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Equipping GPS Satellites with Accelerometers and Satellite-to-Satellite Observables Compass-M1 Broadcast Codes and Their
Application to Acquisition and Tracking


Grace Xingxin Gao, Alan Chen, Sherman Lo, David De Lorenzo, Todd Walter and Per Enge
Stanford University



BIOGRAPHY

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. in Mechanical Engineering in 2001 and
her M.S. in Electrical Engineering in 2003, at Tsinghua
University, Beijing, China. Her current research interests
include Galileo signal and code structures, GNSS receiver
architectures, and GPS modernization.

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 in 2003 and
received his S.B. degree in Aeronautics and Astronautics
from MIT in 2001. His current research interests involves
UXOs, sensor fusion, autonomous helicopters, and GNSS
signals.

Sherman C. Lo is currently a research associate at the
Stanford University Global Positioning System (GPS)
Laboratory. He is the Associate Investigator for the
Stanford University efforts on the Department of
Transportation's technical evaluation of Loran.

David De Lorenzo is a member of the Stanford University
GPS Laboratory, where he is pursuing a Ph.D. degree in
Aeronautics and Astronautics. He received a Master of
Science degree in Mechanical Engineering from the
University of California, Davis, in 1996. David has
worked previously for Lockheed Martin and for the Intel
Corporation.

Todd Walter is a Senior Research Engineer in the
Department of Aeronautics and Astronautics at Stanford
University. Dr. Walter received his PhD. in 1993 from
Stanford and is currently developing WAAS integrity
algorithms and analyzing the availability of the WAAS
signal. He is a fellow of the ION.

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.
Per has received the Kepler, Thurlow and Burka Awards
from the ION for his work. He is also a Fellow of the ION
and the Institute of Electrical and Electronics Engineers
(IEEE). He received his Ph.D. from the University of
Illinois in 1983.

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). The satellite,
sometimes referred to as Compass-2 or Beidou-2, is the
first of the Compass navigation satellite system (CNSS)
that will provide global satellite navigation coverage.
While China has launched several other navigation
satellites, these previous satellites, also termed Compass
or Beidou (in Chinese), provided only regional coverage.
The Compass-M1 differs significantly from these
previous satellites in terms of signal structure, frequency,
and coverage. Most significantly, unlike previous
satellites, it has similar frequencies and signal structure to
other GNSS, making the prospect of interoperation a
tantalizing one.

Understanding the interoperability and integration of
CNSS with GPS, Galileo and GLONASS, requires
knowing and understanding its signal structure,
specifically its codes and code structure. The knowledge
of the code is necessary for designing receivers capable of
acquiring and tracking the satellite. These receivers are
necessary for evaluating the performance and benefits of
CNSS. Just as important is determining if the signal may
degrade performance of GPS/Galileo in the form of
interference. Interference with and degradation of
GPS/Galileo performance are possibilities if
interoperability was not a driving concern in the signal
design. This is of concern to military users as well since
Compass overlays GPS M-code and Galileo Public
Regulated Service (PRS) on E1/E2. So our preliminary
step in studying Compass is a determination and analysis
of the Compass-M1 codes. Additionally, we will implement these codes within our software GNSS
receiver to verify and validate our analysis.

In this paper, we decode the PRN codes of the E2, E5b
and E6 signals broadcast by the Compass-M1 satellite.
The E2 and E5 codes are identical. They are 2046 bits
long and are 11-stage Gold codes. The E6 PRN code is a
10230-bit concatenated Gold code. The head and tail parts
are both 13-stage Gold codes. We then apply the codes
for acquisition and tracking. By using our own software
receiver, we are able to successfully acquire and track the
Compass-M1 satellite. This is useful for evaluating the
performance of the selected codes.

INTRODUCTION

The Beidou or Compass navigation satellite system
(CNSS) is Chinas entry into the realm of GNSS [1, 2].
The current design is have a system comprised of 30
medium earth orbit (MEO) satellites and 5 geostationary
orbit (GEO) satellites. The MEO satellites will operate in
six orbital planes to provide global navigation coverage.

Compass will share many features in common with GPS
and Galileo, providing the potential for low cost
integration of these signals into a GPS/Galileo/Compass
receiver. These commonalities include multiple
frequencies, signal structure, and services.

According to International Telecommunication Union
(ITU) filings by China, Compass will broadcast on four
frequencies centered at 1590 MHz, 1561 MHz, 1269
MHz, and 1207 MHz (rounded). Table 1 provides general
information on the signals in each of these frequencies.
These signals, then, lie in the frequency band of GPS and
Galileo signals.

The Compass navigation signals are code division
multiple access (CDMA) signals similar to the GPS and
Galileo signals. They use binary or quadrature phase shift
keying (BPSK, QPSK, respectively). Further, our
observations and analysis indicate that the codes from the
current Compass-M1 are derived from Gold codes.

Statements from Chinese sources indicate that the system
will provide at least two services: an open civilian service
and a higher precision military/authorized user service.

Frequency Modulation
Type
1589.74 (E1)
1561.1 (E2)
1268.52 (E6)
1207.14 (E5b)
QPSK(2)
QPSK(2)
Q/BPSK(10)
BPSK(2), BPSK(10)
Table 1. Compass Frequencies and Modulation

The Compass-M1 satellite represents the first of this next
generation of Chinese navigation satellites and differs
significantly from Chinas previous Beidou navigation
satellites. Those earlier satellites were considered
experimental, and most were developed for two-
dimensional positioning using the radio determination
satellite service (RDSS) concept pioneered by Geostar
[3].

Compass-M1 is also Chinas first MEO navigation
satellite. Previous Beidou satellites were geostationary
and only provide coverage over China. The global
implications of this satellite and the new GNSS it
represents makes the satellite of great interest to
navigation experts.

The rapid manner in which researchers have already
trained their instruments onto the satellite proves this
point. For example, Centre National dÉtudes Spatiales
(CNES, the French space agency) published an
informative overview of their observations of the
Compass-M1 signals a month after its launch in the
May/June issue of Inside GNSS [4].

The interest has resulted in significant basic information
on the Compass-M1 satellite. Observations by CNES, us,
and other researchers indicate that the current satellite is
only broadcasting on three of the frequencies (E2, E6,
E5b).

To the best of the authors knowledge, no observations of
Compass E1 broadcasts have been made. It also appears
that the Compass satellite is not continuously
broadcasting navigation messages on the other three
frequencies; we have occasionally observed unmodulated
or continuous wave (CW) signals in those bands. Apart
from these basic observations of the Compass-M1 signal
structure, little information has been published on the
actual codes.

The similarity in frequency, signal structure, and services
with GPS and Galileo makes Compass a tantalizing
prospect for GNSS users. These similarities could allow
for the addition of Compass to an integrated GNSS
receiver without additional expensive hardware or
processing. Moreover, the rapid progress of the Compass
development (and the current state of the Galileo
program) offers the intriguing possibility that the system
may become operational before Galileo.

As such, great motivation exists for understanding
Compass and how it may