Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 271

CHAPTERELEVEN

Tracking Weak Signals

11.1 INTRODUCTION

In order to track weak GPS signals, the receiver must be in a low dynamic
range environment. Although a high dynamic range case might be solved with
additional aid, such as an inertial system, it will not be included in this topic
because it cannot be considered as an autonomous receiver. The method discussed
in this chapter is originally designed for processing weak signals. It appears,
however, that this method can also be used to process ordinary (relative strong)
signals without significantly increasing the process time. The difference between
tracking weak and strong signals is that longer data length is needed for tracking
weak signals. The main purpose of tracking is to find three parameters: the
carrier frequency, the navigation data transition, and the accurate C/A code initial
phase. The carrier frequency is used to generate the local signal for continuing the
tracking process. The local signal contains both the C/A and the carrier frequency.
The navigation data transition is determined by the approach discussed in detail
in Section 10.17. The accurate C/A code initial phase is needed to obtain the
pseudorange. The method to obtain the accurate C/A code initial phase is also
referred to as the fine time measurement. This measurement is used to determine
the C/A code initial phase between two digitized points. The acquisition can
only roughly determine the initial C/A code on a digitized data point. If the input
signal is sampled at 5 MHz, the time between two adjacent points is 200 ns,
which corresponds to 60 m, and is not accurate enough for determining the
pseudorange.
There are several possible approaches used to track long input data, and each
approach produces a slightly different result. One approach will be discussed