Radio navigation

Radio navigation or radionavigation is the application of radio frequencies to determine 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.

Bearing-measure systems

These systems used some form of directional radio antenna to determine the location of a broadcast station on the ground. Conventional navigation techniques are then used to take a radio fix. These were introduced prior to WWI, and remain in use today.

Radio direction finding

Amelia Earhart's Lockheed Electra had a prominent RDF loop on the cockpit roof.

Main article: Radio direction finder

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, then 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 RDF systems normally used a loop antenna, a small loop of metal wire that is mounted so it can be rotated around a vertical axis. At most angles the loop has a fairly flat reception pattern, but when it is aligned perpendicular to the station the signal in one side cancels the signal in the other, producing a sharp drop in reception known as the "null". By rotating the loop and looking for the angle of the null, the relative bearing of the station can be determined. Loop antennas can be seen on most pre-1950s aircraft and ships.

Reverse RDF

The main problem with RDF is that it required a special antenna on the vehicle, which may not be easy to mount on smaller vehicles or single-crew aircraft. A smaller problem is that the accuracy of the system is based to a degree on the size of the antenna, but larger antennas would likewise make the installation more difficult.

During the era between World War I and World War II, a number of systems were introduced that placed the rotating antenna on the ground. As the antenna rotated through a fixed position, typically due north, the antenna was keyed with the morse code signal of the station's identification letters so the receiver could ensure they were listening to the right station. Then they waited for the signal to either peak or disappear as the antenna briefly pointed in their direction. By timing the delay between the morse signal and the peak/null, then dividing by the known rotational rate of the station, the bearing of the station could be calculated.

The first such system was the German Telefunken Kompass Sender, which began operations in 1907 and was used operationally by the Zepplin fleet until 1918. An improved version was introduced by the UK as the Orfordness Beacon in 1929 and used until the mid-1930s. A number of improved versions followed, replacing the mechanical motion of the antennas with phasing techniques that produced the same output pattern with no moving parts. One of the longest lasting examples was Sonne, which went into operation just before World War II and used operationally under the name Consol until 1991. The modern VOR system is based on the same principles.

ADF and NDB

Main article: Non-directional beacon

A great advance in the RDF technique was introduced in the form of phase comparisons of a signal as measured on two or more small antennas, or a single highly directional solenoid. These receivers were dramatically smaller, more accurate, and simpler to operate. Combined with the introduction of the transistor and integrated circuit, RDF systems were so reduced in size and complexity that they once again became quite common during the 1960s, and were known by the new name, automatic direction finder, or ADF.

This also led to a revival in the operation of simple radio beacons for use with these RDF systems, now referred to as non-directional beacons (NDB). As the LF/MF signals used by NDBs can follow the curvature of earth, NDB has a much greater range than VOR which travels only in line of sight. NDB can be categorized as long range or short range depending on their power. The frequency band alloted to non-diretional beacons is 190-1750 kHz, but the same system can be used with any common AM-band commercial station.

VOR

VOR transmitter station

Main article: VHF omnidirectional range

VHF omnidirectional range, or VOR, is an implementation of the reverse-RDF system, but one that is more accurate and able to be completely automated.

Instead of a single signal, the VOR transmitter sends out three signals - one is a simple voice channel that sends morse code to identify the station, another is a continuous signal sent in all directions, and the last is a signal that is rotated at 30 RPM. Like the Orfordness concept, the bearing of the station is measured by timing the rotating signal's peak or null. But instead of timing the signal, the rotating signal is changed in phase in synchronicity with its rotation, such that it is in-phase when pointed north, 90 degrees off when it points east, and so forth. By comparing the phase of the received signal with the one being broadcast omnidirectionally, the angle can be determined using simple electronics. 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 easier to use.

As VOR required two VHF receivers as well as a conventional radio for station identification, the system did not become popular until the era of miniaturized electronics, first with small tubes in the 1950s, and then transistorized systems in the 1960s. During this period it quickly took over from the older Radio Range system (see below). Although the system could be used anywhere, as opposed to the beams which were only broadcast in certain directions, in theory the VOR system could be used for free navigation from any to any point. In practice, the older Radio Range procedures were so widely used and standardized that VOR was used to produce a similar set of airways that remain in use today.

The US military also introduced a VOR-like system known as TACAN. It differed from VOR primarily in its modulation system, adding a Lorentz-like signal to accurately define the center of the rotating beam. It requires five receiver channels and additional electronics, an expensive requirement when it was introduced.

Beam systems

Beam systems broadcast narrow signals in the sky, and navigation is accomplished by keeping the aircraft centred in the beam. A number of stations are used to create an airway, with the navigator tuning in different stations along the direction of travel. These systems were common in the era when electronics were large and expensive, as they placed minimum requirements on the receivers - they were simply voice radio sets tuned to the selected frequencies. However, they did not provide navigation outside of the beams, and were thus less flexible in use. The rapid miniaturization of electronics during and after WWII made systems like VOR practical, and most beam systems rapidly disappeared.

Lorenz

Main article: Lorenz beam

In the post-WWI era, the Lorenz company of Germany developed a means of projecting two narrow radio signals with a slight overlap in the center. By broadcasting different audio signals in the two beams, the receiver could position themselves very accurately down the centreline by listening to the signal in their headphones. The system was accurate to less than a degree in some forms.

Known as "Ultrakurzwellen-Landefunkfeuer" (LFF), or simply "Leitstrahl" (guiding beam), little money was available to develop a network of stations. Development was led by the US, where it formed the basis of a wide-area navigation system through the 1930s and 40s (see LFF, below). Development was re-started in Germany in the 1930s as a short-range system deployed at airports as a blind landing aid. Although there was some interest in deploying a medium-range system like the US LFF, deployment had not yet started when the beam system was combined with the Orfordness timing concepts to produce the highly accurate Sonne system.

In the immediate pre-WWII era the same concept was also developed as a blind-bombing system. This used very large antennas to provide the required accuracy over England, and very powerful transmitters. Two beams were used, crossing over the target. Bombers would enter one of the beams and use it for guidance until they heard the second one in a second radio receiver, using that signal to time the dropping of their bombs. The system was highly accurate, and the 'Battle of the Beams' broke out when United Kingdom intelligence services attempted, and then succeeded, in rendering the system useless. Sonne, however, proved just as useful to the UK as Germany, and was left to operate unhindered throughout the war.

Low frequency radio range

LFR ground station

Main article: 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 VOR in the late 1940s. It was used for both enroute navigation as well as instrument approaches.

The ground stations consisted of a set of four antennas that projected Lorenz beams in four cardinal directions. One of the beams was "keyed" with the morse code signal "A", dit-dah, with the second beam "N", dah-dit. Flying down the centreline produced a steady tone. The beams were pointed to the next station to produce a set of airways, allowing an aircraft to travel from airport to airport by following a selected set of stations. 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 US.

ILS

The remaining widely-used beam system is the instrument landing system, or ILS. ILS is essentially a combination of the beam system's narrow fan-shaped broadcasts with a modulation scheme that allows automated display, similar to VOR. ILS uses separate systems to provide horizontal and vertical positioning information, distance to he runway, and airport information. Together, ILS provides enough accuracy and redundancy to allow totally automated landings.

The Lorenz system indicated the centreline of the two signals through the audio pattern, which was listened to by the navigator. In ILS, the two signals are instead AM modulated with slightly different patters, 150 Hz in one and 90 in the other. By comparing the relative strength of the two modulations, the receiver can automatically determine which beam lobe they are in, or if the modulations are equal, along the centreline. For positioning, two such systems are used, the localizer providing horizontal guidance, and the glideslope vertical. The final part of the ILS package is a series of "marker beacons", very low-powered signals that are sent directly upwards from antennas located along the approach to the runway. These are received as the aircraft flies over them, producing an audio signals at specific locations on the approach.

Transponder systems

Positions can be determined with any two measures of angle or distance. The introduction of radar in the 1930s provided a way to directly determine the distance to an object even at long distances. Navigation systems based on these concepts soon appeared, and remained in widespread use until recently. Today they are used primarily for aviation, although GPS has largely supplanted this role.

Radar and transponders

Understanding transponder systems is simple when one considers the operation of conventional radar.

Early systems, like the UK's Chain Home, consisted of large transmitters and separate receivers, and the signals from both were sent to an oscilloscope. The broadcaster periodically sends out a short pulse of a power radio signal, and a small amount of that signal is also sent to the oscilloscope to "trigger" it, sending its beam sweeping quickly across the display. As soon as the pulse is complete, the receiver is turned on, and its signals are amplified and sent to the same display. Any received energy - reflections off distance objects - causes the beam to move upward on the display. This produces a series of "blips" along the display as signals from different objects are received. Since the beam moves at a steady speed, the distance to the object can be determined simply by measuring the blip's distance from the starting point of the trace.

Soon after the introduction of radar, the radio transponder appeared. Transponders are a combination of receiver and transmitter who's operation is automated - upon reception of a particular signal, normally a pulse on a particular frequency, the transponder sends out a pulse in response, typically delayed by some very short time. If that pulse is on the same frequency as the sent signal, it will cause a second blip to appear on the radar display, forming the basis of early IFF systems - those blips that are doubled up have the proper transponder, and are thus friendly aircraft.

Bombing systems

The first distance-based navigation system was the German Y-Gerät blind-bombing system. This used a Lorenz beam for horizontal positioning, and a transponder for ranging. A ground-based system periodically sent out pulses which the airborne transponder retuned. By measuring the total round-trip time on an radar's oscilloscope, the aircraft's range could be accurately determined even at very long ranges. An operator then relayed this information to the bomber crew over voice channels, and indicated when to drop the bombs.

The British introduced similar systems, notably the Oboe system. This used two stations in England that operated on different frequencies and allowed the aircraft to be triangulated in space. To ease pilot workload only one of these was used for navigation - prior to the mission a circle was drawn over the target from one of the stations, and the aircraft was directed to fly along this circle on instructions from the ground operator. The second station was used, as in Y-Gerät, to time the bomb drop. Unlike Y-Gerät, Oboe was deliberately built to offer very high accuracy, as good as 35 m, much better than even the best optical bombsights.

One problem with Oboe was that it allowed only one aircraft to be guided at a time. This was addressed in the later Gee-H system by placing the transponder on the ground and broadcaster in the aircraft. The signals were then examined on existing Gee display units in the aircraft (see below). Gee-H did not offer the accuracy of Oboe, but could be used by as many as 90 aircraft at once. This basic concept has formed the basis of most distance measuring navigation systems to this day.

Beacons

The key to the transponder concept is that it can be used with existing radar systems. The ASV radar introduced by RAF Coastal Command was designed to track down submarines and ships by displaying the signal from two antennas side by side and allowing the operator to compare their relative strength. Adding a ground-based transponder immediately turned the same display into a system able to guide the aircraft towards a transponder, or "beacon" in this role, with high accuracy.

The British put this concept to use in their Rebecca/Eureka system, where battery-powered "Eureka" transponders were triggered by airborne "Rebecca" radios and then displayed on ASV Mk. II radar sets. Eureka's were provided to French resistance fighters, who used them to call in supply drops with high accuracy. The US quickly adopted the system for paratroop operations, dropping the Eureka with pathfinder forces or partisans and then homing in on those signals to mark the drop zones.

The beacon system was widely used in the post-war era for blind bombing systems. Of particular note were systems used by the US Marines that allowed the signal to be delayed in such a way to offset the drop point. These systems allowed the troops at the front line to direct the aircraft to points in front of them, directing fire on the enemy. Beacons were widely used for temporary or mobile navigation as well, as the transponder systems were generally small and low-powered, able to be man portable or mounted on a Jeep.

DME

In the post-war era, a general navigation system using transponder-based systems was deployed as the distance measuring equipment (DME) system.

DME was largely identical to Gee-H in operation, but used new electronics to automatically measure the time delay and display it as a number, rather than having the operator time the signals manually on an oscilloscope. This led to the possibility that DME interrogation pulses from different aircraft might be confused, but this was solved by having each aircraft send out a different series of pulses which the ground-based transponder repeated back.

DME is almost always used in conjunction with VOR, and is normally co-located at a VOR station. This combination allows a single VOR/DME station to provide both angle and distance, and thereby provide a single-station fix. DME is also used as the distance-measuring basis for the military TACAN system, and their DME signals can be used by civilian receivers.

Hyperbolic systems

Hyperbolic navigation systems are a modified form of transponder systems which eliminate the need for an airborne transponder. The name refers to the fact that they do not produce a single distance or angle, but instead indicate a location along any number of hyperbolic lines in space. Two such measurements produces a fix. As these systems are almost always used with a specific navigational chart with the hyperbolic lines plotted on it, they generally reveal the receiver's location directly, eliminating the need for manual triangulation. As these charts were digitized, they became the first true location-indication navigational systems, outputting the location of the receiver as latitude and longitude. Hyperbolic systems were introduced during WWII and remained the main long-range advanced navigation systems until GPS replaced them in the 1990s.

Gee

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 (1.6 km) at longer ranges over Germany. Used after WWII as late as the 1960s in the RAF (approx freq was by then 68 MHz).

LORAN

Other "time based" radio navigation systems were developed from the basic Gee principle. Most capable of these was LORAN, for "LOng-range Aid to Navigation", originally developed for navigation over the Atlantic. The original LORAN system was essentially a version of Gee using longer wavelengths for longer range. During the 1950s a new version, LORAN-C, was introduced that used clever techniques to dramatically improve accuracy.

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-C suddenly became very simple to use, and quickly appeared in civilian systems intended for use on boats starting in the 1980s. It was the most popular navigation system in use though the 1980s and 90s, and its popularity led to many older systems being shut down. However, like the beam systems before it, civilian use of LORAN-C 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. Alpha is still in use.

Satellite navigation

Cessna 182 with GPS-based "glass cockpit" avionics

Main article: Global Navigation Satellite System

Since the 1960s, navigation has increasingly moved to satellite navigation systems. These are essentially DME systems located in space. The fact that the satellites are in orbit and normally move with respect to the receiver means that the calculation of the position of the satellite needs to be taken into account as well, which can only be handled effectively with a computer.

The Global Positioning System, better known simply as GPS, sends several signals that are used to decode the position and distance of the satellite. One signal encodes the satellite's "ephemeris" data, which is used to accurately calculate the satellite's location at any time. Space weather and other effects causes the orbit to change over time so the ephemeris has to be updated periodically. Other signals send out the time as measured by the satellite's onboard atomic clock. By measuring this signal from several satellites, the receiver can re-build an accurate clock signal of its own. Comparing the two produces the distance to the satellite, and several such measurements allows a form of triangulation to be carried out.

GPS has better accuracy that any previous land-based system, is available at almost all locations on the Earth, can be implemented in a few cents of modern electronics, and requires only a few dozen satellites to provide worldwide coverage. As a result of these advantages, GPS has led to almost all previous systems falling from use. LORAN, Omega, Decca, Consol and many other systems disappeared during the 1990s and 2000s. The only other systems still in use are aviation aids, which are also being turned off for long-range navigation while new differential GPS systems are being deployed to provide the local accuracy needed for blind landings.

See also

• Multilateration
• Real-time locating

Radio navigation systems and applications

• 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.
• RAIM
• American Practical Navigator
• Wind triangle
• SCR-277

External links

• Department of Transportation and Department of Defense (March 25, 2002). "2001 Federal Radionavigation Systems" (PDF). http://www.navcen.uscg.gov/pdf/frp/frp2001/FRS2001.pdf. Retrieved November 27, 2005. 
• UK Navaids Gallery with detailed Technical Descriptions of their operation

History

• Some historical and technical aspects of radio navigation in Germany over the period 1907 to 1945