Appendix 03 - 1
APPENDIX 03: How Does GPS Work?
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.
This gives a very accurate way of measuring the time differences between transmission and reception
of the signals generated by the satellites, but does not help with the need for an absolute time
reference. One must bear in mind that if the clock in the receiver were only 100 milliseconds (one
tenth of a second) out with respect to universal time, the calculated distance from the satellite would
have an error of 1,860 miles.
This problem is overcome by a second technique:
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