Radio direction finder
From Wikipedia, the free encyclopedia
A radio direction finder (RDF) is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances "over the horizon", it makes a particularly good navigation system for ships and aircraft that might be flying at a distance from land.
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[edit] Introduction
RDF's work by pointing a directional antenna in "various directions" and then listening for the direction in which the signal from a known station comes through most strongly. This sort of system was widely used in the 1930s and 1940s. RDF antennas are particularly very easy to spot on German World War II aircraft, as loops under the rear section of the fuselage, whereas most US aircraft enclosed the antenna in a small teardrop-shaped fairing.
[edit] Method of Operation
In more recent times the task of finding the signal has been automated in the automatic direction finder, or ADF. In one type of ADF-system the antenna consists of a small cylinder of wire, a solenoid that is highly directional, which is spun by a motor. The electronics listen either for the repeated "peak" in the signal, or most commonly, the "trough" when the signal drops to zero when the antenna is at right angles to the signal. A small lamp attached to a disk is timed to spin at the same speed as the antenna, so when the peak or trough is detected the lamp flashes briefly. To the human eye it appears to be a single spot of light on top of a compass rose.
[edit] Usage in navigation
Signals are provided in the form of radio beacons, the radio version of a lighthouse. The signal is typically a simple AM broadcast of a morse code series of letters, which the RDF can tune in to see if the beacon is "on the air". Most modern detectors can also tune in any commercial radio stations, which is particularly useful due to their high power and location near major cities.
RDF was once the primary form of aircraft navigation, and strings of beacons were used to form "airways" from airport to airport. In the 1950s these systems were generally being replaced by the VOR system, in which the angle to the beacon can be measured from the signal itself, with no moving parts. Since the signal being broadcast in the RDF system is non-directional, these older beacons were referred to as non-directional beacons, or NDB in the aviation world.
Today all such systems are being generally removed in favour of the much more accurate and user-friendly GPS system. However the low cost of ADF systems today has meant a comeback, whereas the expensive VOR systems will likely all be switched off in the near future.
[edit] Function of ADF
The ADF can be used to plot your position, track inbound and outbound, and intercept a bearing. These procedures are used to execute holding patterns and nonprecision instrument approaches.
[edit] Orientation
The ADF needle points TO the station, regardless of aircraft heading or position. The RB indicated is thus the angular relationship between the aircraft heading and the station, measured clockwise from the nose of the aircraft. Think of the nose/tail and left/right needle indications, visualizing the ADF dial in terms of the longitudinal axis of the aircraft. When the needle points to 0°, the nose of the aircraft points directly to the station; with the pointer on 210°, the station is 30° to the left of the tail; with the pointer on 090°, the station is off the right wingtip. The RB does not by itself indicate aircraft position. The RB must be related to aircraft heading in order to determine direction to or from the station.
[edit] Station Passage
When you are near the station, slight deviations from the desired track result in large deflections of the needle. Therefore, it is important to establish the correct drift correction angle as soon as possible. Make small heading corrections (not over 5°) as soon as the needle shows a deviation from course, until it begins to rotate steadily toward a wingtip position or shows erratic left/right oscillations. You are abeam a station when the needle points to the 90° or 270° position. Hold your last corrected heading constant, and time station passage when the needle shows either wingtip position or settles at or near the 180° position. The time interval from the first indications of station proximity to positive station passage varies with altitude—a few seconds at low levels to 3 minutes at high altitude.
[edit] Homing
The ADF may be used to “home” in on a station. Homing is flying the aircraft on any heading required to keep the needle pointing directly to the 0° RB position. To home into a station, tune the station, identify the Morse code signal, then turn the aircraft to bring the ADF azimuth needle to the 0° RB position. Turns should be made using the heading indicator. When the turn is complete, check the ADF needle and make small corrections as necessary.
