It receives data from the satellite network. Each satellite knows its location and emits data continually. The satellites emit signals containing data on the time,
orbits and almanac data (position of all satellites). GPS receivers (on the ground, at sea and in the air) receive the satellite signals and calculate their position
according to this data. A GPS receiver requires at least three satellites to be able to calculate a 2D location (X, Y) on the surface of the earth. A fourth satellite
(sometimes more, depending on signal quality) will provide a 3D location (X, Y, Z) which determines height or altitude. We should note that the initialisation
sequence is essential for a GPS to be able to calculate its position. When it first initialises, you need to wait around 5 minutes. The GPS informs you about the
current initialisation status on the satellite page; it is usually represented by concentric circles.
• What is a location indicated by a GPS?
A position includes:
a latitude value: North-South
a longitude value: East-West
Trafalgar Square has the following coordinates: 51°30'29'' latitude North - 0°7'41'' longitude West (in the ED50 system).
• GPS accuracy
Two factors reduce the accuracy of the GPS: the intentional reduction of the signal and the signal acquisition time.
A. Intentional reduction of the signal
For more than a decade, by presidential decree, the American military intentionally reduced the GPS signal for civil applications by adding a selective availability
code to the signal carrier. This limited accuracy to around 90 metres. However on 1 May 2000, President Bill Clinton cancelled this reduction and offered all users
a theoretical accuracy of 25 metres from any point on the globe.
Currently, given a temporal error of 10
-12
seconds, the GPS system guarantees a horizontal positioning accuracy of at least 36 metres 95% of the time. In reality,
the system performs better. To mitigate against the inherent technological defects in the 1970s and 80s, the GPS system has for several years been using a
system of local and global correction called the "Satellite Based Augmentation System" or SBAS (WAAS in the USA, EGNOS in Europe and MSAS in Asia).
Taking the average of these "augmentations" (a number of higher tracking stations, differential GPS, signal enhancement, etc.), the current accuracy of military
positioning is up to 1mm 95% of the time and 3 metres for civilian use. But not all GPS devices feature this option.
Although the precision of GPS satellites is very high, the accuracy of GPS receivers varies according to when they are updated (every time the almanacs and
ephemerides are initialised) and from model to model.
B. Acquisition time
The time taken to calculate a location with a GPS depends on the acquisition time. This does not depend on the navigation or operating software because
calculating a location (a "fix" in the jargon) depends directly on the signals received by the GPS receiver.
Signal acquisition time depends on several factors:
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