VDF (VHF Direction Finding)

VDF — VHF Direction Finding and Homing

VDF (VHF Direction Finding) is a ground-based bearing facility that determines the direction of a VHF-transmitting aircraft and reports it to the pilot by radio. It is the simplest form of radio navigation and is available at many uncontrolled airfields.

Source: ICAO Annex 10 Vol I §3 Direction Finders; ICAO Doc 4444 PANS-ATM Ch. 8 DF Services.

Operating principle

• Ground station (VDF) has a directional antenna array and determines the direction of arrival of the pilot's transmission.
• Ground station transmits a bearing to the pilot by radio — typically the QDM (magnetic bearing to the station).
• The VDF (VHF Direction Finding) direction finder operates in the VHF range — typically on the airfield's COM frequency.

VDF in daily cockpit operation

There is no dedicated VDF equipment on board — the pilot:

1. Transmits a VHF signal on the ground-DF-station frequency.
2. Receives a bearing in return ("Your QDM is 270°").
3. Steers based on the bearing.

QDM only — INFO service

Aviation supervision office or aerodrome operations office on uncontrolled aerodromes (callsign INFO) equipped with VHF direction finding provide upon request QDM only:

• AFIS/INFO is typically authorised to transmit only QDM (magnetic bearing to the station).
• Not QTE, QDR or QUJ.
• Pilot receives e.g. "DEMRA, your QDM is 270°, request next QDM in 30 seconds".

VDF approach to an airfield

During a VDF approach to an airport, it has to be observed that the approach is always conducted towards the direction finder, which can also be located with an offset from the runway:

• The DF station is often not exactly on the runway centreline, but slightly offset.
• The pilot flies to the DF, not the threshold — visual correction needed near the field.

QDM behaviour during the approach

The QDM during a VDF approach (inbound) decreases — the pilot has to steer smaller:

• If the pilot steers heading 280° at QDM 280°, and the QDM falls to 275° at the next radio call, the pilot must reduce heading to 275° — correction compensates for wind drift.
• In general: closer to the station, sensitivity to wind drift rises → smaller heading corrections needed.

Homing

Homing describes the approach to a bearing station without accounting for wind — the pilot simply flies "into the needle":

• Pilot receives a bearing (e.g. QDM 270°) and steers exactly that heading.
• With wind the aircraft does not arrive on the direct line to the station, but on a curved track (lee drift).
• Use: simple, fast method under good conditions and short distance.

Constant bearing — shortest route

Flying a constant bearing leads the aircraft to the station on the shortest possible route:

• The pilot continuously adjusts heading so the bearing to the station stays constant (= wind correction).
• Result: the aircraft flies a straight line over ground to the station — minimum distance.
• In contrast to homing (no wind correction), which gives a curved track.

Comparison VDF with other procedures

DF accuracy classification (ICAO Doc 4444)

DF stations are graded:

• Class A: ±2° (very rare)
• Class B: ±5°
• Class C: ±10°
• Class D: > 10°

VDF at uncontrolled fields typically meets Class B or C.

Availability in Germany

• Many tower (TWR) and AFIS/INFO stations in Germany are VDF-equipped.
• VDF service is noted in AIP Germany ENR and on VAC charts for each field.
• Pilot can request VDF any time: "Munich Tower, DEMRA, request QDM".

Practical use

• Lost orientation: pilot unsure of position → request VDF.
• Approach in reduced visibility (within VFR minima): VDF gives rough bearing.
• Emergency (lost, low fuel): VDF for assistance.