Global positioning systems are based on the concept of radio navigation. That is, using the characteristics of radio waves transmitted by a fixed point whose characteristics are known to determine the location of a radio receiver. Radio navigation technology got a big boost during World War II. The requirement to guide bombers to their targets and to improve the performance of long-distance fighters resulted in ground-based radio beacons which quickly became indispensible to all aviation and to shipping as well.Historically, land based radionavigation systems have used either of two methods. The simplest is radio direction finding (RDF) systems. Simply, the user 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. The coordinates for the station are ususally published in a book that the operator is user is required to have.
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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.
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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.
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Land Based Radionavigation Systems
DECCA
Frequency Band: Low Frequency Band
Start of Operation: Don't know yet
End of Operation: Don't know yet
Approximate Range: Don't know yet
Limit of Accuracy: Don't know yet
Availability: Don't know yetThe 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
Frequency Band: 20 MHz to 85 MHz (VHF)
Start of Operation: Don't know yet
End of Operation: Don't know yet
Approximate Range: Don't know yet
Limit of Accuracy: Don't know yet
Availability: Don't know yetGEE is a British hyperbolic navigation system, similar to LORAN but using VHF frequencies. This choice of frequency band limits the system to line-of-sight. Transmitting stations were arranged in chains of four stations. The master station transmitted 500 pulses per second, two of the slaves transmitted 250 pulses per second and the third slave transmitted 500/3 pulses per second, synchronized to the master.
LORAN-A
Frequency Band: 1850 kHz to 1950 kHz (MF)
Start of Operation: Don't know yet
End of Operation: Approximately 20 years from start
Approximate Range: 600 nautical miles
Limit of Accuracy: Don't know yet
Availability: Don't know yetThe 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. Because of the propagation changes of medium frequency signals at night, significant errors would occur during nighttime operation.
LORAN-C
Frequency Band: 90 kHz to 110 kHz (LF)
Start of Operation: Late '50s
End of Operation: Still in operation
Approximate Range: Hundreds to thousands of miles
Limit of Accuracy: 0.25 nautical miles repeatable to 18 - 90 meters, 95% confidence
Availability: Quoted 99.7%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
Frequency Band: Low Frequency Band
Start of Operation: Unknown
End of Operation: Unknown
Approximate Range: Don't know yet
Limit of Accuracy: 2 to 4 nautical miles with 95% confidence level
Availability: 95% of the time.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
TACAN
Frequency Band: (US) 960 MHz - 1215 MHz (UHF)
Start of Operation: Unknown
End of Operation: Still in operation
Approximate Range: 200 miles at high altitudes
Limit of Accuracy: Unknown
Availability:TACAN is primarily used by U.S. and other military aircraft. TACAN radio stations are often co-located with civilian VOR systems allowing military aircraft to operate in civil airspace. The system provides the pilot with relative bearing and distance to the radio beacon.
VOR
Frequency Band: (US) 108.0 MHz - 117.95 MHz (VHF)
Start of Operation: 40 + years ago
End of Operation: Still in operation
Approximate Range:250 miles
Limit of Accuracy: As poor as 20 miles
Availability:VOR is a beacon-based navigation system operated in the U.S. by the Federal Aviation Administration (FAA) for civil aircraft navigation. When used by itself, the system allows users to determine their azimuth from the VOR station without using any directional equipment. VOR stations are radio beacons that transmit two signals. The first, called the reference signal, is transmitted with constant phase all around the transmitter. The second signal is phase shifted from the first depending on the compass direction of the user from the station. A simple, inexpensive receiver in the aircraft is used to determine the received phase difference of the two signals, and from that information the direction of the aircraft from the transmitter. By using using two VOR stations, a specific location may be determined.
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
Operating Frequencies: 100 MHz and 400 MHz
Start of Operation: September 17, 1959
End of Operation: December 31, 1996, 2359 GMT
Limit of Accuracy: Don't know yet
Availability: Minimum one observation period per day.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 system has been withdrawn from service as called for in the 1994 Federal Radionavigation Plan.
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.