NDB and ADF
The NDB (Non-Directional Beacon) is the oldest still-active radio navigation system in aviation (since the 1920s). The aircraft receiver is the ADF (Automatic Direction Finder).
Source: ICAO Annex 10 Volume I, Chapter 3.4 NDB.
Operating principle
- NDB ground station: transmits an omnidirectional signal on a frequency. NDBs are used for en-route navigation, approach procedures, reporting point marking.
- ADF on board: antenna determines the direction of arrival → a needle in the cockpit points to the station.
ADF — the radio compass
The radio compass (ADF) is the receiver for the electromagnetic waves transmitted by NDB stations. Further:
- NDBs operate in the LF and MF range, the radio compass (ADF) is the receiver.
- The radio compass (ADF) can receive low and medium frequency transmitters.
Frequency band
- ADF reception range: typically 190 kHz to 1750 kHz — covers LF and MF.
- NDBs in Germany: operate in the frequency band 200 kHz to 526.5 kHz.
- Some onboard ADFs can also use commercial AM broadcast stations (540–1700 kHz) for bearings.
Range and NDB classes
An NDB generally has a range of 25 NM to 100 NM. By class:
| Class | Range | Use |
|---|---|---|
| Approach NDB (locator) | 25-50 NM | Approach aid at airfields |
| En-route NDB | 100-150 NM | En-route navigation in remote areas |
| High-power NDB | up to 200 NM | Rare, special applications |
→ En-route NDB (range 100 NM to 150 NM) have a longer range compared to approach NDB.
Indications — terminology
| Term | Symbol | Definition |
|---|---|---|
| Relative Bearing | RB | The angle between the aircraft longitudinal axis and the needle tip of the radio compass (fixed scale) towards the NDB. The line of reference for the "RB" indication of the ADF is the heading line. |
| Magnetic Bearing TO | MB | Magnetic bearing — the angle between MN and the direction towards the NDB |
| Magnetic Bearing FROM | MBF | Magnetic direction from the station |
| MH | — | Magnetic Heading |
| True Bearing (TB) | TB | True bearing — TB = TH + RB |
Formula: MB = MH + RB (mod 360°)
RMI (Radio Magnetic Indicator) — combined instrument
A combined indicator composed of slaved gyro and radio compass is called RMI:
- The compass rose rotates (driven by the slaved gyro, synced to the magnetic compass).
- The needle always points to the transmitter — reduces pilot workload in NDB tracking.
- On the RMI, the QDR can directly be read off at the tail of the needle.
- QTE cannot directly be read from the RMI — RMI shows only magnetic bearings, not true bearings.
Identifier — BFO / CW mode
To listen to the identifier of A0/A1-modulated NDB stations, the ADF mode selector must be set to BFO or CW. Reason:
- A0/A1-modulated NDBs transmit the identifier as pure carrier keying (Morse without audio tone).
- BFO (Beat Frequency Oscillator) or CW in the ADF receiver generates a beat tone from the unmodulated carrier → pilot hears the Morse.
- A2/A3 NDBs need no BFO/CW (already carry an audio tone).
Sense antenna and mode selector
The ADF mode selector has several positions:
| Mode | Function |
|---|---|
| ADF (or COMP) | Normal bearing mode — needle points to station |
| REC or ANT | If set to REC or ANT, reception occurs via the sense antenna and direction finding is not possible — only identifier listening |
| BFO / CW | Identifier mode for A0/A1 NDBs (see above) |
| TEST | Using the TEST button installed on many ADF devices, the operation of the radio station is checked — typically the needle deflects ~90°; on release it returns to the correct position |
Accuracy and error sources
Due to possible ADF errors (receiver error, ground error, quadrant error), an error of up to ±5° per bearing must be expected.
Limitations / error sources — wave propagation
Twilight / night effect
- Day: signal as ground wave — stable.
- Night: additional skywave (ionospheric reflection) — overlays ground wave → inaccurate or fluctuating bearing, especially at sunrise and sunset.
- Consequence: An ADF can operate inaccurately during twilight.
Coast effect / shoreline effect
- NDB is the most affected by the so-called shoreline effect.
- At the water-land transition the signal refracts — up to ±10°.
- The ADF can deliver false bearings due to the coast effect.
Mountain effect
- Reflections from mountains produce false bearings.
Thunderstorm / lightning
- Lightning produces strong LF/MF emission → the ADF needle can point at the lightning instead of the NDB.
- The ADF delivers unusable indications in the vicinity of thunderstorms.
- Lightning, mountain and coast effect affect the indication of the radio compass.
Ionospheric interference at night
Interference of radio waves due to reflection at or in the ionosphere can be expected in the medium frequency band (NDB) at night.
Use
- Approach aids: some fields have NDB approaches (non-precision approach, NPA).
- En-route: still in use in remote areas (Africa, Pacific).
- Backup to VOR.
- NDBs are used for approach and en-route radio aids.
Phase-out
NDBs are being decommissioned worldwide:
- USA: most of the network already turned off (FAA AIM).
- Europe: many NDBs deactivated, EASA states increasingly replacing with GNSS approaches.
- Germany: only a few NDBs still active.
- Check current AIP / NOTAMs — many NDBs shown on charts are no longer operational.
Worked example
- Aircraft MH = 060°.
- ADF shows RB = 330° (needle 30° left of longitudinal axis).
- MB to station = MH + RB = 060° + 330° = 390° → mod 360° = 030°.
- → Station is magnetically NNE (030°) of the aircraft.