GPS is now used for a variety of purposes-some never even imagined by the makers of GPS. GPS can be used in almost all locations in the world except in places where GPS satellite signals can not be accepted, for example, inside buildings, inside caves, underground, and underwater.
GPS is actually a project the United States Department of Defense (USA) who gave the official name of the NAVSTAR (Navigation Satellite Timing And Ranging). The main part of the system is 24 GPS satellites orbiting the Earth at an altitude of 20,200 kilometers. Satellite orbit is designed so that every point on Earth can see at least four satellites at any time.
Each satellite orbit the earth approximately once in 12 hours with a speed of about 11,000 kilometers per hour. GPS satellites have panels solar power collectors to generate electricity it needs. There was also a battery that stores electric power and used it as a satellite does not get sunlight.
The first GPS satellite was launched in 1978 and the constellation of 24 satellites successfully completed in 1994. After that new satellites routinely launched to upgrade or replace old satellites Satellite damaged / not working anymore. Each satellite transmits navigation data in a signal of CDMA (Code Division Multiple Access)-same as the type of signal for CDMA cellular phone. CDMA signal using a code on the transmission so that the GPS receiver can still recognize the GPS navigation signals, although there is interference at the same frequency. The frequency used is the L1 (1575.42 MHz) and L2 (1227.6 MHz).
Information is transmitted from the satellite to a GPS receiver consists of two types. The first is called "almanac", ie the position of all GPS satellites. The second type of information called "efemeris", ie the almanac correction data. "If the 'almanac' is updated approximately once a week, the data 'eferemis' usually is updated every half hour. Instrument GPS receiver is switched on again after a day off can still use the almanac data before.
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