What Is The Global Positioning System

The Global Positioning System was originally created by the US Department Of Defence. The DoD put 24 satellites into space in orbits 19.000 kilometers or 12,000 miles up that bring each satellite around the World 2 times a day while sending its identity info and a time signal. Each satellite has a very precise nuclear clock on board which is essential to the GPS system precision. The system was made available to the general public in the 80s, and it is completely free to use the system. 24 hours a day, at any location on Earth.

When a GPS receiver receives the signal from a satellite, it looks at the time the signal was sent from the satellite, notices the exact time the signal was received and calculates the distance the signal has travelled. When the gps receiver receives signals from at least 3 satellites, it can use triangulation to calculate exactly where it is on Earth. This is calculated into a latitude and a longitude and displayed on the gps receiver eg. as a "raw" location or as a point on a map. When the gps receiver gets data from 4 or more satellites it can also calculate how high up it is as well - its altitude above sea level. The gps receiver can only use satellites that are within line-of-sight, so it can't communicate with satellites that are behind Earth or shielded by mountains, tunnels or tall buildings, also known as "urban canyons".

Since the system conception, work has gone on to improve the location precision even more. One such improvement, which is being built into most new gps receivers, is the WAAS (Wide Area Augmentation System) in the US or EGNOS (European Geostationary Navigation Overlay Service) in Europe. The EGNOS system uses 3 geostationary satellites and a number of ground stations, and you see the geostationary satellites appear on your GPS receiver as a high number when they are within visibility. When the GPS and the WAAS/EGNOS signals are combined, precision increases, making a precision of plus/minus 5 meters possible.

The US GPS system is not the only one of its kind. The Soviet Union launched its GLONASS system to give their military forces a similar navigational facility, and The European Union is in the process of setting up its Galileo program, using 30 satellites in orbit 24.400 kilometers up and improved technology. Galileo is designed to be interoperable with the 2 other systems and is expected to be operational in 2008.