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 obtain accurate location information for targeting submarine launched ballistic missiles SLBMs, and was also used as a general navigation system by the Navy, as well as hydrographic and geodetic surveying.

History

The system was developed by the Johns Hopkins University Applied Physics Laboratory JHUAPL for the US 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, allowing 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 the frequency would suddenly drop. If the satellite was right overhead the frequency shift would be quite quick as it went from "approaching" to "receding", but with the satellite to one side there would be some time where the range would not be changing and the frequency shift would occur more slowly. The rate of change of the received frequency could be used deduce the slant range (or line-of-sight distance) to the satellite.

Knowing the location of the satellite and the slant range to it allowed users to calculate their position to the side of the satellite's ground track. However, one satellite fix indicates that a user was on one side or the other, not which side. Two satellite 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.

The orbits of the TRANSIT satellites were chosen to cover the entire Earth, and they thus met over the pole and were "spread out" at the equator. Since only one was visible at any given longitude, fixes could be made only when that satellite completed another orbit. At the equator this could be the orbital time if the same satellite was visible on both passes (remember that the Earth turns 15 degrees per hour, the ships along with it) and could be up to several hours if not. At mid-latitudes the delay was more typically an hour or two. For its intended role as an updating system for SLBM launch TRANSIT worked fine, the submarines taking periodic fixes to re-set their inertial guidance platforms, but for near-land navigation TRANSIT was much too slow. In keeping with its Navy role, TRANSIT could not provide an altitude measure, which, combined with the long times needed to calculate the fix after pass, make it useless for air navigation.

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