Navigation Receivers — ADF & DME
Two legacy nav aids still in use, especially internationally and at older airports. ADF (Automatic Direction Finder) receives low-frequency NDB signals and shows relative bearing. DME (Distance Measuring Equipment) interrogates a ground transponder and computes slant range. Both have been largely superseded by GPS in modern operations, but the IFR knowledge test still expects you to know how they work.
ADF — Automatic Direction Finder
- Receives signals from low-frequency NDB (Non-Directional Beacon) transmitters. Largely legacy in the US, but still used internationally and at some airports as a backup.
- Indicator shows relative bearing — the angle between aircraft heading and the station.
- Magnetic Bearing (MB) = Magnetic Heading (MH) + Relative Bearing (RB). If MB exceeds 360°, subtract 360°.
- RMI (Radio Magnetic Indicator) combines the ADF with a heading indicator — needle always points to the station regardless of aircraft heading. Easier to interpret than a fixed-card ADF.
Practical use: NDB approaches are rare in modern US operations but exist. The procedure is fly-through-and-track on the published bearing. Helicopters are often equipped with ADF for international operations or for tracking AM broadcast stations as backup nav.
NDB station identification
NDB stations broadcast continuously. Identify them by listening to the audio — Morse code identifier matching the chart. If you don't hear the identifier, don't trust the needle indication.
NDB signals are vulnerable to several interference sources:
- Thunderstorm interference — lightning produces strong LF radio noise. Needle can swing toward the storm.
- Coastal effect — over water near a shoreline, signals refract along the coastline. ADF accuracy degrades.
- Twilight effect — at dawn and dusk, the ionosphere shifts, and LF signal propagation changes. ADF reception can be erratic.
- Mountain effect — terrain blocks or reflects LF signals.
DME — Distance Measuring Equipment
- Transponder-based — aircraft transmits an interrogation on 1030 MHz; ground station replies on 1090 MHz; aircraft equipment computes slant range from the round-trip time.
- UHF band (960-1,211 MHz). Range up to 199 NM.
- Slant range error: directly over the station, DME displays altitude above station (not zero). Slant range error becomes significant at low altitudes near the station.
- Many airports have collocated VOR/DME or VORTAC stations — same identifier, both in same location.
- GPS may substitute for DME per AIM 1-2-3 with a current database.
Slant range error in practice
DME measures the line-of-sight distance from aircraft to station, not the ground distance. The difference matters when you're high over a station.
Example: at 6,000 ft AGL directly above a DME station, the DME displays 1.0 NM (the slant range from your altitude to the station, since 6,000 ft ≈ 1 NM).
Practically: when an approach is "DME 5 from the FAF," the 5 NM includes some altitude. At pattern altitude near a station, the slant error is small (a fraction of a mile). At high cruise altitudes near a station, it can be a mile or two off ground distance.
Where these still matter
- International operations — many countries still rely heavily on NDB and DME approaches. Your US-trained GPS-only mindset doesn't transfer.
- Backup navigation — when GPS is jammed, spoofed, or unreliable, ground-based nav aids continue to work.
- Specific approaches — some US airports retain NDB or VOR/DME approaches that require these receivers.
- FAA knowledge test — the IFR written test still asks about ADF and DME mechanics. Don't skip the chapter.