GPS is a system which shows the exact position on the earth, anytime in any weather and anywhere. There are 24 GPS satellites orbiting at 11,000 nautical miles above the earth. They are continuously monitored by ground station located worldwide. The satellites transmit signal that can be detected by anyone with a GPS receiver. Using the receiver one can determine the exact position namely longitude, latitude, altitude and the time, with greater precision. GPS has 3 parts: the space segment, user segment and the central segment. Space segments consists of 24 satellites as stated above. User segment consists of receivers which we can hold in hand or mount in a car and this gives the exact location on the earth. The control segment consists of ground stations that make sure that the satellites are working properly.
The satellites are equipped with precise clocks that keep accurate time to within 3 nano seconds (3×10-9 secs). The time signals are transmitted along with their orbital parameters. The receiver detects the time signal and calculates the distance of the satellite. By getting the signals from three different satellites and by doing mathematical calculations, the receiver is able to give the exact position where the receiver’s located. By getting the information from fourth satellite, it is able to give the time also.
Thus four satellites are required to compute the four dimensions of x, y, z (position) and time. At any time and at any location always a minimum of four satellites will be visible for the receiver.
Global Positioning System (GPS) works on set of navigational satellites (18 to 21 nos) around the earth in orbit and their signals to a receiver on earth. The signal from each satellite provides the position of the satellite at a specific time.
The receiver software analyses the signals and displays the position of the receiver with reference to a specific spherical coordinates, which has, its centre of earth as its origin. The navigational satellites are so orbiting that at any given time and spaces on the earth, a minimum of three are visible to the receiver.
The accuracy of the position increases with the number of satellites the positional accuracy is around 200 NV, with a different correction (a method of using two or more receiver) the accuracy can be increased to centimetres. Ifs are has been increasing since Iraq war, especially in all modes transportation.
The Global Positioning System is a satellite based Radio Navigation System developed by the United States Department of Defence. It provides continuous, all weather worldwide navigation capability for sea, land and air applications.
The user can easily determine his position, i.e. latitude, longitude and altitude using the GPS receiver which receives signals from various satellites orbiting the earth. ‘Tie complete GPS constellation consists of 21 operational satellites and 3 spares, orbiting the earth about once every 12 hours at a distance of roughly 10,900 nautical miles.
Unlike ground based navigational system signal generators the satellites are able to cover a very large area of the earth because of their altitude and the fact that their signals are free of interference from local geography. The error in GPS at times may come up to 30 to 100 Mts.
An advanced system known as GDPs or differential Global Positioning System evaluates this error with an accuracy of within 5 Mts.