Land based radionavigation systems have been historically been either radio direction finding (RDF) systems or "hyperbolic" systems. RDF systems are pretty easy to understand. Simply, the operator tunes to a radio station that has known coordinates, A directional antenna is used to get a bearing to the radio station. The procedure is repeated with another radio station. The operator's position can then be calculated.Hyperbolic systems are a bit more difficult to understand. A hyperbolic system uses coordinated transmissions from two or more radio stations. It relies on the fact that all the points where the difference between radio signals from different stations is constant form a hyperbola (artwork coming eventually). It is possible to make a map showing several hyperbolas (remember, the hyperbolas are lines indicating a constant difference). The user uses radio equipment to listen to the radio signals, then matches the difference heard to the appropriate hyperbola on the map. This puts the user somewhere along one arc on the ground. This process is repeated by using a second pair of stations to determine another hyperbola. The user determines position by finding the point on the map where the two parabolas intersect.
The NAVSTAR and GLONASS systems use the principle of trilateration. That is, the user's receiver determines the distance from the user to each of several satellites. Since the positions of the satellites are known, either through previous publication or as part of the satellite's broadcast information, the user's position can be calculated.
DECCA
The DECCA system was a low-frequency hyperbolic navigation system covering much of western Europe, parts of Canada, the Persian Gulf and the Bay of Bengal. DECCA worked by comparing the phase difference of radio signals emitted by several radio stations.GEE
GEE is a British system, similar to LORAN but using VHF frequencies. This limits the system to line-of-sight.LORAN-A
The LORAN-A (standard LORAN) system was developed during World War II at the Massachusetts Institute of Technology. LORAN stands for LOng Range Navigation and was developed in response to the need for precise navigation for military ships and aircraft. Operation was in the 1850 kHz - 1950 kHz radio band. The system had a 600 mile range.LORAN-C
The LORAN-C system was developed in the 1950s. Current land-based radio navigation system operating in the 90 kHz to 110 kHz band. Loran-C is a pulsed hyperbolic system that provides 0.25 nm predictable accuracy, 18 - 90 m repeatable accuracy, 95% confidence and 99.7% availability. It was developed to provide the Department of Defense (DOD) with a radionavigation capability with longer range and much greater accuracy than its predecessor, Loran-A. Loran-C is the federally provided radionavigation system for civil marine use in U.S. coastal waters. The U.S. Coast Guard is responsible for system operation and maintenance in the U.S. and certain overseas locations. Loran-C provides coverage for the continental U.S. and its coastal waters, the Great Lakes, and most of Alaska. Many other countries are also involved in the providing of Loran-C (or Loran-like) services, or are in negotiations with their neighbors to expand coverage. These countries include India, Norway, France, Ireland, Germany, Spain, Italy, Russia, China, Japan, the Philippines and others.OMEGA
Current land-based radionagivation system, somewhat older than Loran-C. Developed by the United States, it is operated in conjunction with six other nations. OMEGA is a very low frequency, phase comparison, worldwide radionavigation system which provides positioning within 2 to 4 nautical miles at a 95% confidence level with a 95% availability.
GLONASS
Current russian satellite-based positioning system - counterpart to NAVSTARSECOR
SECOR (Sequential Collation of Range) was a U.S. Army satellite navigation and positioning system. Thirteen satellites were launched between 1964 and 1969. Most of the satellites were small (17 kg - 20 kg) and boxy.TRANSIT
Transit was the first operational satellite navigation system. Developed by the Johns Hopkins Applied Physics Laboratory, the system was intended as an aid to submarine navigation.The Transit system allowed the user to determine position by measuring the doppler shift of a radio signal transmitted by the satellite. The user was able to calculate position to within a few hundred meters as long as the user knew his altitude and the satellite ephemeris.
The system has several drawbacks. First, the system is inherently two dimensional. Second, the velocity of the user must be taken into account. Third, mutual interference between the satellites restricted the total number of satellites to five. Thus, satellites would only be visible for limited periods of time. These drawbacks pretty much eliminated aviation applications and severely limited land-based applications.
TIMATION
Developed in 1972 by the Naval Research Laboratory (NRL), Timation satellites were intended to provide time and frequency transfer. The original satellite flew with stable quartz crystal oscillators. Later models flew with the first space-borne atomic clocks. The third satellite acted as a GPS technology demonstrator.NAVSTAR
The NAVSTAR GPS system is a satellite-based radionavigation system developed and operated by the U.S. Department of Defense (DOD). The NAVSTAR system permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other radionavigation systems available today.The NAVSTAR systems performs another function besides positioning and time transfer. Starting with satellite vehicle 8, Navstar satellites carry nuclear explosion detection equipment. The GPS Nuclear Detection (NUDET) system is a joint program between the U.S. Air Force and the Department of Energy. The NUDET system replaces the older VELA system in enforcing the nuclear Non-Proliferation Treaty (NPT) and the Limited Test Ban Treaty (LTBT).
PARUS (TSIKADA-M)
Russian six-satellite military navigation system.TSIKADA
Russian four-satellite civil navigation system.TSYKLON
First navigation satellite launched by the Soviet Union in late 1967. The satellite is based on Doppler techniques demostrated by U.S. Transit system.
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