Radio compass

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ADF receiver and MDI

A radio compass , also ADF ( automatic direction finder ), RDF ( radio direction finder ), or radio compass is an on-board receiving system for radio navigation using radio direction finding to a non-directional radio beacon (NDB) or to other radio transmitters, which are also in the frequency range 190– Send 1750 kHz. The ADF is used for position determination in shipping and aviation, as a support for flight path control and is used for instrument approaches. It also provides additional information on other navigation equipment such as the VOR (rotary radio beacon) and the DME (range finder). A radio compass is also used in terrestrial , applied geophysics . It does not serve as a normal compass (display of the direction in which the vehicle is currently moving), but for aiming at a partial destination.


The radio waves from NDBs (Non-Directional Beacons, non-directional radio beacons), which transmit in the long and medium wave range, have a relatively large range. The received signals are evaluated in the ADF display device. The complete on-board system of the ADF consists of the antenna system with receiving antennas, the receiver with control panel and the separate display device.

Antenna system

To avoid ambiguities in direction, two different antenna types are required for the radio compass.

Various aircraft antennas and their installation
  1. A rotatable ferrite rod antenna ( loop , formerly a loop antenna , H -field antenna) picks up the radio waves emitted by an NDB. The magnetic field coming from the transmitter induces a voltage in it, the magnitude and phase of which depend on the direction from which the waves come. However, this results in a 180 ° ambiguity of the direction, because the transmitter can also be in the opposite direction.
  2. The side determination antenna ( sense , a long wire antenna , E- field antenna) has no preferred direction, it receives equally well from every direction. Together with the ferrite rod antenna, it eliminates the 180 ° ambiguity of magnetic bearing.

In aircraft, the ferrite rod antenna is attached in a flat casing under the fuselage. The rudder antenna, which used to be a long-wire antenna stretched from the fuselage to the tip of the rudder unit or ran on the ground, is now also housed in the casing.


In the early days of flying, a transmitter (NDB) was sighted by rotating a loop antenna with a vertical axis until the induced frame voltage has a minimum ( minimum bearing ). The antenna is rotated in such a way that the received signal arrives at the receiver as weakly as possible, since a minimum bearing can be much sharper than a maximum bearing. Accuracies of 3 to 5 degrees can be achieved without external interference. There remains a 180 ° ambiguity of the direction, because the transmitter can also be in the opposite direction.

Illustration of the ADF cardioids

To resolve this ambiguity, one uses the voltage that is supplied by the auxiliary antenna (E-field antenna) from the electrical component of the electromagnetic field. A clear direction of propagation of the radio waves can only be determined by superimposing the field sizes of both antennas with a 90 ° phase shift.

The adjacent picture shows the coil axis of loop antenna A in NS direction; Maximum voltage is induced when the transmitter is either in the east or in the west. The reception diagram has the shape of a horizontal 8.

The reception diagram of the wire antenna is a circle with no preferred direction. If you add both diagrams in the correct phase, the total voltage, the resultant, is especially large if the transmitter is in the east and especially small if it is in the west. The result is a heart curve ( cardioid ), which now has a clear minimum. A servomotor rotates the loop antenna so that the signal combined from both antennas reaches the minimum. The indicator needle of the radio compass is coupled to the servomotor so that its position corresponds to the detected minimum reception of the antenna system.

The evaluation described works optimally in the horizontal plane and fails if the antennas are above the NDB. That is the cause of the silence .

Receiver and control unit

Depending on the device manufacturer, the long and medium wave receivers with different control panels are (engl. Control panel ) provided. The frequency range extends from 190 to 1799 kHz in a frequency grid of 0.5 kHz. Therefore, you can also listen to radio stations in the long and medium wave range and use them for direction finding purposes. When using radio transmitters for navigational purposes, however, caution is advised, as these transmitters often work at different locations on the same frequency in so-called single- frequency operation . Stations that are operated with other stations in the same wave mode may not be used for direction finding, as no usable direction finding is possible, especially in areas of confusion where the signal arrives from several locations with similarly high field strengths.

During the Second World War , while enemy bomber groups were flying over Germany, the radio stations were switched to single-frequency operation so that they could not be used by the bombers as a radio navigation aid.

Display devices

The display device is housed in the cockpit separately from the control panel. Various display devices can be used with the ADF receiver. All show a bearing (engl. Bearing ) relative to the operating device set in the transmitter (NDB). The tip of the needle points in the direction of the transmitter, turning over a graduation rose .

There are three types of display devices:

RBI ( relative bearing indicator )

The RBI is the classic ADF display device. The 360 ​​° scale is not adjustable on the RBI, the zero degree mark is at the top in the direction of the aircraft's longitudinal axis. The needle of the RBI points to the radio beacon (NDB). The angle between the aircraft's longitudinal axis and the direction to the NDB is read off the scale. This angle is the side bearing ( relative bearing ).

With this relative bearing read off, the pilot knows his direction to the NDB in relation to his aircraft longitudinal axis. To get to the station, i.e. to get a QDM to the NDB, add the flown heading (MH, Magnetic Heading) and the relative bearing (RB) according to the following formula:

MH + RB = MB (= QDM)
Moving Dial Indicator (MDI)

More modern devices have a manually rotatable (MDI) and thus adjustable to the heading or an automatically controlled by a gyro compass (RMI) compass rose . The conversion is not necessary here and the QDM (magnetic heading to the transmitter) can be read directly.

MDI ( moving dial indicator )

The MDI is similar to the RBI, but the scale can be adjusted manually using the adjustment head (HDG or SET). You have to set the flown heading (MH) according to the gyro on the MDI, then you can read the QDM at the needle tip.

RMI ( radio magnetic indicator )

The most recently developed RMI offers a number of advantages over the other two devices. The RMI is a combined display device consisting of a radio compass and a magnetic compass. As with the MDI, the scale of the RMI (hereinafter referred to as the compass rose) is movable. However, it turns by itself with the help of a long-range compass , which is located in the tip of the aircraft wing. So it is a course top that you don't have to readjust in flight. A synchro serves as a receiver to drive the compass rose of the RMI . A synchro that acts as a transmitter is coupled to the compass gyro and electrically connected to the receiver.

Radio Magnetic Indicator (RMI)

In addition to the gyro function, the RMI has two pointers that can be assigned signals from other radio navigation receivers, for example either two VOR or one ADF and one VOR. The NAV-1 receiver is usually connected to the RMI. There are also devices that allow switching from NAV-1 to NAV-2.

There is also a synchro in the device to drive the pointers. Since the pointers sit on the axis of the compass rose, they are connected to the pointers with a 1: 1 gear.

The RMI provides the pilot with three pieces of information that make the device ideal for cross bearing :

  1. Compass rose: magnetic heading (MH), above below the course mark (red triangle in the adjacent figure)
  2. Pointer 1 (broad, yellow pointer in the picture): magnetic bearing (QDM) to the first set ground station, i.e. VOR or NDB
  3. Pointer 2 (in the picture narrow, green pointer): magnetic bearing (QDM) to the second set ground station, i.e. VOR or NDB


At the end of the needle that the QDM shows for the set VOR or NDB, you can read the radial or the QDR directly.

  • Reading the QDM

In your mind you project the needle of the ADF onto the scale of the course top. So you can read the QDM directly on the gyro.

Digital successors

Analog pointer instruments are no longer available in modern passenger aircraft. Instead, images are displayed on two computer monitors that correspond to the usual display instruments. The necessary signals are provided by a digital navigation computer .

Problems and glitches

It is worth mentioning the high susceptibility of this system to interference: During twilight and at night, the natural increase in range in the long and medium wave range means that transmitters of the same frequency can be received, which can interfere with reception and greatly falsify the display. Rain and especially strong thunderstorms can cause the direction needle no longer to point to the selected transmitter, but to the center of the thunderstorm. Even when the weather is nice, errors due to reflection and bending of the waves on mountains are possible.

Optimal reception performance can only be achieved by an antenna system that is technically correctly designed and correctly installed in the right place on the aircraft. Performance losses occur in the event of corrosion, bending, loose fastening (vibration), shading by components of the aircraft and mutual interference between the antennas.

The DF errors, which can occur due to atmospheric disturbances and path diversion above the ground, are not directly related to the function of the antenna system. However, the errors that arise from field distortions in the close vicinity of the DF antenna and those that are caused by changes in the path of the reference axis of the antenna system in different flight positions.

  • Quadrant error (Engl. Quadrantal error )

This is due to the deflection of the radio waves on the outer skin of the aircraft together with the result of the mixture of reflected waves with the newly arriving waves. This also includes the deflection that the radio waves experience from the aircraft's own magnetic field. Impulse interference caused by the ignition system, the generator, any servo motors and unshielded, interference-prone supply lines from inverters also have a negative impact on reception.

The size of the quadrant error depends on the direction of incidence of the radio waves and the intensity of the aircraft's own magnetic field. This error can be mechanically and / or electrically compensated for, as long as it is a fixed error source and error size. An existing quadrant error falsifies the bearings in the direction of the aircraft's longitudinal axis. The error is greatest when the radio wave comes from the quarter circles in relation to the longitudinal axis of the aircraft - i.e. with side bearings of 045 °, 135 °, 225 ° and 315 °.

  • Tilt error (Engl. Error dip )

It occurs in flight when there are deviations in position from the horizontal (climbing, descending, turning). If the aircraft only inclines the longitudinal axis ( roll ) or the transverse axis ( pitch ), the effect is the same as that of the quadrant error in the quarter circle directions.

By tilting the aircraft, e.g. B. in the curve, the loop antenna is moved from its MINIMUM position. By automatically turning the DF antenna into the current MINIMUM position, the display is incorrect. This effect is particularly strong in the vicinity of the transmitter. The error is reduced with greater distances to the NDB. So, similar to the magnetic compass, you can only take bearings in a horizontal straight flight at a distance from the NDB.

History and Applications

Advertisement for the Kolster radio compass, 1930

DF methods for determining direction using electromagnetic waves are among the oldest methods of radio location. Heinrich Hertz made the first attempts to determine the direction of an incident wave with dipoles and loops at the end of the 19th century . A pioneer in the use of the radio compass for seafaring was Frederick A. Kolster at the time of the First World War .


  • Peter Dogan: The Instrument Flight Training Manual. 1999, ISBN 0-916413-26-8 .
  • Jeppesen Sanderson: Private Pilot Study Guide. 2000, ISBN 0-88487-265-3 .
  • Jeppesen Sanderson: Private Pilot Manual. 2001, ISBN 0-88487-238-6 .
  • Wolfgang Kühr: The private pilot. Technology II, Volume 3 1981 ISBN 3-921270-09-X .
  • Luftfahrt-Bundesamt (LBA): ADF navigation. 1991.
  • Rod Machados: Instrument Pilot's Survival Manual. 1998, ISBN 0-9631229-0-8 .
  • Jürgen Mies: Radio navigation. 1999, ISBN 3-613-01648-6 .
  • US Department of Transportation, Federal Aviation Administration: Instrument Flying Handbook. AC61-27C, 1999.

Individual evidence

  1. Automatic Direction Finder ( Memento from October 23, 2006 in the Internet Archive )