GPS System Segments - Understanding GPS: Principles and Applications - page 67

CHAPTER 3
GPS System Segments

Arthur J. Dorsey and Willard A. Marquis
Lockheed Martin Corporation
Peter M. Fyfe
The Boeing Company
Elliott D. Kaplan and Lawrence F. Wiederholt
The MITRE Corporation

3.1 Overview of the GPS System

GPS is comprised of three segments: satellite constellation, ground-control/moni-
toring network, and user receiving equipment. Formal GPS JPO programmatic
terms for these components are space, control, and user equipment segments,
respectively. The satellite constellation is the set of satellites in orbit that provide the
ranging signals and data messages to the user equipment. The control segment (CS)
tracks and maintains the satellites in space. The CS monitors satellite health and sig-
nal integrity and maintains the orbital configuration of the satellites. Furthermore,
the CS updates the satellite clock corrections and ephemerides as well as numerous
other parameters essential to determining user PVT. Finally, the user receiver equip-
ment (i.e., user segment) performs the navigation, timing, or other related functions
(e.g., surveying). An overview of each system segment is provided next, followed by
further elaboration on each segment starting in Section 3.2.

3.1.1 Space Segment Overview
The space segment is the constellation of satellites from which users make ranging
measurements. The SVs (i.e., satellites) transmit a PRN-coded signal from which the
ranging measurements are made. This concept makes GPS a passive system for the
user with signals only being transmitted and the user passively receiving the signals.
Thus, an unlimited number of users can simultaneously use GPS. A satellite's trans-
mitted ranging signal is modulated with data that includes information that defines
the position of the satellite. An SV includes payloads and vehicle control subsys-
tems. The primary payload is the navigation payload used to support the GPS PVT
mission; the secondary payload is the nuclear detonation (NUDET) detection sys-
tem, which supports detection and reporting of Earth-based radiation phenomena.

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