Radio navigation
Radio navigation or radionavigation is the application of radio waves to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles are measurements from/to electric beacons, especially
- Angular directions, e.g. by bearing, radio phases or interferometry,
- Distances, e.g. ranging by measurement of time of flight between one transmitter and multiple receivers or vice versa,
- Distance differences by measurement of times of arrival of signals from one transmitter to multiple receivers or vice versa
- Partly also velocity, e.g. by means of radio Doppler shift.
Bearing-measurement 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 World War I, and remain in use today.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, one could determine the direction to the broadcasting antenna. A second measurement using another station was then taken. Using triangulation, the two directions can be plotted on a map where their intersection reveals the location of the navigator. 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, especially near airports and harbours.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 received on one side of the loop 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 Zeppelin 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 was used operationally under the name Consol until 1991. The modern VOR system is based on the same principles.
ADF and NDB
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 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. 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 allotted to non-directional beacons is 190–1750 kHz, but the same system can be used with any common AM-band commercial station.
VOR
, or VOR, is an implementation of the reverse-RDF system, but one that is more accurate and able to be completely automated.The VOR station transmits two audio signals on a VHF carrier – one is Morse code at 1020 Hz to identify the station, the other is a continuous 9960 Hz audio modulated at 30 Hz, with the 0-degree referenced to magnetic north. This signal is rotated mechanically or electrically at 30 Hz, which appears as a 30 Hz AM signal added to the previous two signals, the phasing of which is dependent on the position of the aircraft relative to the VOR station.
The VOR signal is a single RF carrier that is demodulated into a composite audio signal composed of a 9960 Hz reference signal frequency modulated at 30 Hz, a 30 Hz AM reference signal, and a 1020 Hz 'marker' signal for station identification. Conversion from this audio signal into a usable navigation aid is done by a navigation converter, which takes the reference signal and compares the phasing with the variable signal. The phase difference in degrees is provided to navigational displays. Station identification is by listening to the audio directly, as the 9960 Hz and 30 Hz signals are filtered out of the aircraft internal communication system, leaving only the 1020 Hz Morse-code station identification.
The system may be used with a compatible glideslope and marker beacon receiver, making the aircraft ILS-capable
Beam systems
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 World War II made systems like VOR practical, and most beam systems rapidly disappeared.Lorenz
In the post-World War I 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.Originally known as "Ultrakurzwellen-Landefunkfeuer", or simply "Leitstrahl", little money was available to develop a network of stations. The first widespread radio navigation network, using Low and Medium Frequencies, was instead led by the US. Development was restarted 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 all of these roles, the system was generically known simply as a "Lorenz beam". Lorenz was an early predecessor to the modern Instrument Landing System.
In the immediate pre-World War II era the same concept was also developed as a blind-bombing system. This used very large antennas to provide the required accuracy at long distances, and very powerful transmitters. Two such beams were used, crossing over the target to triangulate it. 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 through electronic warfare.
Low-frequency radio range
In 1926 the National Bureau of Standards began developing the low-frequency radio range, or four course range system.The ground stations consisted of a set of four antennas that projected two overlapping directional figure-eight signal patterns at a 90-degree angle to each other. One of these patterns was "keyed" with the Morse code signal "A", dit-dah, and the second pattern "N", dah-dit. This created two opposed "A" quadrants and two opposed "N" quadrants around the station. The borders between these quadrants created four course legs or "beams" and if the pilot flew down these lines, the "A" and "N" signal merged into a steady "on course" tone and the pilot was "on the beam". If the pilot deviated to either side the "A" or "N" tone would become louder and the pilot knew to make a correction. The beams were typically aligned with other stations 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 over 400 LFR stations in the US.