BendixKing SKYMAP IIIC - Manual (Page 132)

HOW DOES GPS WORK?
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APPENDIX D HOW DOES GPS WORK?
INTRODUCTION TO GPS
What Is GPS?
Many of the radio-navigation aids used in aviation were originally developed for military
use but have now been made freely available to civil users. The Global Positioning System
(GPS) has a similar history. The GPS constellation of satellites is an American military
facility operated by the US Department of Defence (DoD). This constellation consists of 21
operational satellites plus three spares held in reserve to replace any of the active ones,
which might fail. They orbit approximately 10,900 N.M. above the earth in six planes
inclined at a 55 degree angle to the Equator and circle the earth twice daily. The orbits are
so arranged that a minimum of four satellites will always be visible from any part of the
earth's surface at any time. This allows suitable receivers to make extremely accurate
determination of latitude, longitude, altitude, velocity and time from satellite signals
received by an aircraft, airborne or on the ground.
How Does It Work?
Each satellite carries an extremely accurate atomic clock and continuously transmits
precise timing waveforms and data concerning its health status and almanac, orbital
information and clock timing corrections. The signals have been designed to be extremely
resistant to interference from terrestrial radio transmissions, electronic equipment and
weather.
The technique used to establish position using these satellites is one of simple ranging. In
other words, position is calculated from measurements of the distances between the
observer and a number of satellites. The exact position of each satellite at any given time
is known. This information is actually transmitted by each satellite every few minutes. The
precise time (in relation to Universal Time) that each satellite starts to transmit its coded
signal is also known.
Given this information and the speed of radio waves through space, it is possible to
establish the exact distance from each satellite by simple mathematics. For example, if a
car leaves point A at exactly 1 o'clock and travels at sixty miles an hour to point B which it
reaches at exactly 2 o'clock, we know the distance between points A and B must be sixty
miles.
In order to achieve the required degree of accuracy from this technique it would normally
be necessary to have an extremely accurate atomic clock built into each receiver as well
as each satellite. This would not be viable, as each receiver would then cost tens of
thousands of pounds. Instead, an ingenious system is used whereby GPS receivers
require to be fitted with only reasonably accurate (and thus low-cost) clocks. This involves
two important techniques:
1) Code matching or synchronisation. Each satellite transmits, for civilian use, a unique
1023 bit code. This code is repeated every millisecond (one thousandth of a second).
In the receiver, circuitry generates an identical code that is then slid back and forth in time
until it exactly matches the code being received from the satellite.
When a perfect match is achieved, the time at which the receiver is generating the start bit
of the code is recorded. The difference in time (or offset) between the receiver generating
the code and the satellite generating the code is therefore measurable because the time at
which the satellite should have generated the code is known.
Rev 9 Jul 2006 D-1 Skymap/Tracker IIIC Pilot's Guide