How many satellite navigation systems are there?
The US and Russia provide the two global satellite navigation systems now available. A third system, Galileo, being built and operated in Europe, is due to come on line early in the next decade. The three systems will be fully interoperable, which means that users on will be able to determine a position by picking up signals from any combination of satellites belonging to any of the three systems.
A satellite navigation system consists of a space segment, typically 24 navigational satellites, a ground or control segment and a user segment. The satellites are placed in circular orbits at heights of about 20 000 km so that at least four are visible from any point on the Earth’s surface at any time.
The ground or control segment consists of several tracking stations placed at different locations over the Earth’s surface, and a central ground station. The tracking stations monitor the position and health of the satellites, send data back to the central station for processing and then relay accurate measurements of each satellite’s position to the satellite for incorporation into its navigational signal.
The user segment consists of the users of the GPS system who are equipped with terminals to receive the satellites’ navigational signals.
The first navigation satellite was launched in 1978 and was the forerunner of the US GPS system, which was originally developed for the US Department of Defense. The GPS system is now operated from the Shriever Air Force Base in Colorado.
The first satellite in the Russian GLONASS system was launched in 1982. Following the dissolution of the Soviet Union, the operational capabilities of the GLONASS system degraded due to lack of funding. The system is now in the process of being restored to full functionality. GLONASS is operated from a system control centre near Moscow.
Europe’s approach to Global Navigation Satellite Systems has begun with the European Geostationary Navigation Overlay Service (EGNOS), which complements and improving GPS. EGNOS entered pre-operational service in 2006 and will undergo certification for safety-of-life applications.
Technical and operational experience with EGNOS is paving the way for Galileo, Europe’s Global navigation satellite System. The first Galileo satellite, GIOVE-A – Galileo In-Orbit Validation Element A, was launched at the end of 2005.
Galileo is a joint initiative between ESA and the European Commission. When fully deployed in the early years of the next decade, it will be the first civilian positioning system to offer global coverage.
The receiver measures travel times by comparing ‘time marks’ imprinted on the satellite signals with the time recorded on the receiver’s clock. The time marks are controlled by a highly accurate atomic clock on board each satellite.
These clocks, however, are too expensive to incorporate into standard receivers, which have to make do with small quartz oscillators like those found in a wristwatch. Quartz oscillators are very accurate when measuring times of less than a few seconds, but rather inaccurate over longer periods. The solution is to re-set the receiver’s time to the satellite’s time continuously. This is done by the receiver’s processor using an approximation method involving signals from at least four satellites.
For this system of measurement to work, all satellites need to be synchronised so that they can start transmitting their signals at precisely the same time. This is achieved by continuously synchronising all on-board atomic clocks with a master clock on the ground. These super-accurate clocks can keep time to within one second in 100 million years!