Geosciences and Remote Sensing

GPS AND PHOTOGRAMMETRY

The GPS is a Global Navigation Satellite System (GNSS) developed by United States Department of Defence. It is only fully functional GNSS in the world. It uses a constellation of between the 24 and 32 earth orbit satellites that transmit precise radio signals, which allow the GPS receivers to determine their current location, the velocity, and the time. These satellites are the high orbit, circulating at 14,000km/hr and 20,000km above the earth's surface. The signal being sent to earth at the speed of light is what is picked up by any GPS receiver that are now common place worldwide. The first satellite navigation system, used by United States Navy, was first successfully tested in 1960. Using a constellation of the five satellites. A GPS receiver calculates its position by the precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits the messages containing the time the message was sent, precise orbital information, and the general system health, current date and time of all GPS satellites. The receiver measures the transit time of each message and computes the distance to the each satellite. A form of triangulation is used to combine these distances with the location of the satellites to determine receiver's location. The position is displayed, perhaps with a moving map display or longitude and latitude; elevation information may be included. Many GPS units are also show information such as direction and speed, calculated from the position changes.