Updating satellite orbit data
integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. It was initially developed for use by the United States military and became fully operational in 1995. When the Soviet Union launched the first artificial satellite (Sputnik 1) in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory (APL) decided to monitor its radio transmissions.
Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit.
In addition, the satellite’s antennas direct the signal over a specific geographic area.
It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
The GPS system is provided by the United States government, which can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time.
As a result, a number of countries have developed or are in the process of setting up other global or regional navigation systems.
The Director of the APL gave them access to their UNIVAC to do the heavy calculations required.
The next spring, Frank Mc Clure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the user's location, given that of the satellite.
This approach is particularly advantageous for global mobile telephone services in which signal delays during two-way communications can be disruptive and confusing.