The Global Positioning System (GPS)
The Global Positioning System (GPS) is the only fully deployed satellite positioning system with global coverage. The system was developed by the United States Department of Defense and is maintained by the US Air Force, however since the early 1990’s it has been freely available for civilian use.
The fully-deployed system comprises at least 24 satellites (currently over 30 are in orbit), a set of ground stations around the World which are used to monitor and control the satellites, and any number of GPS receivers which can calculate their precise location based on signals received from the satellites. Each satellite orbits the Earth about once every 12 hours at an altitude of 20,200 kilometres (about 12,500 miles) and transmits a unique signal containing timing information which can be decoded at the receiver.
Timing is the key to GPS. The satellites are time-synchronised with each other by very precise atomic clocks which are regularly calibrated by the ground stations. A GPS receiver detects differences in timing between signals received from the satellites and these differences can be used to calculate the distance to each satellite. The satellites also transmit their precise location and trajectory, hence with an understanding of the location of the satellites and the distances from a number of satellites it is possible to triangulate to calculate the location of the receiver. To calculate a precise location fix, four variables need to be resolved: x, y, z and t, (time). Hence measurements need to be made from a minimum of four satellites.
For many people, GPS is synonymous with satellite navigation – however this is a mistake. GPS is an example of a location technology, and navigation is only the tip of the iceberg in terms of potential consumer applications. In future when all electronic devices become aware of where they are, a huge variety of new uses will emerge.