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 provide accurate location information to 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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[edit] History
The system was developed by the Johns Hopkins University Applied Physics Laboratory JHU-APL 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 about 600 nautical miles (1,100 km), with an orbital period of about 106 minutes. A constellation of five satellites was required to provide reasonable 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 their orbits crossed-over the poles and were "spread out" at the equator. Since only one was usually visible at any given time, fixes could be made only when one of the satellites was above the horizon. At the equator this delay between fixes could be up to several hours. 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 single-pass 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 their transmitter is on the ground and the receiver is 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 navigation service in 1991. 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 1991, the satellites were kept in use as spaceborne 'mailboxes' and for the Navy's Ionospheric Monitoring System.
[edit] Description
The TRANSIT system satellites broadcast two UHF carrier signals that provided precise time hacks (every two minutes), plus the satellite's six orbit elements and orbit perturbation variables. The orbit ephemeris and clock corrections were uploaded twice each day to each satellite from one of the four Navy tracking and injection stations. This broadcast information allowed a ground receiver to calculate the location of the satellite at any point in time. Use of two carriers permitted ground receivers to reduce navigation errors caused by ionospheric refraction. The Transit system also provided the first world-wide time-keeping service, allowing people everywhere to synchronize their clocks with 50 micro-second accuracy.
The critical information that allowed the receiver to compute location was a unique frequency curve caused by the doppler effect. The doppler effect caused an apparent compression of the carrier's wavelength as the satellite approached the receiver, and stretching of wavelengths as the satellite receded. The spacecraft traveled at about 17,000 mph, which could increase or decrease the received carrier signal by as much as 10 kHz. This doppler curve was unique for each location within line-of-sight of the satellite. For instance, the earth's rotation caused the ground receiver to move toward or away from the satellite's orbit, creating a non-symmetric doppler shift for approach and recession, allowing the receiver to determine whether it was east or west of the satellite's north-south ground track.
Calculating the optimal receiver location was not a trivial exercise. The navigation software used the satellite's motion to compute a 'trial' doppler curve, based on an initial 'trial' location for the receiver. The software would then perform a least squares curve fit for each two-minute section of the doppler curve, recursively moving the trial position until the trial doppler curve 'most closely' matched the actual doppler received from the satellite for all 2-minute curve segments.
If the receiver was also moving relative to the earth, such as aboard a ship or airplane, this would cause mis-matches with the idealized doppler curves, and degrade position accuracy. However, positional accuracy could usually be computed to within 100-meters for a slow-moving ship, even with reception of just one two-minute doppler curve. This was the navigation criteria demanded by the U.S. Navy, since American submarines would normally expose their UHF antenna for only 2-minutes to obtain a usable Transit fix.
It is noteworthy that surveyors used Transit to locate remote benchmarks by averaging dozens of Transit fixes, producing sub-meter accuracy. In fact, the elevation of Mount Everest was corrected in the late 1980s by using a Transit receiver to re-survey a nearby benchmark. Literally thousands of warships, freighters and private watercraft used Transit from 1967 until 1991. Even some Soviet warships were known to be equipped with Motorola NavSat receivers.
[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.
[edit] External links
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| Historical | Transit (USA) |
| Operational | GLONASS (USSR / Russia) · GPS (USA) · Beidou (China) |
| Developmental | COMPASS (China) · Galileo (Europe) · IRNSS (India) · QZSS (Japan) |
| Related topics | EGNOS · GAGAN · GPS·C · LAAS · MSAS · WAAS |
