Non-Directional Beacon and ADF

A non-directional beacon (NDB) is the simplest radio navigation aid still in service: a ground transmitter in the low- and medium-frequency (LF/MF) bands that radiates the same signal in every direction. The beacon itself carries no directional information. The work is done in the aircraft, where the automatic direction finder (ADF) measures the direction from which the signal arrives and drives a needle to point at the station. Together, NDB and ADF have long served en-route navigation, holding and non-precision approaches, and they remain in the syllabus of every licence from the PPL to the ATPL.

LF/MF signals follow the curvature of the Earth, so an NDB can be received beyond line of sight and at low level, where a VOR cannot. The same propagation leaves the bearing exposed to the ionosphere at night, to coastlines, to thunderstorms and to the airframe itself, and the ADF gives no warning when its needle is wrong. Many states are withdrawing beacons, but NDB approaches, locators and holds over beacons are still published, and relative bearing problems remain a staple of EASA and FAA exams.

NDBs and locator beacons

ICAO Annex 10 allocates NDBs the band 190 to 1750 kHz. In the United States they normally operate between 190 and 535 kHz, and the FAA's AIM describes a continuous carrier modulated at 400 or 1020 Hz for identification. Every beacon except a compass locator transmits a continuous three-letter Morse identifier, interrupted only by voice transmissions; an FAA class designator containing W, such as HW, marks a beacon without voice.

The signal is vertically polarised and travels mainly as a surface wave (ground wave), which follows the Earth's curvature. Range therefore depends on transmitter power and on the surface beneath. A rule of thumb used in EASA training gives about 3√P NM over the sea and 2√P NM over land, P being the power in watts: a 1 kW beacon reaches roughly 95 NM over the sea and 63 NM over land, and quadrupling the power only doubles the range.

ICAO states publish a beacon's range as its designated operational coverage (DOC), within which a bearing accuracy of about ±5° can be expected by day. The DOC applies by day only, because of night effect. The FAA instead classifies beacons by standard service volume, a radius that is the same at all altitudes:

A locator beacon (L) is a low-power NDB serving an aerodrome's approach procedures, typically usable to 10 to 25 NM. Sited at an ILS marker, it is what the FAA calls a compass locator: under 25 W, with a range of at least 15 miles, although some outer locators use up to 400 W. At the outer marker it forms a locator outer marker (LOM), which transmits the first two letters of the localiser identifier; a middle locator sends the last two (see instrument landing system).

How the ADF finds direction

Loop and sense antennas

A loop antenna, or loop aerial, is a coil whose output depends on its orientation to the incoming wave. When the plane of the loop is in line with the station, its two vertical sides receive the wave at slightly different moments and the difference produces a maximum signal. When the loop is broadside to the station, both sides receive the wave in phase and the output falls to a sharp null. The loop's polar diagram is therefore a figure of eight with two nulls 180° apart: a precise direction, but no way of telling on which side the beacon lies.

The sense antenna removes that ambiguity. It is a simple vertical aerial, or its electrical equivalent, with a circular, omnidirectional polar diagram. Added to the loop's output in the correct phase and amplitude, it reinforces one lobe of the figure of eight and cancels the other, producing a heart-shaped cardioid polar diagram with a single null. The ADF uses the cardioid to decide which side the beacon is on and the loop's sharp null to measure the bearing precisely.

BFO and NDB identification

Emission designators describe what the beacon sends. N0N is an unmodulated carrier; A1A means that carrier is keyed on and off in Morse; A2A means a keyed audio tone amplitude-modulates the carrier. Textbooks and charts usually write them as NON A1A and NON A2A.

An unmodulated carrier produces no sound in an AM receiver. The beat frequency oscillator (BFO) generates a signal offset from the incoming carrier by an audio frequency, and mixing the two produces an audible beat note. For a NON A1A beacon the BFO must be selected for tuning, identification and monitoring, because the identifier exists only as interruptions of the carrier. For NON A2A the tone is audible without it, so the BFO is used, if at all, to find the carrier while tuning and is switched off to identify.

The identifier is the ADF's only integrity check, because an ADF has no warning flag: tuned to the wrong beacon or a broadcast station, or out of range, the needle still settles convincingly. The AIM notes that most disturbances affecting the bearing also affect the identification: a noisy identifier usually accompanies an erratic needle, and voice, music or a wrong identifier may be heard while a steady false bearing is displayed. The identifier is therefore monitored for as long as the beacon is in use.

Relative bearing and ADF indicators

Relative bearing is the angle measured clockwise from the aircraft's nose to the beacon. It is all the ADF measures. It is a radio bearing, one derived from a radio signal, and because radio waves travel along great circles, a bearing plotted over a long distance on a Mercator chart needs a conversion angle correction. Adding heading turns it into a bearing to the station:

Magnetic heading + relative bearing = magnetic bearing to the station (QDM), subtracting 360° if the sum exceeds 360°.

Adding true heading instead gives the true bearing to the station. Because heading is measured at the aircraft, conversion between true and magnetic uses the variation at the aircraft; a VOR radial, by contrast, is referenced to the variation at the station.

Three indicators present the bearing:

• The fixed-card ADF, or relative bearing indicator (RBI), has zero permanently at the top. Its needle shows relative bearing only, and the pilot adds heading.
• The movable-card ADF has a card the pilot turns by hand to the current heading. The needle head then reads QDM and the tail QDR, but only until the heading changes, when the card must be reset.
• The radio magnetic indicator (RMI) has a card slaved to the compass system, so it turns automatically. The head reads QDM and the tail QDR at all times, and relative bearing is the angle between the lubber line and the needle. The bearing pointers on an HSI or navigation display work the same way.

Keeping the needle on the nose is homing: in a crosswind the aircraft follows a curved path downwind of the direct track and reaches the beacon by a longer route. Tracking holds a wind correction angle instead: with the correct heading inbound, the needle sits off the nose by that angle, on the downwind side, and stays there. With starboard drift the heading is track minus drift; with port drift, track plus drift. Close to the beacon the needle becomes sensitive, and at station passage it swings towards the tail; directly overhead lies a cone of silence where the needle is unreliable.

Night effect and coastal refraction

ADF night effect comes from the ionosphere. By day the D layer, about 75 km up, absorbs LF and MF energy, so only the surface wave reaches the aircraft. At night the D layer disappears and sky waves returned by the E layer come back to Earth, arriving with random phase and polarisation. Mixed with the surface wave, they make the signal fade and the needle wander. The effect is worst around dusk and dawn, while the ionosphere is changing, and at long range, where the surface wave is weak compared with the sky wave. Night bearings should come from beacons well within range, averaged and cross-checked.

Coastal refraction, called shoreline effect in FAA material, affects the surface wave where it crosses a coastline. The wave travels slightly faster over water than over land, so on passing from land to sea it bends away from the normal, towards the line of the coast. An aircraft over the sea measures the direction of the refracted wave, and the position line plotted from an inland beacon lies closer to the coast than the aircraft really is. The error is nil when the signal crosses the coast at 90° and grows as the crossing becomes more oblique. It is minimised by using beacons on or close to the coast, and bearings that cross the coastline at or near a right angle.

Quadrantal error, dip error and interference

Quadrantal error (ADF) comes from the airframe. The fuselage and wings pick up and re-radiate the incoming signal, and the loop receives the sum of the direct and re-radiated fields, which distorts the indicated bearing. The error is nil when the beacon lies on a cardinal relative bearing (000, 090, 180 or 270) and greatest near the quadrantal ones (045, 135, 225 and 315).

ADF dip error, or bank error, appears in turns. The aerial system senses the vertically polarised wave correctly only while the aircraft is level; banking tilts it, and the bearing error grows with bank angle. ADF bearings are read with the wings level.

The remaining errors come from outside the aircraft:

View full schematic

NDB procedures today

NDBs define en-route reporting points, holding fixes and non-precision approaches (see holding patterns and instrument approach procedures). An NDB approach gives lateral guidance only and ends at a minimum descent altitude, and its protected areas reflect the aid's imprecision.

In the United States, AIM paragraph 1-2-3 lets a suitable RNAV system, including IFR-approved GPS, substitute for an ADF, or for an NDB or compass locator that is out of service: to determine position relative to the beacon, navigate to or from it, and hold over it, even where a chart notes "ADF required". It may not replace the NDB that provides lateral guidance on the final approach segment, unless the title includes "or GPS"; an RNAV system may fly that final segment only while the NDB is operating and monitored for course alignment.

In Europe, the performance-based navigation rule, Commission Implementing Regulation (EU) 2018/1048, allows conventional procedures to be kept only as a contingency from 6 June 2030, and many beacons are already being withdrawn; the AIP and NOTAMs say which remain. GNSS and DME now provide most of the position fixing that NDBs once did.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.