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 U.S. Navy to provide accurate location information to its Polaris ballistic missile submarines, and it was also used as a navigation system by the Navy's surface ships, as well as for hydrographic and geodetic surveying. Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use as well.
Contents |
The TRANSIT satellite system was developed by the Applied Physics Laboratory (APL) of Johns Hopkins University for the U.S. Navy. Just days after the Soviet launch of Sputnik 1, the first man-made earth-orbiting satellite on October 4, 1957. Two physicists at APL, William Guier and George Weiffenbach, found themselves in discussion about the microwave signals that would likely be emanating from the satellite and were able to determine Sputnik's orbit by analyzing the Doppler shift of its radio signals during a single pass.[1] Frank McClure, the chairman of APL's Research Center, suggested that if the satellite's position were known and predictable, the Doppler shift could be used to locate a receiver on Earth.
Development of the TRANSIT system began in 1958, and a prototype satellite, Transit 1A, was launched in September 1959.[2] That satellite failed to reach orbit.[3] A second satellite, Transit 1B, was successfully launched April 13, 1960, by a Thor-Ablestar rocket.[4] The first successful tests of the system were made in 1960, and the system entered Naval service in 1964.
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.
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.
The TRANSIT system was made obsolete by the Global Positioning System, and ceased navigation service 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 to be used continuously, whereas TRANSIT provided a fix only every hour or more.
After 1996, the satellites were kept in use as spaceborne 'mailboxes' and for the Navy's Ionospheric Monitoring System.
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. Note that these OSCAR satellites were not the same as the OSCAR series of satellites that were devoted to use by amateur radio operators to use in satellite communications.
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 was several hours; at mid-latitudes the delay decreased to an hour or two. For its intended role as an updating system for SLBM launch, TRANSIT sufficed, 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 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 transit satellite broadcast on 150 and 400 MHz. The two frequencies were used to allow the bending of the satellite radio beacons by the ionosphere to be canceled out, thereby improving location 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 most likely 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 mismatches 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 criterion 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. The U.S. Submarine version of the Transit System also included a special encrypted (and more accurate) version of the downloaded satellite's orbital data. This enhanced data allowed for considerably enhanced system accuracy (not unlike Selective Availability <SA> under GPS). Using this enhanced mode accuracy was typically less than 20 meters. (Between LORAN C and GPS.) Certainly, the most accurate navigation system of its day.
A network of ground stations, whose locations were accurately known, continually tracked the transit satellites. They measured the Doppler shift on 5 hole paper tape using standard teletype hole convention. This data was sent to the Satellite Control Center at Applied Physics Laboratory in Laurel, Maryland using commercial and military teletype networks. The data from the fixed ground stations provided the location information on the transit satellite orbit. Locating a transit satellite in earth orbit from a known ground station using the Doppler shift is simply the reverse of using the known location of the satellite in orbit to locate an unknown location on the earth, again using the Doppler shift .
A typical ground station occupied a small Quonset hut. The accuracy of the ground station measurements was a function of the ground station master clock accuracy. Initially a quartz oscillator in a temperature controlled oven was used as the master clock. The master clock was checked daily for drift using a VLF receiver tuned to a US Navy low frequency VLF station. The VLF signal had the property that the phase of the VLF signal did not change from day to day at noon along the path between the transmitter and the receiver and thus could be used to measure oscillator drift. Later rubidium beam and cesium beam clocks were used. Ground stations had number names; for example, Station 019 was McMurdo Station, Antarctica. For many years during the 1970s this station was staffed by a graduate student and an undergraduate student, typically in electrical engineering, from the University of Texas at Austin. Other stations were located at New Mexico State University, the University of Texas at Austin, Sicily, Japan, Seychelles Island, Thule Greenland and a number of other locations. The Greenland and Antarctica stations saw every pass of every transit satellite because of their near pole location for these polar orbiting satellites.
A portable version of the ground station was called a Geoceiver and was used to make field measurements. This receiver, power supply, punched tape unit, and antennas could fit in a number of padded aluminum cases and could be shipped as extra cargo on an airline. Data was taken over a period of time, typically a week, and sent back to the Satellite Control Center for processing. Therefore, unlike GPS, there was not an immediate accurate location of the geoceiver location. A geoceiver was permanently located at the South Pole Station and operated by USGS personnel. Since it was located on the surface of a moving ice sheet, its data was used to measure the ice sheet movement. Other geoceivers were taken out in the field in Antarctica during the summer and were used to measure locations, for example the movement of the Ross Ice Shelf.
Since no computer small enough to fit through a submarine's hatch existed (in 1958), 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 Corporation[5] (later TRW) for the Lafayette class SSBNs. 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.
There were 37 other satellites in the Transit series that were assigned the Transit name by NASA.[6]
Transit 4A, launched June 29, 1961, was the first satellite to use a radioactive power source.[7] Transit 4B was notable for sustaining damage in a nuclear explosion, specifically the United States Starfish Prime high-altitude nuclear test in 1962.[8]
The US Air Force also periodically launched short lived satellites equipped with radio beacons of 162 MHz and 324 MHz at much lower orbits to study orbital drag. The Transit ground tracking stations tracked these satellites as well, locating the satellites within their orbits using the same principles. The satellite location data was used to collect orbital drag data, including variations in the upper atmosphere and the Earth's gravitational field.
|
|||||||||||||||||