Global Positioning System

GPS is available in two basic forms: the standard positioning service (SPS), or civilian signal, and the precise positioning service (PPS), or military signal. Prior to 2000 the U.S. military intentionally corrupted or degraded the SPS signal for national security purposes by using a process known as Selective Availability. As a result, the SPS signal was much less accurate than PPS. In May 2000 President Bill Clinton announced that the military would stop Selective Availability. This increased the accuracy and reliability of SPS by a factor of ten. Today, the military and civilian GPS signals are believed to be of the same accuracy. For national security reasons the Defense Department retained the ability to jam the SPS signal on a regional basis if necessary. Both the SPS and the PPS signals provide a horizontal position that is accurate to about 10 m (about 33 ft).

Several techniques have been developed to enhance the performance of GPS. One technique, known as differential GPS (DGPS), employs two fixed stations on Earth as well as satellites. DGPS provides a horizontal position accurate to about 3 m (about 10 ft). Another technique, known as WAAS, or Wide Area Augmentation System, was developed by the Federal Aviation Administration (FAA) to improve the safety of aircraft navigation. WAAS monitoring stations around the United States catch GPS signals, correct errors, and send out more-accurate signals. A technique involving the use of carrier frequency processing (see Carrier Wave), known as survey grade GPS, was pioneered by surveyors to compute positions to within about 1 cm (about 0.4 in). SPS, DGPS, WAAS, and carrier techniques are accessible to all users.

The European Union (EU) launched the first satellite in its planned space-based radio-navigation system in December 2005. The EU system will eventually have 30 satellites and is expected to become operational in 2009. It will have an accuracy of 1 m (3.3 ft) and will be operated as a strictly civilian enterprise. Other countries outside the EU, such as China and Israel, are contributing financially to the creation of the system. Known as the Galileo program and also as the Global Navigation Satellite System (GNSS), the system will be compatible with GPS. The United States negotiated an agreement under which satellite signals could be jammed over a battlefield area without shutting down the entire GNSS or GPS system. The two systems, however, were expected to compete for commercial applications.