Global Navigation Satellite System

Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allows small electronic receivers to determine their position, a few meters within a time using the signals transmitted along a line of sight by radio from satellites. The receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

Starting in 2007, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of restoring the full exploitation. The European Union’s Galileo positioning system is a new generation of GNSS in the first phase of deployment, scheduled to be operational in 2010. China has indicated it may extend its Beidou navigation system in a regional global system. India’s IRNSS, the next generation of GNSS is in the development stage and should be operational by 2012.

History and Theory: Early predecessors were the ground DECCA, LORAN and Omega systems, using radio transmitter’s long land instead of satellites. These systems of radio broadcast from an impulse known master location, followed by a repetition of the pulse of a number of slave stations. The time between receiving and sending the signal to the slave is carefully controlled, allowing receivers compare the time between receipt and the time between shipments. From this point of the distance to each of the slaves could be determined by providing a patch.

The first satellite navigation system has been Transit, a system deployed by the United States Army in the 1960's. Transit 'operation was based on the Doppler Effect: the satellites traveled on the known paths and broadcast their signal on a frequency well known. The frequencies are slightly different from the frequency of circulation due to the movement of the satellite with respect to the receiver. By monitoring the frequency of this change within a short span of time, the receiver can determine the location on one side or the other of the satellite, and several of these measures combined with a precise knowledge of satellite’s orbit may fix a particular position.

Part of a satellite in orbit around l 'broadcast included precise information on its orbit. To ensure accuracy, the US Naval Observatory (USNO) continuously observed precisely the orbit of the satellites. As a satellite’s orbit deviated, the USNO would send updated information to the satellite. Subsequently broadcast from a satellite will contain updates the most recent and accurate information on its orbit.

Modern systems are more direct. The satellite broadcasts a signal containing the satellite position and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message which is superimposed on a code that serves as reference timing. The satellite uses an atomic clock to maintain synchronization of all satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission of receipt measured by an internal clock, thereby measuring the time of flight of the satellite. Several of these measurements can be made simultaneously at different a satellite, which allows a continuous set to be generated in real time.