GPS ( Global Positioning System )
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GPS was initially meant for military applications and was built by the American Department of Defence (DOD) in 1978. It was originally called NAVSTAR and was introduced with the launch of the first satellite.
Today, around thirty fully operational satellites orbit the earth covering a distance of 20200 km. These GPS satellites transmit signals which help locate the precise location of a GPS receiver. The latest in efficient satellite technology ensures that the GPS signal can be used without any fee by any individual in possession of a GPS receiver.
The predecessor of GPS used to be fixed radio stations spread across the globe in known locations. First, a master station sends out signals after which the slave stations across the globe start to respond. The slave stations send out these signals after a precise amount of time. The receivers then start to evaluate the time delay between the reception of the master and slave signals, thus determining a position relative to the slave stations.
Fixed radio station broadcasting was a problem for the military. This is the main reason why Transit, the first navigation satellite, was introduced in the 1960s. The location was determined with the help of a receiver, which calculated the Doppler Effect on the frequency broadcasted by the satellite to the frequency actually received. After this, the receiver closest to the satellite would get information and subsequent readings would precisely single out a location relative to the position of the satellite.
Modern satellites work differently to determine exact positions. A satellite signal would include the satellite’s position and the time of signal transmission. With this crucial information, the ground unit would be able to successfully locate a target swiftly and efficiently. Every satellite signal places the ground unit on a sphere from the satellite. The location of the GPS receiver is then identified as the intersection of the spheres (for additional satellite signals).