Transit (satellite)

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Operational 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 by ballistic missile submarines, and was also used as a general navigation system by the Navy, as well as hydrographic and geodetic surveying.

1 History
2 Description
- 2.1 The AN/UYK-1 Computer
3 External links

[edit] 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 (1,100 km), 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 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, since submarines took periodic fixes to re-set their inertial guidance system, but TRANSIT lacked the ability to provide high-speed, real-time position measurements.

With later improvements, the system provided accuracy of roughly 200 meters, and also provided time synchronization to roughly 50 microseconds. TRANSIT satellites also broadcast encrypted messages, although this was a secondary function.

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.

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.

After 1996, the satellites were kept in use and moved on to other things: they are used for the Navy Ionospheric Monitoring System.

[edit] Description

The TRANSIT system satellites broadcast a continuous signal which included the precise time, as well as the orbital parameters of the satellite. These were regularly sent to each satellite's memory from the Naval Observatory and re-transmitted by the satellite continuously. The signals could be used 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, and likewise the signal would be lower frequency as the satellite receded. As it passed overhead the frequency would suddenly drop from the approaching to receding frequencies. If the satellite was right overhead the frequency shift would be quite quick, 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 "speed" of the frequency shift thus indicated how far the satellite was to one side or the other of the receiver. Traditional navigational systems could then be used to select which of the two possibilities, right or left, was correct.

Calculating the range was not a trivial exercise: it required a least squares deviation of the predicted frequencies (computed from knowing the estimated position of the ship and the orbital parameters of the satellite) from the observed frequencies. This was generally beyond the abilities of a human operator, and thus required the development of a navigational computer to handle the task.

[edit] The AN/UYK-1 Computer

Since no computer small enough to fit through a submarine's hatch existed, a new computer was designed, named the AN/UYK-1. It was built with rounded corners to fit through the hatch and was about five feet tall and sealed to be water-proof. The principal design engineer was then-UCLA-faculty-member Lowell Amdahl, brother of Gene Amdahl. The AN/UYK-1 was built by the Ramo-Wooldridge division of TRW for the Lafayette class SSBN's. It was equipped with 8,192 words of 15-bit core memory plus parity bit, threaded by hand at their Canoga Park factory. Cycle time was about one microsecond.

The AN/UYK-1 was a "micro-programmed" machine with a 15-bit word length that lacked hardware commands to subtract, multiply or divide, but could add, shift, form one's complement, and test the carry bit. Instructions to perform standard fixed and floating point operations were software subroutines and programs were lists of links and operators to those subroutines. For example, the "subtract" subroutine had to form the one's complement of the subtrahend and add it. Multiplication required successive shifting and conditional adding.

The most interesting feature of the AN/UYK-1 instruction set was that the machine-language instructions had two operators that could simultaneously manipulate the arithmetic registers, for example complementing the contents of one register while loading or storing another. It also may have been the first computer that implemented a single-cycle indirect addressing ability.

During a satellite pass, a GE receiver would receive the orbital parameters and encrypted messages from the satellite, as well as measure the doppler shifted frequency at intervals and provide this data to the AN/UYK-1 computer. The computer would also receive from the ship's inertial navigation system (SINS), a reading of latitude and longitude. Using this information the AN/UYK-1 ran the least squares algorithm and provided a location reading in about fifteen minutes.