062 Radio Navigation topic guide
VOR: Principles and Errors
VOR remains the backbone of conventional radio navigation because it gives a continuous, direct bearing rather than a single needle swing. The ground station radiates two 30 Hz signals: one omnidirectional and fixed in phase, called the reference, and one that rotates around the compass so that its phase at any point in space depends on the aircraft's bearing from the station, called the variable. The receiver compares the two phases and converts the difference directly into a radial.
That radial is always defined outward from the station and is always measured relative to magnetic north at the station, never at the aircraft. Confusing that reference point, or confusing the outbound radial with the inbound course a pilot actually flies, produces the single most common VOR orientation error in the exam and in the cockpit alike.
Radials, QDMs and the reciprocal
A radial is a magnetic bearing measured outward from the VOR: radial 090 means the aircraft sits somewhere on the line running due east of the station. QDM is different. It is the magnetic bearing that takes the aircraft to the station, so if you are established on radial 090 in nil wind, the QDM home is the reciprocal, 270. The exam exploits this directly, giving a radial and asking for a course, or the reverse, on the assumption that a rushed pilot will treat the two labels as one number.
The safest habit is to draw it rather than calculate it under pressure: plot the station, mark the given radial as a line running away from it, and read the return course straight off the opposite end of that line. In the cockpit the CDI and the TO/FROM flag do this translation automatically, but the exam question strips that display away and asks for the raw number underneath it.
Site, propagation and scalloping errors
Site error comes from the ground around the transmitter: buildings, rising terrain or even large aircraft taxiing nearby reflect the signal and distort the pattern the receiver decodes, which is why VOR stations are sited on cleared, level ground wherever practical. Propagation error is the broader term for distortion introduced between station and aircraft, including terrain screening the line-of-sight VHF signal. Scalloping is the rapid, small needle oscillation caused by reflected paths arriving out of phase with the direct signal; it is worse at low altitude and long range over rough terrain, and it eases as the aircraft climbs or closes on the station.
Directly above the station, the vertical radiation pattern collapses into a cone rather than a sharp point, and inside it the bearing becomes unreliable and the TO/FROM flag can flip rapidly. This is the cone of confusion, and it is expected, not a fault: the correct response is to hold heading and let the indications settle beyond the cone rather than chase a swinging needle.
Doppler VOR and range
A conventional VOR simulates its rotating pattern from what is effectively a single point, which is exactly what makes it sensitive to nearby reflectors. Doppler VOR instead uses a ring of fixed antennas commutated in sequence to simulate a much larger rotating aperture; reflected signals then arrive at a shallower relative angle, sharply reducing site error at difficult locations without changing anything the pilot reads in the cockpit.
Range is limited by line of sight because VOR is a VHF signal: the higher the aircraft, the further the direct wave reaches before the earth's curvature blocks it, so published VOR range is altitude-dependent. This is the same limit that governs ILS and VHF communications, and it explains why low-level reception of a distant VOR is patchy while the same station is solid at cruise altitude.
Worked example
Worked example: radial versus QDM
An aircraft is established outbound on the 090 radial from a VOR, in nil wind. Ignoring the cone of confusion, what QDM will take it directly back to overhead the station?
- A090
- B180
- C270
- D360
Show the answer and walkthrough
Correct answer: C
- A. This is the outbound radial itself, measured FROM the station. Treating it as the return course confuses the radial with the QDM, the single most common VOR orientation error.
- B. This adds only 90 degrees, a quarter turn, to the radial instead of the full 180 degree reciprocal needed to reverse a bearing exactly.
- C. Correct. QDM is the magnetic bearing TO the station, the reciprocal of the radial: 090 plus 180 gives 270.
- D. This comes from adding 270 instead of 180 to the radial, a slip that surfaces when a pilot half-remembers that the correct answer is 270 and mistakes it for the amount to add rather than the resulting course itself.
Step by step
- Identify what radial 090 means: the aircraft is on the line running magnetic 090 degrees FROM the station.
- QDM is the bearing TO the station, always the reciprocal of the radial you are established on.
- Add 180 degrees to reverse the bearing: 090 + 180 = 270.
- Sanity check: 270 sits directly opposite 090 on the compass rose, exactly 180 degrees apart, confirming the reciprocal.
Common mistakes
Treating the radial and the QDM as the same number
A radial is defined FROM the station and a QDM is defined TO it. Using one where the other is required in an intercept or holding question flies the aircraft the wrong way entirely.
Chasing the needle inside the cone of confusion
Erratic swings and flag flicker overhead the station are expected, and correcting against them wastes attitude and heading discipline for no navigational benefit; the correct response is to hold heading and wait it out.
Assuming any needle scalloping means the VOR is unserviceable
Scalloping from reflected signal paths is a known low-altitude, long-range artefact, not a failure code. Misreporting it can mask an actual navigation problem or trigger an unnecessary diversion.
Related topic guides
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Last reviewed July 2026