In the middle of the ocean or out of sight of land, one can't determine an accurate position by looking out the window. The idea of using satellites for navigation began with the launch of Sputnik 1 on October 4, 1957. Scientists at Johns Hopkins University's Applied Physics Laboratory monitored that satellite. They noticed that when the transmitted radio frequency was plotted on a graph, a characteristic curve of Doppler shift appeared. By studying this apparent change of radio frequency as the satellite passed overhead, they were able to show that the Doppler curve, when properly used, described the orbit of the satellite.
Most navigation systems use time and distance to figure location.
Early on, scientists recognized the principle that, given velocity and the time required for a radio signal to be transmitted between two points, the distance between the two points can be computed. In order to do this calculation, a precise, synchronized time of departure and measured time of arrival of the radio signal must be obtained. By synchronizing the signal transmission time to two precise clocks, one in a satellite and one at a ground-based receiver, the transit time could be measured and then multiplied by the exact speed of light to obtain the distance between the two positions. Data transmitted from the satellite provides the user with time, the precise orbital position of the satellite and position of other satellites in the system. Currently, there is a full constellation of 24 orbiting satellites devoted to navigation.