1. Global Positioning through Trilateration

Constellation of GPS Satellites

The entire GPS consists of at least 24 satellites that are constantly circling around the Earth!

The satellites are divided among six orbits, each with four satellites sharing a common orbit.

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24 Satelliten auf sechs verschiedenen Bahnen
24 GPS satellites on six different orbits.

Visibility of GPS Satellites

Owing to the great height of their location, several GPS satellites are simultaneously visible from many differerent parts of the Earth. This makes it possible to ensure the visibility of at least four satellites at any point on Earth at any time.

The visibility angle of a satellite, as well as the effective visibility angle of four satellites, are best explained in the following illustration.

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Effective visibility angle of GPS satellites on the Earth's surface.

To be able to use global positioning with the least interruptions, a constant number of five to six visible satellites must be maintained. This is necessary to prevent unwanted interruptions in an on-going global positioning. Such interruptions may occur when a satellite starts to disappear behind the horizon and the signal has to be taken over by another satellite.