The Global Positioning System, usually called GPS, and originally named NAVSTAR, is a satellite navigation system used for determining one's precise location almost anywhere on earth. A GPS unit receives time signal transmissions from multiple satellites, and calculates its position by triangulating this data.
The GPS normally consists of 24 satellites in 6 orbital planes. Each satellite circles the earth twice every day. The satellites carry atomic clocks and constantly broadcast the precise time, along with some other administrative information. The receiver does not need a precise clock, but it needs to have contact to four satellites in order to unambiguously compute its own latitude, longitude, elevation and the precise time. By comparing the signals from two satellites, the receiver can compute the difference between the distances to those two satellites. Since the receiver also knows the precise locations of the four satellites, three such differences yield a system of three (nonlinear) equations for the three unknowns latitude, longitude and elevation. Once the receiver's position and therefore distance to the satellites is known, the time signal sent out by one of the satellites allows to compute the exact time at the receiver's position. If elevation information is not required, only three satellites need to be contacted.
The calculations are complicated by the fact that the speed of radio waves is not constant in the atmosphere and is affected by atmospheric conditions and angle of entry, among other factors. The receiver contains a mathematical model to account for these influences and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation.
The GPS was designed by and is controlled by the United States Department of Defense. GPS satellites are manufactured by Boeing. The first GPS satellite was launched in 1978 and the final (24th) satellite was launched in 1994. Additional satellites are avalible to replace those that fail.
The systems is used by countless civilians as well, who can use the GPS's Standard Positioning Service worldwide free of charge. In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military and other, mostly government users. However, on May 2, 2000, Bill Clinton announced that this "Selective Availability" would be turned off, and so now all users enjoy the same level of access, allowing a precision of position determination of less than 20 meters.
The Russian counterpart to GPS is called GLONASS and is used as a backup by some commercial GPS receivers.