Acquisition of Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 224

CHAPTERTEN

Acquisition of Weak Signals

10.1 INTRODUCTION

In this and the following chapters the reception of weak signals will be studied.
This chapter deals with acquisition of weak signals, and the next chapter discusses
signal tracking. Because the software approach is very flexible, there are many
different ways to deal with this problem. Only one approach will be included here.
First the sensitivity of a receiver will be discussed. Because the GPS sig-
nal is a spread spectrum, the processing gain must be used to obtain a decent
signal-to-noise ratio ( S / N ). There are two ways to improve the S / N : coherent
and noncoherent integrations. This problem has been well studied by the radar
designers, and the results will be used here. With the combination of coherent
and noncoherent integrations, the sensitivity of a receiver can be determined.
From the sensitivity, the probability of detection and probability of false alarm
will be obtained.
In general, there are three categories of acquiring a GPS signal. The first cate-
gory is performed without a priori information. The second category is performed
with a priori information, and the third category is with aided information. Per-
formance without a priori information means that the user does not know his
approximate location nor the time of day. Thus the acquisition must search all
the satellites with all possible Doppler frequencies. Obviously this method is
time-consuming. Acquisition with a priori information means that one knows
approximately the location and time of the day. From the almanac data the
number of available satellites and the approximate Doppler frequency can be
obtained. Thus with a priori information the acquisition can be simplified, saving
time. Most situations are between these two categories. For example, one knows
the available satellites but not the approximate Doppler frequency.