Global Positioning System (GPS)

The Global Positioning System (GPS) is a network of radiolocation/radionavigation apparatus that operates on a worldwide basis. The system employs several satellites, and allows determination of latitude, longitude, and altitude.
All GPS satellites transmit signals in the microwave part of the radio spectrum. The signals are modulated with codes that contain timing information used by the receiving apparatus to make measurements. A GPS receiver determines its location by measuring the distances to several different satellites. This is done by precisely timing the signals as they travel between the satellites and the receiver. The process is similar to triangulation, except that it is done in three dimensions (in space) rather than in two dimensions (on the surface of the earth).
There is some reduction in the propagation speed of EM microwaves in the ionosphere, as compared with the propagation speed in free space. The extent of this reduction depends on the signal frequency. The GPS employs dual-frequency transmission to compensate for this effect. A GPS receiver uses a computer to process the information received from the satellites. From this information, it can give the user an indication of position down to a few meters.
Increasingly, automobiles, trucks, and pleasure boats come with GPS receivers installed as standard equipment. If you are driving in a remote area and you get lost, you can use GPS to locate your position. Using a cell phone, citizens band (CB) radio transceiver, or amateur (ham) radio transceiver, you can call for help and inform authorities of your exact position, which is displayed on a detailed map of the area in which you are located.