Tracking GPS Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 160

CHAPTEREIGHT

Tracking GPS Signals

8.1 INTRODUCTION

One might think that the basic method of tracking a signal is to build a narrow-
band filter around an input signal and follow it. In other words, while the
frequency of the input signal varies over time, the center frequency of the fil-
ter must follow the signal. In the actual tracking process, the center frequency
of the narrow-band filter is fixed, but a locally generated signal follows the
frequency of the input signal. The phases of the input and locally generated
signals are compared through a phase comparator. The output from the phase
comparator passes through a narrow-band filter. Since the tracking circuit has a
very narrow bandwidth, the sensitivity is relatively high in comparison with the
acquisition method.
When there are phase shifts in the carrier due to the C/A code, as in a GPS
signal, the code must be stripped off first as discussed in Section 7.5. The tracking
process will follow the signal and obtain the information of the navigation data.
If a GPS receiver is stationary, the expected frequency change due to satellite
movement is very slow as discussed in Chapter 3. Under this condition, the
frequency change of the locally generated signal is also slow; therefore, the
update rate of the tracking loop can be low. In order to strip off the C/A code
another loop is needed. Thus, to track a GPS signal two tracking loops are
required. One loop is used to track the carrier frequency and is referred to as the
carrier loop. The other one is used to track the C/A code and is referred to as
the code loop.
In this chapter the basic loop concept will be discussed first. Two tracking
methods will be discussed. The first one is the conventional tracking loop. The