Transit (satellite)

The TRANSIT system, also known as NAVSAT (for Navy Navigation Satellite System), was the first satellite navigation system to be used operationally. The system was primarily used by the US Navy to take accurate fixes for programming the launches of SLBMs from submarines, but was also used as a general navigation system by both the Navy and US Air Force, as well as hydrographic and geodetic surveying.

History

The system was developed by the Applied Physics Laboratory of Johns Hopkins University for the Navy. The first successful tests of the system were made in 1960. The satellites (known as OSCAR or NOVA satellites) used in the system were placed in low polar orbits[?], at an altitude of 600 nautical miles, with an orbital period of about 106 minutes. A constellation of five satellites was required to provide global coverage. While the system was operational, at least ten satellites – one spare for each satellite in the basic constellation – were usually kept in orbit.

The TRANSIT system was made obsolete by the Global Positioning System, and ceased operation in 1996. Improvements in electronics allowed the GPS system to effectively take several fixes at once, thereby greatly reducing the complexity of deducing a position. In addition the GPS system uses many more satellites than were used with TRANSIT, making the system able to be used continually, whereas TRANSIT provided a fix only every hour or more.

Description

The TRANSIT system satellites broadcast a contiuous signal which included the precise time, as well as the orbital parameters of the satellite. Ships would measure this signal and use the orbital parameter data to calculate the location of the satellite at any point in time.

As a satellite approached a ground receiver, the received frequency would be higher than the transmitted frequency due to the doppler effect, but as it passed over frequency would suddenly drop. The rate of change of the received frequency could be used deduce the slant range (or line-of-sight distance) to the satellite. If the satellite was right overhead the shift would be quite quick as it went from "approaching" to "receeding", but with the satellite to one side this would occur more slowly.

Calculating a fix consisted of two steps. First the signals from the satellite were used to determine the position of the satellite, then the slant range was calculated from the measurement of the doppler shift. The slant range thus indicated that the ship was on one side or the other of the satellite, although the system didn't tell you which side. Two fixes could be used to determine that information, but more typically the TRANSIT fix could be compared with less accurate systems to determine the side.

The system provided accuracy of roughly 200 meters, and also provided time synchronization to roughly 50 microseconds. Since only one measurement could be taken per pass, TRANSIT was considered fairly accurate, but slow. The interval between satellite passes at mid-latitudes was typically an hour or two, but near the equator the interval between fixes could be several hours.

The principle is similar to the system used by emergency locator transmitters, except there the transmitter is in orbit and the receiver is on the ground. Details on the signal are forwarded directly to ground stations, which then generate a fix on the transmitter using a process similar to TRANSIT.