Very High Frequency (VHF) Omnidirectional Range (VOR)

[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 16-22, ACE The Technical Pilot Interview 101/348, AVIA4019]

(Last Update : 2020 Apr 28 1610Z)

VOR stands for Very High Frequency (VHF) Onmidirectional Range.

VOR provides magnetic bearing information to and from the station.

VOR signals transmitted are subject to line-of-sight restrictions.

The prefix “onmi-“means all – the VHF radio transmitting ground station projects (sends out) straight line courses (radials) from the station in all directions. On aeronautical charts, radials are superimposed (depicted) by compass rose which take reference to magnetic north from 000 to 359.

Indeed, every VOR radials combine both 1) FM (frequency modulation) reference-phase signal and 2) AM (amplitude modulation) variable-phase signal. The FM signal is same in all directions for a particular VOR radio ground station and the AM signal is changing the phase at a constant rate. In short the unique signals for each of the 360 radials are produced.

VHF’s band is actually ranging from 30–300 MHz

However, for aviation navigation application , VOR ground stations transmit the signal within 108.0-117.95 MHz (108-118 MHz); In terms of radio communication application in aviation field, voice is transmitted through VHF 118-137 MHz.

Be noted that the VOR signals is VHF signal, so it is subject to line-of-sight restrictions – VOR signals only be received at increasingly higher altitudes as the distance of the aircraft from the station increases.


Range of VOR?

Average 25NM to approx. 140 NM

VOR is normally used within approximately 130 NM of the station – i.e a short-range navigation aid.

These distance VOR radials (radio signals) are projected depends mainly upon the power output of the transmitter. In addition, as mentioned about the VOR signals are subject to line-of-sight restrictions.The theoretical maximum range based on 1) the height of the ground station’s transmitter, 2) the height of the receiver

Maximum range (in NM) = 1.25 [ Transmitter‘s height (in AMSL) +√ Receiver‘s height (in AMSL)]


3 common types of navigation instrument of VOR:

  1. Course Deviation Indicator (CDI)
  2. Horizontal Situation Indicator (HSI)
  3. Radio Magnetic Indicator (RMI)

Note: Either any one of the instruments, VOR ground station can be positively identified by its Morse code in the recorded voice.   

Course Deviation Indicator (CDI)

A typical CDI consists of :

  1. Omnibearing Selector (OBS)
  2. CDI needle (both left and right)
  3. TO/FROM indicator

The CDI is an azimuth dial. By using the OBS – that is a course selector to select a desire radial (course). After positively identify the correctness of the ground station by the Morse code in recorded voice, by centering the CDI needle, the aircraft should fly TO/FROM the chosen facility according to the status of the “TO/FROM” indicator.






Horizontal Situation Indicator (HSI)

HSI combines the magnetic compass with navigation signals and a glideslope. HSI uses the output from a flux valve to drive the compass card.

The desire radial (course) is represented by Course select pointer selected by rotating the course select knob (≈ OBS in CDI). The course deviation bar displays the aircraft’s position relative to the selected course. And TO/FROM indicator shows whether the aircraft is flying toward or away from the chosen facility.

The main feature of HSI (difference from CDI) is that it provides both left right deviation along with (plus) glide slope information as well as heading indication in turns (by reading lubber line against the compass card reading).





Radio Magnetic Indicator (RMI)

RMI combines the compass with two bearing pointers. The magnetic reading fed by remote indicating compasses and the two pointers are driven by any two combinations of a GPS, an ADF and/or a VOR.



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