How Does GPS Work?
APPENDIX 3 - HOW DOES GPS WORK?
WHAT IS GPS?
Many of the radio-navigation aids used in aviation were originally devel-
oped 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 Defense (DOD). This constellation consists of 21
operational satellites plus three spares held in reserve to replace any of
the active ones that might fail. They orbit approximately 10,900 nm
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
HOW DOES IT WORK?
Each satellite carries an extremely accurate atomic clock and continu-
ously 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 informa-
tion is actually transmitted by each satellite every few minutes. The pre-
cise 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
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.
Rev 1 Mar/2000
KMD 150 Pilot's Guide