Automatic Direction Finder (ADF): Definition, Function, Use, Test, History

An Automatic Direction Finder (ADF) is an aircraft radio-navigation instrument that continuously displays the relative bearing from the airplane to a non-directional radio beacon transmitting in the MF or LF bandwidth. Because it replaced the manual rotation of a loop antenna required by older DF sets, the system is termed automatic. The ADF thus serves as an electronic aid to navigation by pointing to the station and indicating the aircraft's directional heading toward it.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is an ADF in aviation?

An automatic direction finder (ADF) is an aircraft navigation product which automatically calculates the relative bearing of the aircraft to the radio station. An ADF identifies the relative bearing of an aircraft from a radio beacon transmitting in the MF or LF bandwidth. The onboard ADF Receiver detects and processes signals from a Non Directional Beacon, then displays the relative bearing on a compass so the flight crew can see at a glance the direction to the station. The tail of the bearing pointer indicates the bearing of the aircraft from the beacon, and the system continuously calculates relative bearing without further input from the pilot. By adding this relative bearing to the aircraft's magnetic heading, the crew obtains the magnetic bearing to the NDB. Adding or subtracting 180 yields the magnetic bearing from the station, allowing direct tracking or homing in still air.

What are the components of an Automatic Direction Finder?

The automatic direction finder is built from three principal units: two antennas, the ADF receiver, and the ADF instrument. The antenna system itself is made of a loop antenna and a sense antenna. The loop antenna can be rotated physically or electronically to sense the direction of the station while the sense antenna is omnidirectional and eliminates the 180 ambiguity that the loop alone gives. In modern aircraft these two antennas are separate units or combined into one compact loop/sense assembly.

The loop aerial is normally mounted in the vertical stabilizer or in a small fairing on the upper fuselage, while the sense antenna is placed on the bottom of the aircraft, sometimes appearing as a couple of rods jutting out from each side of the vertical stabilizer at a 45-degree angle in the so-called ‘cat-whisker’ layout.

Inside the flight deck, the ADF control panel brings the crew to the system: a frequency-select knob sets the desired NDB channel in 0.5, 10 or 100 kHz steps. A function switch offers the positions OFF-ANT-ADF, and a beat-frequency-oscillator switch marked TONE lets the operator enable the audible Morse identifier.

The ADF receiver, located in the avionics bay, detects, amplifies and processes the incoming AM signal from the NDB and powers the bearing indicator. That indicator is a relative-bearing instrument or a radio-magnetic indicator (RMI) whose needle and compass card show the bearing of the station relative to the aircraft heading.

What are the types of ADF in aviation?

There are two main types of ADF indicators in use: the fixed-card ADF and the moveable-card ADF. A fixed-card ADF is the simplest type, with a fixed azimuth dial on which 0° always represents the aircraft nose. A movable-card ADF is an upgrade to the fixed-card ADF. Its dial can be rotated manually so the pilot can align the azimuth with the aircraft heading.

The two advanced ADF cockpit displays are the Relative Bearing Indicator (RBI) and the Radio Magnetic Indicator (RMI). An RBI is a single-needle instrument that shows only the relative bearing to the Non-Directional Beacon. The RMI is an alternate ADF display providing more information than a standard ADF: it is normally a single-needle Radio Magnetic Indicator, but dual-needle versions exist. The needle can be selected to show information from VOR equipment as well as from the ADF.

How does an ADF work in aviation?

The ADF system operates with an ARINC 712 antenna like the Collins DFA-901, whose loop antenna produces two possible directions 180 degrees apart, while the sense antenna eliminates this ambiguity. Inside the ADF-900, digital technology enhances performance and increases reliability by comparing phases of the signals from the two antennas, then steering a servo motor that positions the bearing pointers. The pilot tunes in a station's frequency, most often a radio beacon like a Non-Directional Beacon that transmits signals in all directions. The NDB identifies by Morse code identifier broadcast on the carrier wave so the pilot can confirm the correct station.

To make the identifier audible, the pilot depresses the BFO button. The BFO produces a signal slightly removed from the received frequency then mixes with the received signal. This mixing produces an audible beat frequency equal to the difference of the two frequencies, so BFO permits the Morse code identifier to be heard even when there is no modulation. The same technique makes the unmodulated parts of the A1A signal audible for on-off keyed signals.

ADF needles can be rotated around the card, or the compass card can be rotated to airplane heading. In ANT mode the bearing pointer is parked at 90 degrees relative bearing so that loop directionality is cancelled while the sense antenna continues to receive the carrier for audio monitoring. Regardless of mode, the needle points to the beacon and the RMI indicates the bearing of the station relative to the aircraft. Wind corrections are made similar to tracking to the station so that the symbolic airplane on the indicator as well as the actual aircraft are kept on course toward the beacon.

An Automatic Direction Finder works by locking its needle toward the base station. Once I set the radio to the non-directional beacon's frequency, the needle gives continual direction data irrespective of our direction. Radio guidance assistance came without need for particular aircraft attitude.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What is the frequency range of an ADF in aviation?

The frequency range of an ADF is between 190 kHz and 1750 kHz, covering both the low and medium frequency bands. Within this range, aviation NDBs typically transmit between 200 kHz and 415 kHz, while marine and other NDBs are located between 190 kHz and 535 kHz in the long-wave band. Most receivers are also able to tune the standard AM broadcast stations from 540 kHz to 1620 kHz, but these signals are not relied upon for navigation because they lack the required two- or three-letter Morse identifier and are not aligned with the published beacon coordinates.

What is the difference between ADF and VOR in aviation?

An ADF uses a low- or medium-frequency non directional beacon and the ground station transmits a single omnidirectional signal and the aircraft's receiver measures the bearing to that NDB. VOR is a short-range radio navigation system: it stands for very high frequency (VHF) omni-directional range and is an aviation term that broadcasts 360 different signals, one for each degree of azimuth. Because the ground station emits a unique signal for every bearing, the cockpit indicator can show the magnetic bearing to the station regardless of aircraft heading. Measuring the phase difference in VOR is more accurate and less affected by environmental factors. The accuracy of course alignment of the VOR is generally plus or minus 1 degree and the system is accurate to within one degree, whereas NDB signals are distorted by terrain and weather. Pilots can positively identify a VOR by its Morse code identification or by the recorded automatic voice identification whereas NDBs are identified only by their Morse code. Although ADF and VOR can be co-located, the two systems are fundamentally different: NDB provides relative bearing and needs the aircraft's magnetic heading to calculate the required track, while VOR provides magnetic bearing directly and gives azimuth information without reference to aircraft heading.

How to use an ADF in aviation?

To use an ADF in aviation the pilot tunes in the station frequency, identifies the station, sets the compass card to the heading currently flown, and notes whether the beacon is left or right of the nose. The needle then points to the bearing the airplane is on to or from the Non-Directional Beacon. If the needle is at 9 o'clock the station is off the left wing. Adding the relative bearing to the airplane's magnetic heading gives the magnetic bearing to the NDB, and the pilot can determine bearings and home on the station. To intercept a desired inbound bearing the pilot turns to parallel that bearing, then turns toward the desired magnetic bearing the number of degrees determined for the intercept angle. The standard intercept angle equals two times the drift angle. Maintain the interception heading until needle deflection equals the intercept angle minus an appropriate lead, as track is intercepted and turned back in the direction of the needle until the relative bearing equals the estimated wind correction angle. For tracking inbound or outbound the same substitution rule applies: substitute 180 degree needle position for zero position, and wind corrections are made similar to tracking to the station. If the needle deflects off-course, re-intercept the track and turn to a new relative bearing equal to the new estimated wind correction angle. These procedures are used to execute holding patterns and non-precision instrument approaches.

How to read ADF in aviation?

To read an ADF, begin with the first rule: the pointer end always points to the station. Watch the compass card; it is a directional gyro, so it rotates automatically as the aircraft turns and keeps north at the top of the dial. At the instant the needle stabilizes, read the exact number beneath it and that degree value is the relative bearing.

Relative bearing alone is not a map track and must be combined with magnetic heading. Magnetic heading is the direction your nose is pointed, taken from the heading you are currently flying. Add the RB to the MH and the sum is the un-corrected magnetic bearing to the beacon. When the aircraft is heading 345 and the RB read from the ADF dial is 45, the calculation is 345 + 45 = 390. Subtract 360 and the magnetic bearing to the station is 030. If the result exceeds 360, simply deduct 360. If the sum is the reciprocal you need, apply +/- 180 for FROM. The reverse operation works similarly. Given magnetic bearing to beacon 060 and aircraft heading 345, the required RB is 060 - 345 = 075. The needle will settle at 3 o'clock, showing the station off the right wing. Whether the indication is a 45 degree needle or the station off left wing with needle at 9 o'clock, the same two numbers - RB and MH - deliver the exact bearing to fly to the beacon in still air.

How to test ADF in aviation?

To test ADF in aviation, press the TEST button and watch the ADF needle, while maintaining heading. Note when needle deflection appears, then adopt a standard intercept angle equal to twice the drift angle, verifying receiver accuracy.

I set the ADF radio to a recognized non-directional beacon frequency identified on the sectional map. I chose a secure elevation aside from governed airspace. I observed the direction pointer; it pointed toward the NDB without delay or undulation. I analyzed this indicated direction to the magnetic heading. This in-flight trial afforded assurance that the whole arrangement was functioning as intended.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the limitations of an ADF?

The limitations of an ADF are outlined below.

• Automated direction finders are only reliable in relative close proximity to NDBs.
• ADFs rely on line of sight communication; obstacles like mountains or large structures can obstruct signals, leading to erroneous readings.
• ADF lacks a flag to warn pilots when erroneous bearing is displayed.
• ADFs provide directional guidance but do not inherently offer precise distance measurements.
• The night effect limits an ADF’s useful range for navigation independently of the NDB transmission power.
• Signal refraction near shorelines can affect ADF accuracy.
• Radio beacons are subject to disturbances that may result in erroneous bearing information in an ADF.
• Radio beacon identification may be noisy when the ADF needle is erratic.
• If an electrical storm is nearby, the ADF needle points to the source of lightning rather than to the selected station.
• Man-made interferences like nearby electronic devices or even other aircraft's transmissions may distort or mask genuine signals for an ADF.
• When the aircraft is banked the ADF needle reading will be offset; to keep the needle on zero the aircraft must be turned slightly resulting in a curved flight path to the station.

ADF’s provide directional guidance but do not inherently offer precise distance measurements, so the pilot must infer range from time and groundspeed. Signal integrity is fragile. Shorelines refract low frequency radio waves as they pass from land to water, giving shoreline error that bends the apparent bearing, especially if the aircraft track is close to parallel to the shore. Bank error and loop alignment error add further cockpit level inaccuracies. Although airborne equipment is capable of accuracies in the region of +2, the absence of a flag or alert means that every limitation must be anticipated and corrected by pilot technique: fly higher to reduce terrain and night effect problems, avoid NDBs on the coast to minimize coastal reflection, and never use a beacon outside the published service volume.

What are ADF errors in aviation?

What ADF errors in aviation are is explained below.

• When an aircraft is in a turn, the loop antenna position is compromised, causing the ADF instrument to be off balance leading to a bank error
• Loop alignment error can result if the ADF system is not aligned with the longitudinal axis
• In areas of high electrical activity, like a thunderstorm, the ADF needle will deflect toward the source of electrical activity, causing erroneous readings
• Dip error is introduced when the aircraft is in a banked attitude as the ADF needle dips down in the direction of the turn
• A quadrantal error is calibrated in most aircraft

ADF bearings suffer considerable errors, so pilots must know why the needle may show errors. Both bank and dip effects are normally compensated on installation, yet residual deflection still appears during prolonged or steep manoeuvres. Static noise is caused by nearby thunderstorms, and reflections arrive at the antenna and mix with direct waves, giving a steady false bearing. When that happens, voice, music, or erroneous identification will usually be heard, even though the receiver is not tuned to the station. Night effect occurs if outside 70 Nm, further increasing spread and scatter of the combined accuracy of the system, which is already limited to about 5 degrees. Therefore, the pilot must continuously monitor the NDB identification and cross-check position by other means.

What is the history of the Automatic Direction Finder?The Automatic Direction Finder (ADF) traces its roots back to the late 1920's. Ships began experimenting with RDF in 1903, and Germany's Telefunken Kompass Sender came into service around 1912. Engineer Fredrich Kolster invented the Radio Compass, and the US Navy demonstrated its use for navigation on July 6, 1920, using a Radio Compass equipped Curtis F 5 L flying boat. That same year a US Navy seaplane located a battleship 100 miles (160.93 kilometers) offshore with RDF. By the start of World War II the automatic radio compass was developed that had a sense ability to resolve the 180 ambiguity, and Aircraft Radio Corporation built early models.

The receiver evolved throughout the thirties. Vibrating reeds were being replaced with more modern systems of navigation by the mid thirties, and the last contracts are from 1941 for the RU 19. Affordable NDB navigation had to wait for most small aircraft until the 1960's when transistorization brought the compact ‘set and forget’ Automatic Direction Finder. Since then, the airborne equipment has progressed, yet the early system of airborne direction finding from Non Directional Beacons (NDBs) has remained almost unchanged, and ADF is retained for training purposes.