NDB & ADF Navigation
Non-directional beacons and the Automatic Direction Finder — bearing terminology, homing, tracking, and the limitations that pilots must understand before using NDB approaches.
NDB
Non-Directional Beacon
A ground-based LF/MF radio transmitter (190–1750 kHz) that broadcasts an omnidirectional signal. Aircraft equipped with an ADF (Automatic Direction Finder) use NDBs for bearing and homing navigation. NDBs are identified by a 2–3 letter Morse code ident and are progressively being replaced by GPS/GNSS.
NDB Fundamentals
A Non-Directional Beacon (NDB) is a ground-based radio transmitter operating in the frequency band 190–1750 kHz (the AM broadcast band). Unlike VOR, it transmits an omnidirectional signal with no phase or directional information of its own.
- The aircraft's ADF (Automatic Direction Finder) receiver detects the direction of the incoming signal using a loop antenna
- The ADF needle always points toward the NDB — it indicates the relative bearing from the aircraft's nose to the station
- NDBs are identified by a 2 or 3 letter Morse code ident; always verify ident before use
- Some NDBs also broadcast voice weather information or are combined with marker beacons
- Frequencies below 200 kHz are susceptible to the most interference; avoid or apply extra caution
ADF vs RMI: A basic ADF shows a fixed compass card with a needle indicating relative bearing from the nose. A Radio Magnetic Indicator (RMI) combines heading information from the gyro compass — the needle indicates the magnetic bearing to the station directly, eliminating manual calculation.
Bearing Terminology
NDB navigation uses several related bearing concepts. Confusing them is a common exam and flight error. Study the table carefully.
| Term | Abbreviation | Definition | Reference |
|---|---|---|---|
| Relative Bearing | RB | Angle from aircraft nose to NDB, measured clockwise 0–360° | Aircraft nose (not magnetic north) |
| QDM | QDM | Magnetic bearing TO the station — the heading to steer to reach the NDB (nil wind) | Magnetic North |
| QDR | QDR | Magnetic bearing FROM the station — equivalent to a VOR radial | Magnetic North |
| QTE | QTE | True bearing FROM the station to the aircraft | True North |
| QUJ | QUJ | True bearing TO the station from the aircraft | True North |
Calculating QDM from ADF Indication
The fundamental formula for converting an ADF relative bearing to a magnetic bearing to the station (QDM):
(If result > 360°, subtract 360°)
Worked Example 1
- Aircraft magnetic heading: 090°
- ADF needle (relative bearing): 045°
- QDM = 090 + 045 = 135°M
→ Fly heading 135°M (nil wind) to reach the NDB
Worked Example 2
- Aircraft magnetic heading: 310°
- ADF needle (relative bearing): 080°
- QDM = 310 + 080 = 390 → 390 − 360 = 030°M
→ Fly heading 030°M to reach the NDB
Deriving QDR (bearing FROM station)
- QDR = QDM ± 180° (the reciprocal)
- From example 1: QDR = 135 + 180 = 315°M
→ Aircraft is on the 315° radial FROM the NDB (QDR 315)
Homing
Homing is the simplest NDB technique: continuously turn to keep the ADF needle pointing to 0° (dead ahead). The aircraft will always be heading directly toward the beacon.
- In still air, homing produces a straight track to the station
- With a crosswind, homing produces a curved ground track — the aircraft continually weathercocks into the wind but still drifts downwind
- Eventually reaches the station, but the curved track consumes more distance than a straight track
- Used when rough navigation is acceptable or as a quick last-resort technique
Homing is not tracking. Holding the ADF needle at 0° does not guarantee a straight-line track over the ground when wind is present. For IFR approaches or precise navigation, use the tracking technique with a calculated wind correction angle.
Tracking
Tracking maintains a precise straight ground track to or from an NDB by applying a wind correction angle (WCA). The procedure differs for inbound and outbound tracks.
Inbound Tracking (Homing to QDM)
- Identify the required QDM (track to station)
- Turn to intercept the track
- Observe ADF needle drift — if needle moves left of nose, wind is from the left
- Apply a WCA into wind (e.g. turn 10° left if wind is from left)
- The ADF needle will be offset from 0° by an amount equal to the WCA when on track
- Correct as needed — "double and halve" technique for regaining track
Outbound Tracking (Flying QDR)
Outbound tracking from an NDB is more complex as the ADF needle points behind the aircraft (toward 180° when directly on track and flying outbound).
- Set heading = desired outbound track (QDR heading)
- On track, ADF needle should point to 180° (tail of aircraft toward station)
- If needle drifts from 180°, drift is occurring — apply correction
- WCA applied in opposite sense to inbound (wind from right → turn right when outbound)
NDB Limitations
NDB is the least accurate of the conventional navaids. Pilots must be aware of the following error sources:
| Limitation | Cause | Effect on ADF |
|---|---|---|
| Static interference | Lightning, precipitation static, nearby transmitters | Random needle swings; may point to lightning strike rather than NDB |
| Coastal refraction | Radio waves bend when crossing coastline at oblique angles | Bearing errors; worst when crossing coast at angles less than 30° |
| Night effect | Ionospheric skywave returns at night on LF/MF frequencies | Multiple signals cause needle flutter and bearing errors; worst at dawn/dusk |
| Station passage | Aircraft directly overhead the NDB | Needle swings erratically — can flip 180°; disregard briefly until past station |
| Mountain effect | Terrain reflection and diffraction of radio waves | Erratic needle; treat all NDB fixes in mountainous areas with extra caution |
| Quadrantal error | Airframe and antenna interaction at certain relative bearings | Systematic error at 45°, 135°, 225°, 315° relative bearings; corrected by deviation card |
Accuracy comparison: NDB/ADF in ideal conditions achieves approximately ±5° accuracy. VOR achieves ±2–3° and GPS/GNSS achieves well under 1°. NDB approaches therefore have higher minima (MDA) than equivalent ILS or RNAV approaches. NDBs are being progressively decommissioned worldwide in favour of GPS/GNSS navigation.