GPS explanation

Global Positioning Satellites

A network of satellites that continuously transmit coded information, which makes it possible to precisely identify locations on earth by measuring distance from the satellites.

US department of defense satellites constantly circling the Earth. The satellites transmit very low power radio signals allowing anyone with a GPS receiver to determine their location on Earth, whether on land on sea or in the air.

The first ones where put into orbit in 1978 with a full compliment of 24 being reached in 1994.
The satellites became available for civilian use in the 1980s.

The NAVSTAR ( Navigation Satellite Timing And Ranging) system consists of 3 parts.
The satellites, at least 24 satellites (21 active plus 3 spare) in high orbit about 12,000 miles above the Earth, arranged so that 4 are "visible" with a GPS receiver on the earth at any one time. Travelling at 7,000 mph, this allows them to circle the earth every 12 hours. Power is solar energy with a backup battery and they are built to last about 10 years.
Frequency is 1575.42 MHz.
The time it takes the signal to reach the receiver multiplied by the speed of light gives the distance from the satellite. Receive from 3 or 4 satellites and your receiver can calculate it's 3 dimensional position on the Earth.
Ground stations, the ground stations consist of one master control station and 4 monitoring stations. The monitoring stations collect data from the satellites regarding their position and the time they have on board. This information is sent to the master station, corrected and then sent back to the satellites and so keeps the satellites as accurate as possible.
The user, users are anybody who carries a GPS receiver, including the military, boaters, pilots, hikers and motorists.

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