Initial developments of the Global Positioning System (GPS) were launched during the Cold War for military purposes. In 1983, the Soviet Unionshot down a South Korean civilian airliner entering their airspace. To prevent such a tragedy ever happening again, U.S. President Ronald Reagan ordered the development of a system that would be made available for civilian use. The firstsatellite in the Global Positioning System was launched in 1989, its twenty-fourth satellite in 1993. GPS satellites orbit 20,200 km (12,550 mi) above the Earth on six tracks.
Positioning is done by triangulation; the receiver determines the satellite’s distance based on the signals it has emitted. The first satellite draws a circular arc on the Earth’s surface. Every point of this arc is located at the same distance from the receiver. A second satellite draws another circular arc; the receiver is found somewhere in the intersection of these two arcs. The position of the receiver can be precisely determined by using a thirdsatellite. In practice, a fourthsatellite is required to measure elevation for a greater accuracy of measurement.
Distance measurement is based on the following principle; the signal emitted by the satellite encodes the time of emission, which the receiver compares with the time of reception. After that, it calculates the time it took for the signal to arrive. The signal travels at the speed of light, at almost 300,000 kilometers per second (186,400 miles per second). For instance, if 0.1 seconds have elapsed between the emission and reception of the signal, the distance is approximately 30,000 kilometers (18,640 miles). In practice, the process of calculation is more complicated, since according to Albert Einstein’s theory of relativity, time is affected by motion.
Today inexpensive GPS receivers are available to anyone. Most mobile phones also contain GPS receivers. In addition to civilian use, military use is also highly important, since in modern warfare, great accuracy is essential for minimizing civilian casualties. High-tech missiles and cruise missiles can continuously measure their positions and are able to adjust their trajectory accordingly.