GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 113

CHAPTER 4
GPS Satellite Signal Characteristics

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

4.1 Overview

In this chapter, we examine the properties of the GPS satellite signals, including fre-
quency assignment, modulation format, navigation data, and the generation of
PRN codes. This discussion is accompanied by a description of received signal
power levels as well as their associated autocorrelation characteristics. Cross-
correlation characteristics are also described. The chapter is organized as follows.
First, background information on modulations that are useful for satellite radio-
navigation, multiplexing techniques, and general signal characteristics including
autocorrelation functions and power spectra are discussed in Section 4.2. Section
4.3 describes the legacy GPS signals, defined here as those signals broadcast by the
GPS satellites up through the Block IIR SVs. Section 4.4 presents an overview of the
GPS navigation data modulated upon the legacy GPS signals. As discussed in Chap-
ter 3, new civil and military signals will be broadcast by the Block IIR-M and later
satellites. These new signals are discussed in Section 4.5. Finally, Section 4.6
summarizes the chapter.

4.2 Modulations for Satellite Navigation

4.2.1 Modulation Types
Binary phase shift keying (BPSK) is a simple digital signaling scheme in which an RF
carrier is either transmitted “as is” or with a 180º phase shift over successive inter-
vals in time depending on whether a digital 0 or 1 is being conveyed (e.g., see [1]). A
BPSK signal, as illustrated in Figure 4.1, can be thought of as the product of two
time waveforms—the unmodulated RF carrier and a data waveform that takes on a
value of either

1/ R b seconds, where R b is
the data rate in bits per second. The data waveform amplitude for the k th interval of
T b seconds can be generated from the k th data bit to be transmitted using either the
mapping [0, 1]

+ 1or − 1 for each successive interval of T b =

→ [ − 1, + 1] or [0, 1] → [ + 1, − 1]. In many systems, forward error cor-

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