Global Positioning System (GPS), More Technical Background
More GPS tech background
The system consists of a "constellation" of at least 24 satellites in six orbital planes.
The GPS satellites were initially manufactured by Rockwell; the first was launched in February, 1978, and the most recent was launched November 6, 2004.
Each satellite circles the Earth twice every day at an altitude of 20,200 kilometres (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.
The receiver does not need a precise clock, but does need to have a clock with good short-term stability and receive signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time.
The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a "pseudorange"). It then decodes the satellites' locations from their radio signals and an internal database.
The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals.
The receiver does not have a very precise clock and thus cannot know the time delays; however, it can measure with high precision the differences between the times when the various messages were received.
This yields three hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver.
This is why at least four satellites are needed because fewer than four satellites yield two hyperboloids, whose intersection is a curve; it is impossible to know where the receiver is located along the curve without supplemental information; such as, elevation.
If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude).
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