A ground direction-finding (DF) station needs nothing special on the aircraft: the pilot transmits on an ordinary VHF (very high frequency) communication frequency, the station's direction-finding antenna measures which way the transmission came from, and the controller passes the answer back by voice.
The service
That measured direction is the whole service: the use of a ground DF is as a bearing service any radio-equipped aircraft can call on, asking which way to steer to reach the station, or for help when unsure of position, with nothing carried on board beyond the ordinary radio. Everything in this lesson is about that measured line: its two names, what several stations can do with their lines together, how far the service reaches, what can pull the measurement off its line, and how accurate its answer is declared to be.
QDM and QDR
One measured line through station and aircraft carries two names, depending on which end it is read from:
QDM — the magnetic bearing to the station: the direction the aircraft would fly, measured from magnetic north, to go straight to the station;
QDR — the magnetic bearing from the station: the direction from the station out to the aircraft, the radial the aircraft sits on.
They are the same line read from its two ends, so they differ by exactly 180 degrees: an aircraft on QDR 090 from the station, due east of it, flies QDM 270 to reach it.
Check you understood
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
1. Warm-up
The term QDM means:
2. Applying it
The range of a ground DF (direction-finding) service is limited because:
3. Pushing it
A DF (direction-finding) station's antenna stands 400 ft above the terrain. An aircraft calls it from 2,500 ft. Using R=1.23hTX+1.23hRX, where hTX is the station's antenna height and hRX the aircraft's height in feet, the maximum range is about:
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What you are looking at: left, one station, one aircraft, and the single measured line between them, carrying its two names: the arrowhead at the station end is the QDM, the head pointing outward from the station is the QDR, reciprocals of each other against the magnetic north reference. Right, two stations, each contributing its own bearing line: the lines cross at the aircraft, and the crossing is the fix.
A fix from two stations
One bearing puts the aircraft somewhere on a line; it says nothing about where along it. But by using more than one ground station, the position of an aircraft can be determined and transmitted to the pilot: each station measures its own bearing to the same transmission, the bearing lines are laid over a chart, and where they cross is the aircraft. The service can therefore hand a lost pilot not just a direction but a position, with the aircraft carrying nothing more than its ordinary radio.
How far the service reaches
The whole service rides on the aircraft's VHF transmission, and its range is limited by the path of the VHF signal: as the propagation lesson established, VHF travels as a space wave, essentially line of sight, with no surface wave and no ionospheric return. The station can only measure what it can hear, so the horizon sets the service's reach; height at either end extends it. That geometry has a formula the syllabus expects you to use:
VHF line-of-sight range
R=1.23hTX+1.23hRX
R
the maximum range, in nautical miles (NM)
hTX
the transmitter height, in feet
hRX
The 1.23 is not an arbitrary number: it packages the geometry of the radio horizon. The straight-line distance to the horizon grows with the square root of both the height and the Earth's radius, and VHF bends very slightly downward in the standard atmosphere, stretching the purely geometric horizon by roughly 15 percent; fold the Earth's radius, that refraction allowance and the feet-to-nautical-mile conversion into one constant and 1.23 is what comes out. The exam only asks you to use it, but knowing it encodes Earth curvature plus a little atmospheric bending makes the formula memorable rather than magic.
What can corrupt the bearing
Two effects can corrupt the measurement itself:
Synchronous transmissions — these cause errors: if two aircraft transmit on the frequency at the same time, the station receives both at once and its antenna measures one blended arrival direction belonging to neither aircraft;
A DF bearing arrives with an honesty declaration. VDF (VHF direction finding) information is divided into classes according to ICAO (International Civil Aviation Organization) Annex 10, by the accuracy the station is prepared to guarantee:
Class A — accurate to within plus or minus 2 degrees;
Class B — accurate to within plus or minus 5 degrees;
Class C — accurate to within plus or minus 10 degrees;
Class D — accurate to less than Class C.
The controller states the class with the bearing, and the pilot knows how much to lean on it: a Class A bearing can anchor a position, a Class D bearing is a hint.
What the exam asks
The two definitions, the range formula as a calculation, the class table and the two corruption mechanisms carry the marks. The overview, following the learning objectives: