GPS Satellites (original) (raw)

UNIT-VIII SATELLITE NAVIGATION & THE GLOBAL POSITIONING SYSTEM INTRODUCTION

The Global Positioning Satellite System (GPS) has revolutionized navigation and position location. It is now the primary means of navigation for most ships and aircraft and is widely used in surveying and many other applications. The GPS system, originally called NAVSTAR, was developed as a military navigation system for guiding missiles, ships. And aircraft to their targets. GPS satellites transmit L-band signals that are modulated by several codes. The C/A (coarse acquisition) code was made available to the public in the mid-1980s. The secure high accuracy P code allows authorized users (mainly military) to achieve positioning accuracy of 3m. This was the accuracy that the military users wanted for targeting smart bombs and cruise missiles, but such accuracies are also useful for auto-landing aircraft in fog and for docking ships in bad weather. The GPS system has been successful because it provides a direct readout of the present position of a GPS receiver with a typical accuracy of 30 m. There are other position location systems, such as LORAN, (a contraction of long range navigation) that can also provide direct readout of position, but not with the accuracy and reliability of GPS. The success of GPS is an excellent example of what satellites do best: broadcasting. An unlimited number of GPS receivers can operate simultaneously because all that a GPS receiver has to do to locate itself is to receive signals from four GPS satellites. The GPS space segment consists of 24 satellites in medium earth orbit (MEO) at a nominal altitude of 20,200 km with an orbital inclination of 55°. The satellites are clustered in groups of four, called constellations, with each constellation separated by 60° in longitude. The orbital period is approximately one-half a sidereal day (11 h 58 min) so the same satellites appear in the same position in the sky twice each day. The satellites carry station-keeping fuel and are maintained in the required orbits by occasional station-keeping maneuvers, just like GEO satellites. The orbits of the 24 GPS satellites ensure that at anytime, anywhere in the world, a GPS receiver can pick up signals from at least four satellites. Up to 10 satellites may be visible at some times, and more than four satellites are visible nearly all of the time. Replacement satellites are launched as needed, so there may be more than 24 operational GPS satellites at any given time. Figure 1: GPS block II-F satellite