The VOR

The VHF omnidirectional radio range (VOR) transmits a signal from which a receiver works out which magnetic bearing from the station the aircraft sits on: its radial. A radial is the QDR of the last module, measured by the aircraft itself, continuously, with no ground operator in the loop. This is also the lesson where the HSI's course arrow and deviation bar from the Instrumentation course finally get their signal: everything the course arrow, deviation bar and TO/FROM flag display, this station provides.

How the signal encodes direction

The trick is phase. The station transmits two signals at once: a reference phase, identical in every direction, and a variable phase, whose timing shifts with the direction in which it leaves the station. The receiver compares the two: the phase difference between the reference and variable signals equals the aircraft's bearing from the station, one degree of phase per degree of radial, which is why the propagation lesson's phase vocabulary matters here. On the aircraft, the VOR airborne equipment has three main components:

the antenna — hears both signals;
the receiver — measures the phase difference;
the indicator — draws the result.

Frequencies and neighbours

Per ICAO (International Civil Aviation Organization) Annex 10, the VOR's allocated band is VHF, using 108.0-117.975 MHz. The bottom of that allocation is shared territory: within 108.0-111.975 MHz, the frequencies with an odd number in the first decimal place are used by the instrument landing system (ILS), so 108.10 or 109.30 is an ILS while 108.20 or 109.40 can be a VOR. One more service rides on the band: ATIS (automatic terminal information service) information is transmitted on VOR frequencies, the aerodrome's recorded broadcast using the station's voice channel.

The VOR family

Several types of VOR are in operation, listed for the exam:

conventional VOR (CVOR) — the first generation, emitting its signals by means of a rotating antenna;
Doppler VOR (DVOR) — the second generation, emitting its signals by means of a combination of fixed antennas utilising the Doppler principle: the propagation lesson's frequency shift with relative motion, the motion here simulated by switching the transmission around the ring of fixed antennas; less sensitive to reflections from its surroundings;
en-route VOR — the full-range station the IFR (instrument flight rules) airway structure is built on;
terminal VOR (TVOR) — a station with a shorter range, used as part of the approach and departure structure at major aerodromes;
test VOR (VOT) — a station emitting a signal used to test the VOR indicators in an aircraft.

Identification and failure

A VOR identifies itself with Morse-code letters, and some stations add plain text, a recorded voice naming the station between ident cycles. Per ICAO Annex 10, a VOR station has an automatic ground monitoring system, and unlike the NDB (non-directional beacon) of the last module, its self-watching reaches the signal itself: on failure of the station to stay within its required limits, the identification and navigation components are removed from the carrier, or the radiation ceases altogether. A VOR that has lost its ident is telling you its navigation signal is no longer to be trusted, which is precisely why the ident is checked before the needle is used.

Reading the VOR

On an RMI (radio magnetic indicator), the radial is read at the needle's tail: the head points to the station (the QDM), so the tail sits on the bearing from it, which is the radial, with no arithmetic at all. On an HSI (horizontal situation indicator) or OBI (omnibearing indicator), the pilot instead preselects a course with the OBS (omnibearing selector), and the instrument shows the angular displacement from it: the deviation bar or needle moves across a dot scale, full deflection at 10 degrees, 2 degrees per dot, so a two-dot deflection means the selected course lies 4 degrees away. On the dedicated OBI instrument this needle-and-dots presentation is the CDI (course deviation indicator), the same face under its own name.

The TO/FROM indicator answers the question the deviation cannot: which side of the station the aircraft is on. Draw a line through the station perpendicular to the selected course: on the side the selected course points away from, the indicator shows TO (flying the selected course would take you to the station); on the far side it shows FROM. Combined with the deviation, this fixes the aircraft's position relative to the VOR into one of four quadrants around the station, and here the heading deserves its own sentence: the display is built only from the selected course and the aircraft's position, never from its heading, so the picture must be interpreted with the heading in mind: fly with the selected course roughly ahead and the needle reads in the natural sense; fly the reciprocal and every deflection reads reversed.

Flying the VOR

The in-flight VOR procedures are the ADF procedures of the last lesson flown against a better display:

tracking — holds a selected radial with the deviation centred, and wind is handled exactly as before: a wind correction angle into the wind, the difference being that the VOR shows the track error directly as deviation instead of leaving it to be inferred from a bearing change;

intercepting a radial inbound or outbound — flown by setting the desired course, taking a cut toward it, and rolling out as the needle centres;

changing from one radial to another — the same capture from a track already held;

station passage — announced by the TO indication flipping to FROM, the needle swinging busily through the overhead confusion as it happens;

the abeam point — where the radial 90 degrees to the track is crossed, read directly as the RMI needle passing the wingtip position.

Radial to true

A radial is magnetic by definition, and the magnetic reference is the station's, not the aircraft's: when converting a radial into a true bearing, the variation at the VOR station has to be taken into account, because the station's radials were aligned to magnetic north where the station stands. Over long distances the variation at the aircraft can differ; the station's value is the one wired into the signal.

Accuracy and scalloping

Two closing facts. First the flying tolerance: per ICAO Doc 8168, to be considered established on a VOR track in approach procedures, the required bearing must be flown within the half-full-scale deflection of the required track. The criterion is defined on the instrument, half of full deflection; on the standard 10-degree scale that means within 5 degrees, the needle less than halfway out. Second the terrain: reflections from terrain can bend radials, leading to wrong or fluctuating indications, which is called scalloping: the needle wanders slowly or oscillates as the aircraft flies through regions where the direct and reflected signals disagree.