Global Positioning System

T-6A INSTRUMENT NAVIGATION

CHAPTER ONE

GLOBAL POSITIONING SYSTEM

Global Positioning System (GPS) is a U.S. satellite-based radio navigational, positioning, and

time transfer system operated by the DoD. The system provides highly accurate position and

velocity information plus precise time on a continuous global basis to an unlimited number of

properly equipped users. The system is unaffected by weather and provides a worldwide

common grid reference system based on the earth-fixed coordinate system. For its earth model,

GPS uses the World Geodetic System of 1984 datum.

GPS operation is based on the concept of ranging and triangulation from a group of satellites in

space that act as precise reference points.

The GPS constellation of 24 satellites is designed so at least five are always observable by a user

anywhere on earth. The receiver uses data from a minimum of four satellites above the mask

angle (the lowest angle above the horizon at which it can use a satellite).

The GPS receiver verifies the integrity (usability) of the signals received from the GPS

constellation through Receiver Autonomous Integrity Monitoring (RAIM) to determine if a

satellite is providing corrupted information. At least one satellite, in addition to those required

for navigation, must be in view for the receiver to perform the RAIM function; thus, RAIM

needs a minimum of five satellites in view to detect an integrity anomaly. For receivers capable

of doing so, RAIM needs six satellites in view to isolate the corrupt satellite signal and remove it

from the navigation solution.

RAIM messages vary somewhat between receivers; however, generally there are two types. One

type indicates there are not enough satellites available to provide RAIM integrity monitoring and

another type indicates the RAIM integrity monitor has detected a potential error that exceeds the

limit for the current phase of flight. Without RAIM capability, aircrews have no assurance of

GPS position accuracy.

INTRODUCTION TO NAVIGATION SYSTEMS 1-11