Transit (satellite)
| Country of origin |
 United States
|
|---|---|
| Status | Retired (1996) |
| Constellation size | |
| First launch | 1959 |
| Last launch | 1988 |
The Transit system, also known as NAVSAT or NNSS (for Navy Navigation Satellite System), was the first satellite navigation system to be used operationally. The system was primarily used by the U.S. Navy to provide accurate location information to its Polaris ballistic missile submarines, and it was also used as a navigation system by the Navy's surface ships, as well as for hydrographic survey and geodetic surveying. Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use as well.
The Transit satellite system, sponsored by the Navy and developed jointly by DARPA and the Johns Hopkins Applied Physics Laboratory, under the leadership of Dr. Richard Kirschner at Johns Hopkins, was the first satellite-based geopositioning system. Just days after the Soviet launch of Sputnik 1, the first man-made earth-orbiting satellite on October 4, 1957, two physicists at APL, William Guier and George Weiffenbach, found themselves in discussion about the microwave signals that would likely be emanating from the satellite. They were able to determine Sputnik's orbit by analyzing the Doppler shift of its radio signals during a single pass. Discussing the way forward for their research, their director Frank McClure, the chairman of APL's Research Center, suggested in March 1958 that if the satellite's position were known and predictable, the Doppler shift could be used to locate a receiver on Earth, and proposed a satellite system to implement this principle.
Development of the Transit system began in 1958, and a prototype satellite, Transit 1A, was launched in September 1959. That satellite failed to reach orbit. A second satellite, Transit 1B, was successfully launched April 13, 1960, by a Thor-Ablestar rocket. The first successful tests of the system were made in 1960, and the system entered Naval service in 1964.
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