Global Positioning System
The Global Positioning System (GPS), originally developed for U.S. military applications, is the most widely used part of the Global Navigation Satellite System. It provides both precision position and time information.
Principle of operation
The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.
Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
This consists of a minimum of 24 by satellites in full operation capability (FOC) status that orbit in six different planes. The exact number of satellites varies as satellites are replenished when older ones are retired.
They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees, making a complete orbit in approx. 11 hours, 58 minutes. All Satellites are dual-frequency, in the L-band and broadcast on L1 and L2 using spread spectrum modulation. The satellites are tracked by a world-wide network of monitor stations. The tracking data is sent to a master control station that continuously updates position and clock estimates for each satellite. The updated data is then uplinked to the satellite via one of several S-band ground antennas.
GPS equipment is acquired by the Global Positioning Systems Wing of the Air Force Space Command, and operated, once in orbit, by the 2nd Space Operations Squadron at Schriever Air Force Base, Colorado.
The latest version being launched are the GPS Block IIF. built by Boeing, with an extended design life of 12 years, faster processors with more memory, and a new civil signal on a third frequency (L5). The first GPS Block IIF satellite is scheduled to launch in 2010. Starting in 2014, they will be followed by GPS IIIA, with a "new civilian signal (L1C), which is designed to be highly interoperable with the European Galileo satellite navigation system signal and intended to be fully compatible and interoperable with those signals planned for broadcast on Japan's Quazi-Zenith Satellite System (QZSS). For military users, GPS IIIA satellites will provide further increases in the anti-jam capability of the M-Code signals." 
A "selective availability" mode can be invoked for specific areas, in which the high-precision information will be available to receivers only with military cryptographic keys, but the general trend to assume that it is more useful to have accurate signals available than selective ones. By not going into SA mode, commercial equipment can quickly be adapted to military use, as was done with handheld navigation receivers in the Gulf War. Civilian ships and aircraft also depend on accurate signals.
Basic satellite GPS can have its information improved with information from local correction systems, such as Differential GPS (DGPS) or Wide Area Augmentation System (WAAS), which can determine position within less than 3 feet/1 meter.