Transit (satellite)
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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.
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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 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 continuous 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.
Johns-Hopkins University Applied Physics Laboratory was the prime contractor for the earliest Transit Navigation System (1961-63), which was developed in Canoga Park, CA by Ramo-Wooldridge div of TRW for the Lafayette class SSBN's. Because no computer small enough to fit through the 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 AN/UYK-1 Computer
Even the core memory was threaded on-site in Canoga Park. Principal design engineer was then-UCLA-faculty-member Lowell Amdahl, brother of Gene Amdahl, who was the principal architect of the IBM-360 computers at the same time. The AN/UYK-1 was a "micro-programmed" machine with a 16-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. Cycle time was about one microsecond. "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.
System Operation
During a satellite pass, a GE receiver would receive the orbital parameters and encrypted messages from the satellite. These were regularly injected into each satellite's memory from the Naval Observatory and re-transmitted by the satellite continuously by frequency modulation. The GE receiver measured the received (doppler shifted) frequency at intervals and provided this data to the AN/UYK-1 computer. The computer (which had 8,192 words of 16 bit iron-core memory) would also receive from the ship's inertial navigation system (SINS), a reading of latitude and longitude.
The program to compute a correction of the ship's position adjusted these latitude and longitude values to minimize the least squares deviation of the predicted frequencies (computed from knowing the estimated position of the ship & the orbital parameters of the satellite) from the observed frequencies. On the AN/UYK-1, this process took about 15 minutes. The AN/UYK-1 system had no peripheral random access memory available to it. The entire "operating system", micro-programmed instruction subroutines, data and application had to fit within the 8K words of core memory -- a remarkable feat even when judged by programming standards decades later. The actual positional accuracy was classified "Top Secret" for many years.
With later improvements, 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, since submarines took periodic fixes to re-set their inertial guidance platforms, but TRANSIT lacked the ability to provide high-speed, real-time position measurements. In addition to the lack of real-time navigation, TRANSIT could not provide an altitude measure, which made it useless for air navigation. TRANSIT paved the way for the real-time, high-precision Global Positioning System, GPS.
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.
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