GPS (Global Positioning System)
The Global Positioning System (GPS) is a system consisting of 24 operational satellites in six circular orbits that lie in non-synchronous orbits at inclinations of 55 degrees, 20,200km above the earth. The constellation circles the earth once every 12 hours and consists of four groups of six satellites, including 21 that provide the positioning service and three back-ups.
The GPS satellites are used to calculate the position of a GPS receiver on or above the surface of the earth by applying simple geometry together with computing algorithms that assist the receiver in determining which satellites to use and resolve any ambiguity related to location. GPS allows users to determine their three-dimensional position, velocity and time (regulated by atomic clocks) 24 hours a day across the world. GPS reached full operational capability on 17 July 1995. The three segments to GPS are space, control and user.
The space segment consists of the 24 satellites mentioned above. Each weighs around 2,000lbs and measures 17ft across from one solar panel wing tip to the other. By having 21 in use at any one time and three back-ups, it enables up to three to be switched off simultaneously for maintenance and older ones to be replaced when they reach the end of their working lives.
The control segment consists of a master control station in Colorado Springs, plus a further five monitor stations and three ground antennas located across the world. The five monitor stations track the GPS satellites that are in view, with a minimum of six in view at any one time, and collect ranging information from the satellite broadcasts. These stations then send back the information they collect to the master control station that precisely calculates the satellite orbits. The information is then converted into updated navigation messages for each satellite and transmitted via the three ground antennas, which also transmit and receive satellite control and monitoring signals.
The GPS user segment consists of the receivers, processors and antennae that enable the user community to receive satellite broadcasts and convert signals into their precise position, velocity and time. A total of four satellites are required to compute the three-dimensional position and the time.
Although the system was developed by the US Department of Defense, the system can be used by anyone. Typically the positioning system is used in aircraft, ships, ground vehicles, and also for hand carrying by individuals. As well as land, sea and airborne navigation, GPS is also used for surveying, geophysical exploration, mapping and geodesy and vehicle location systems. The time system is used by astronomical observatories and telecommunications facilities, while laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers.
There are two levels of service, a Standard Positioning Service (SPS) and a Precise Positioning Service (PSP). The SPS is for general public use and is intentionally degraded to protect US national security interests through a process called Selective Availability (SA) which controls the availability of the system's full capabilities. It is accurate to within 100 metres (2drms) horizontal, 156 meters (2 Sigma) vertical, 300 metres (99.99% probability) horizontal, 340 nanoseconds time (95% probability).