designing receivers capable of ac- quiring and tracking the satellite. More important is determining if the signal may degrade performance of the current GNSS in the form of interference. Finally, we are eager to learn from the code and signal design of our Chinese colleagues. For this research, we set up a 1.8-m dish antenna to collect the broadcast Compass-M1 signals. Even with the dish antenna, the received signal is still weak and buried in thermal noise. We then apply signal processing and are able to extract the PRN code chips out of the noise in all three frequency bands. The PRN codes are thousands of bits long. In ad- dition, we find that the Compass-M1 PRN codes in all frequency bands are Gold codes. We also derive the Gold code generators to represents thousands of code chips with fewer than a hundred bits. Finally, we implement these codes in our software receiver to verify and validate our analysis. Index Terms—Global navigation satellite system (GNSS), pseu- dorandom noise codes, spread spectrum. I. INTRODUCTION T HE Beidou or Compass navigation satellite system (CNSS) is China’s entry into the realm of global navi- gation satellite systems (GNSS). The current design plans for 30 medium earth orbit (MEO) satellites and five geostationary orbit (GEO) satellites. The MEO satellites will operate in six orbital planes to provide global navigation coverage [1]. Compass will share many features in common with U.S. GPS and European Galileo systems, providing the potential for low cost integration of these signals into a GPS/Galileo/Compass receiver. These commonalities include multiple frequencies, signal structures, and services. Statements from Chinese sources indicate that the system will provide at least two services: an open civilian service and a Manuscript received July 01, 2008; revised June 01, 2009. Current version published July 17, 2009. This work was supported in part by the U.S. Fed- eral Aviation Administration under Cooperative Agreement 95-G-005. The as- sociate editor coordinating the review of this manuscript and approving it for publication was Dr. Gerard Lachapelle. The authors are with the GPS Lab, Stanford University, Stanford, CA 94305 USA (e-mail: gracegao@stanford.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTSP.2009.2025635 TABLE I COMPASS FREQUENCIES AND MODULATION. BPSK DEFINES A BPSK MODULATION WITH A CHIP RATE OF MHz. FOR EXAMPLE, BPSK(2) DENOTES 2.046-MHz CHIP RATE Fig. 1. Frequency occupation of GPS, Galileo, and Compass. higher precision military/authorized user service [1]. According to International Telecommunication Union (ITU) filings by China, Compass will broadcast in four frequency bands known as E1, E2, E6, and E5b [2]. Table I provides center frequency and modulation information on the signals in each of these bands. Fig. 1, taken from [2], shows the overlap in frequency of the Compass signals with those of GPS and Galileo. Like GPS and Galileo, the Compass navigation signals are code division multiple access (CDMA) signals. They use binary phase shift keying (BPSK) [3]. BPSK defines a BPSK modulation with a chip rate of MHz. For example, BPSK(2) denotes 2.046-MHz chip rate. Further, our observations and analysis indicate that the codes from the current Compass-M1 are derived from Gold codes. The Compass-M1 satellite represents the first of the next generation of Chinese navigation satellites and differs signif- icantly from China’s previous Beidou navigation satellites. Those earlier satellites were considered experimental, and were developed for two-dimensional positioning using the radio determination satellite service (RDSS) concept pioneered by Geostar [4]. Compass-M1 is also China’s first MEO navigation satellite. Previous Beidou satellites were geostationary and only provided coverage over China. The global implications of this satellite and the new GNSS it represents makes the satellite 1932-4553/$26.00 © 2009 IEEE Authorized licensed use limited to: Stanford University. Downloaded on January 11, 2010 at 15:06 from IEEE Xplore. Restrictions apply.