Page 53 - Electronic Navigation Cyber Book

CHAPTER 4 GLOBAL POSITIONING SYSTEM

The Global Positioning System (GPS) is a space-based satellite
navigation system that provides location and time information in all
weather conditions, anywhere on or near the Earth where there is an

unobstructed line of sight to four or more GPS satellites. The system
provides critical capabilities to military, civil and commercial users around
the world. It is maintained by the United States government and is freely

accessible to anyone with a GPS receiver.

Introduction

In the early 1950s, with the shock launching by the USSR of a man-
made satellite into low orbit, the United States space program was born.
Although a tiny vehicle by present day standards, the USSR's 'Sputnik'
had a radio transmitter on board, the frequency of which exhibited a
pronounced Doppler shift when observed from any fixed point on the

earth's surface. The Doppler phenomenon was well documented but
this was the first time the effect had been produced by and received
from a man-made orbiting satellite. Space engineers soon recovered

from the initial shock and were quick to see that the effect could be
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