Radio Navigation Aviation Pdf

[Kenneth]

Navigationby radio aids includes navigation mainly by reference to indications of bearing and distance indicated on VOR, DME and ADF equipment located on the aircraft. This information is derived from ground radio beacons (VOR, DME and NDBs or broadcast stations.)

The simplest form of navigation involves flying a direct track between a succession of VOR or NDB beacons which lie on or close to the desired track (see Figure 1.) This is the form of navigation employed in conventional airways navigation. The magnetic track and distance betwen successive beacons is measured and the calculated heading is flown, with corrections to allow for the effect of wind, until the next beacon is reached. Distance (and elapsed time) to the next beacon is read from DME or calculated by dead reckoning.

When direct beacon to beacon navigation is not possible (e.g. to avoid entering controlled airspace or danger, prohibited or restricted areas), navigation is more difficult. Two alternative methods are often used:

1. The route is plotted between radio beacons, some of which may be within controlled airspace; but the aircraft turns onto a pre-calculated track from or to another beacon before the area to be avoided is reached (see Figure 2.)

Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction.[1][2] Like radiolocation, it is a type of radiodetermination.

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.[citation needed]

The first system of radio navigation was the Radio Direction Finder, or RDF.[3] 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.[4][5] 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.[citation needed]

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.[3] 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.[citation needed]

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.[citation needed]

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.[citation needed]

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.[6] 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 (see below).[citation needed]

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.[citation needed]

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.[citation needed]

The system may be used with a compatible glideslope and marker beacon receiver, making the aircraft ILS-capable (Instrument Landing System)}. Once the aircraft's approach is accurate (the aircraft is in the "right place"), the VOR receiver will be used on a different frequency to determine if the aircraft is pointed in the "right direction." Some aircraft will usually employ two VOR receiver systems, one in VOR-only mode to determine "right place" and another in ILS mode in conjunction with a glideslope receiver to determine "right direction." }The combination of both allows for a precision approach in foul weather.[7]

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.[citation needed]

Originally known as "Ultrakurzwellen-Landefunkfeuer" (LFF), or simply "Leitstrahl" (guiding beam), 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 (see LFF, below). 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.[citation needed]

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 (over England), 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.[citation needed]

The low-frequency radio range (LFR, also "Four Course Radio Range" among other names) 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 en route navigation as well as instrument approaches.[citation needed]

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.[8]

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.[citation needed]

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