The Basics Of Gps - Interphase Star Pilot GPS Operation Manual

The Basics of GPS
GPS, or Global Positioning System, is a satellite navigation system
designed to provide worldwide positioning and time information with
all weather capability. quick response time and accuracy to within
several yards. GPS is maintained by the United States Department of
Defense 24 hours a day.
GPS is a system of 24 satellites orbiting at an altitude very high
above the earth. Because of the great altitude, the satellites' signals
are not interfered with by the terrain or geography, and the high
frequency of the signals ( l-500 MHz) results in practically no
interence from shipboard engines or other electronic systems.
GPS uses the principle of ranging to give you your position. This is
done by measuring the distance fiom your position to the position of
at least three satellites. The distance to a satellite is calculated by
measuring how long it takes radio si-unals to arrive fiom the known
position of each of the respective satellites in their orbits.
The satellites and the Star Pilot must be precisely tirned to generate
the exact same radio signal at the exact same time in order to measure
the time and distance calculations with a high degree of accuracy.
This is accomplished by the use of atomic clocks on all the satellites,
and by comparing what part of the radio signal code is received at the
exact same time from all the satellites.
To complete its calculations for your position, the Star Pilot looks up
in its data base the known position of each of the satellites at that
exact time. Because the GPS satellites are in such a high orbit, their
positions at any given time are quite predictable. Now. all the Star
Pilot needs to do is calculate all this tor three or four satellites every
few seconds, and you have continuous position updatin-e!
GPS satellites are constantly monitored by Department of Defense
ground stations. Satellite clocks and exact positioning information is
computed and any variations are noted or comected as necessary. and
transmitted to the appropriate satellites. Other possible sources of
errors, such as atmospheric drag and geometric position measurement
discrepancies, are eliminated by the sophisticated, mathematical
programming within the Star Pilot GPS system.
20