Radio navigation or radionavigation is the application of radio frequencies to determining a position on the Earth. Like radiolocation, it is a type of radiodetermination.
The basic principles are measurements from/to electric beacons, especially
- directions, e.g. by bearing, radio phases or interferometry,
- distances, e.g. ranging by measurement of travel times,
- partly also velocity, e.g. by means of radio Doppler shift.
Radio Direction Finding
The first system of radio navigation was the Radio Direction Finder, or RDF. By tuning in a radio station and then using a directional antenna to find the direction to the broadcasting antenna, radio sources replaced the stars and planets of celestial navigation with a system that could be used in all weather and times of day. By using triangulation, two such measurements can be plotted on a map where their intersection is the position. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were also set up specifically for this task. Early systems used a loop antenna that was rotated by hand to find the angle to the signal, while modern systems use a much more directional solenoid that is rotated rapidly by a motor, with electronics calculating the angle. These later systems were also called Automatic Direction Finders, or ADF.
Low frequency radio range
The low frequency radio range (LFR), also known as the four-course radio range, LF/MF four-course radio range, A-N radio range, Adcock radio range, or commonly "the Range", was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s in the U.S. and other countries, until the advent of the Very High Frequency Omni-directional radio range (VOR), beginning in the late 1940s. It was used for both enroute navigation as well as instrument approaches. The ground stations emitted directed radio waves into four quadrants, with one opposing quadrant pair sending out a stream of "di dah" Morse codes (A), and the other, "dah dit" codes (N). The intersections between the four quadrants defined four straight course lines, or airways, along which the signal was a combination of the A and N codes, resulting in a uniform audio hum. When deviating from the airway, one of the Morse codes, A or N, became distinctly audible, which told the pilots to turn left or right, depending on their location and direction. Directly over the station was a "null" — no audible sound, the so-called Cone of Silence — which helped establish a definite position over the ground. The on board receiver was a simple AM radio, tuned to the appropriate range frequency. Effective course accuracy was about three degrees, which near the station provided sufficient safety margins for instrument approaches down to low minimums. At its peak deployment, there were nearly 400 LFR stations in the U.S. alone.
In the 1930s German radio engineers developed a system called the "Ultrakurzwellen-Landefunkfeuer" (LFF), or simply "Leitstrahl" (guiding beam) but referred to outside Germany as Lorenz, the name of the company manufacturing the equipment. In Lorenz two signals were broadcast on the same frequencies from highly directional antennas with beams a few degrees wide. One was pointed slightly to the left of the other, with a small angle in the middle where they overlapped. The signals were chosen as dots and dashes, timed so that when the aircraft was in the small area in the middle the sound was continuous. Planes would fly into the beams by listening to the signal to identify which side of middle they were on, and then corrected until they were in the center.
Originally developed as a night and bad-weather landing system, in the late 1930s they also started developing long-range versions for night bombing. In this case a second set of signals were broadcast at right angles to the first, and indicated the point at which to drop the bombs. The system was highly accurate and a battle of the beams broke out when United Kingdom intelligence services attempted, and then succeeded, in rendering the system useless.
The next major advance in "beam based" navigation system was the use of two signals that varied not in sound, but in phase. In these systems, known as VHF omnidirectional range, or VOR, a single master signal is sent out continually from the station, and a highly directional second signal is sent out that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle can be determined without any physical motion in the receiver. This angle is then displayed in the cockpit of the aircraft, and can be used to take a fix just like the earlier RDF systems, although it is, in theory, easier to use and more accurate.
Systems based on the measurement of the difference of signal arrival times from two or more locations are called hyperbolic systems due to the shape of the lines of position on the chart. These include:
The British GEE system was developed during World War II. GEE used a series of transmitters sending out precisely timed signals, and the aircraft using GEE, RAF Bomber Command's heavy bombers, examined the time of arrival on an oscilloscope at the navigator's station. If the signal from two stations arrived at the same time, the aircraft must be an equal distance from both transmitters, allowing the navigator to determine a line of position on his chart of all the positions at that distance from both stations. By making similar measurements with other stations, additional lines of position can be produced, leading to a fix. GEE was accurate to about 165 yards (150 m) at short ranges, and up to a mile (1.6km) at longer ranges over Germany. Used after WWII as late as the 1960s in the RAF (approx freq was by then 68 MHz).
Other "time based" radio navigation systems were developed from the basic GEE principle. Most capable of these was LORAN, for "LOng-range RAdio Navigation", originally developed for navigation over the Atlantic. In LORAN a single "master" station broadcast a series of short pulses, which were picked up and re-broadcast by a series of "slave" stations, together making a "chain". Since the time between the reception and re-broadcast of the pulses by the slaves was tightly controlled, the time it took for the radio signal to travel from station to station could be measured by listening to the signals. Since the time for the re-broadcasts to reach a remote receiver varies with its distance from the slaves, the distance to each slave could be determined. By plotting the circles representing the ranges on a map, the area where they overlapped formed a fix.
At first the electronics needed to make these accurate measurements was expensive, and using it was difficult. As the sophistication of computer systems grew to the point where they could be placed on a single chip, LORAN suddenly became very simple to use, and quickly appeared in civilian systems intended for use on boats starting in the 1980s. However, like the beam systems before it, civilian use of LORAN was short-lived when newer technology quickly drove it from the market.
Other hyperbolic systems
Similar hyperbolic systems included the British/US Decca Navigator System used in the English Channel area, the US global-wide VLF/Omega Navigation System, and the similar Alpha deployed by the USSR. The expensive to maintain Omega system was shut down in 1997 as the US military migrated to using GPS, while Alpha is still in use.
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The most recent are satellite navigation systems. From early Transit (satellite) (doppler effect) systems, where one satellite provided a fix of varying quality dependent on a number of factors (one being altitude of the observer), we now see the Global Positioning System's constellation of satellites providing high quality positions based on high frequency signals providing near constant highly accurate positions in three dimensions.....
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- VHF omnidirectional range (VOR)
- Distance measuring equipment (DME)
- Tactical air navigation (TACAN)
- Non-directional beacon (NDB)
- Instrument landing system (ILS)
- Marker beacon (three-light marker beacon system)
- Transponder Landing System (TLS)
- Microwave landing system (MLS)
- Long-range navigation (LORAN)
- Global Positioning System (GPS)
- Local Area Augmentation System (LAAS)
- Wide Area Augmentation System (WAAS)
- Differential GPS (DGPS)
- EGNOS (European Geostationary Navigation Overlay Service)
- Global Navigation Satellite System (GLONASS)
- Galileo positioning system (Galileo)
- Space Integrated GPS/INS (SIGI) or (SIGI) for short.
- American Practical Navigator
- Wind triangle
- Department of Transportation and Department of Defense (March 25, 2002). "2001 Federal Radionavigation Systems" (PDF). http://www.navcen.uscg.gov/pubs/frp2001/FRS2001.pdf. Retrieved November 27, 2005.
- UK Navaids Gallery with detailed Technical Descriptions of their operation