How the Global Positioning System (GPS) Works
GPS (Global Positioning System) is a network of about 30 satellites orbiting around the Earth, which are used to tell any GPS receiver on the ground where it is located. The satellites orbit about 20,000 kilometers above the surface of the Earth where it is possible for them to orbit without needing extra energy to move them. They orbit twice every day travelling at a speed of nearly 7,000 km per hour. The computers and signal transmitters inside each satellite are solar powered. The satellites constantly send out radio signals containing information on the current time and their position at that point in time.
A GPS receiver on Earth can use the signal from four satellites to pinpoint its exact location. The receiver must be in line of sight to these satellites because they use radio wave signals to communicate their information. Radio is a type of light invisible to humans, so an outdoor receiver works best. Finding four satellites is never a problem, however, as their orbits were designed such that at least six satellites are always within line of sight to any point on Earth. In fact, there are usually about nine satellites available. This makes it a very reliable system with lots of redundancy.
The United States Government developed and maintains the GPS network of satellites and monitoring stations. Occasionally a new satellite must be sent up to replace older ones or to fortify the system; however, a few backup satellites are already in orbit, ready and waiting to replace any that might fail.
The U.S. Department of Defense is in charge of monitoring and sustaining the GPS. Originally, the system was designed solely for military use so that military movements and targets could be more accurately determined than with previous radio technologies. However, a civilian plane, Korean Air Lines Flight 007, entered prohibited airspace over the USSR in 1983 and was shot down, killing 269 passengers. In response, then President Ronald Regan decided that the GPS should be available to the public. Once it was completed in 1994, the GPS was made available for both private and military use all over the world.
The GPS is now freely available to anyone with a receiver. A receiver costs about $100 and is a convenient hand-held device. Until 2000, military use of the GPS took precedence over civilian use, offering a weaker and less precise signal to private users. However, it is now accurate enough to provide cars with instantly updated driving directions and current speeds, using only GPS information.