Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 153

CHAPTER 5
Satellite Signal Acquisition, Tracking, and
Data Demodulation

Phillip W. Ward
NAVWARD GPS Consulting
John W. Betz and Christopher J. Hegarty
The MITRE Corporation

5.1 Overview

In practice, a GPS receiver must first replicate the PRN code that is transmitted by
the SV being acquired by the receiver; then it must shift the phase of the replica code
until it correlates with the SV PRN code. When cross-correlating the transmitted
PRN code with a replica code, the same correlation properties occur that occurs for
the mathematical autocorrelation process for a given PRN code. As will be seen in
this chapter, the mechanics of the receiver correlation process are very different
from the autocorrelation process because only selected points of the correlation
envelope are found and examined by the receiver. When the phase of the GPS
receiver replica code matches the phase of the incoming SV code, there is maximum
correlation. When the phase of the replica code is offset by more than 1 chip on
either side of the incoming SV code, there is minimum correlation. This is indeed the
manner in which a GPS receiver detects the SV signal when acquiring or tracking the
SV signal in the code phase dimension. It is important to understand that the GPS
receiver must also detect the SV in the carrier phase dimension by replicating the
carrier frequency plus Doppler (and usually eventually obtains carrier phase lock
with the SV signal by this means). Thus, the GPS signal acquisition and tracking
process is a two-dimensional (code and carrier) signal replication process.
In the code or range dimension, the GPS receiver accomplishes the cross-corre-
lation process by first searching for the phase of the desired SV and then tracking the
SV code state. This is done by adjusting the nominal spreading code chip rate of its
replica code generator to compensate for the Doppler-induced effect on the SV PRN
code due to LOS relative dynamics between the antenna phase centers of the
receiver and the SV. There is also an apparent Doppler effect on the code tracking
loop caused by the frequency offset in the receiver's reference oscillator with respect
to its specified frequency. This common mode error effect, which is the time bias
rate that is ultimately determined by the navigation solution, is quite small for the

153