Development Of VOR

VOR was developed due to the following reasons

  • Flaws in ADF/NDB
  • Unreliability of ADF/NDB
  • No failure warning device
  • Static interference in the upper LF/ Lower MF Band


VHF Omnidirectional Radio Range (VOR):

It is a short range radio navigational aid on VHF band. The airborne equipment, in conjunction with ground station indicates QDM. It employs the principle of bearing by phase comparison.

  • It is in VHF Band so static is eliminated
  • Failure warning device incorporated as an Off Flag
  • Better user interface
  • Flexibility of selecting and maintaining any track.











General Principle:  A ground VOR station transmits a variable signal and a reference signal.

Variable Signal: A rotating polar diagram of figure of eight, amplitude modulated (AM) to 30hz on station carrier frequency is radiated.

Reference Signal: An omni-directional reference signal on station frequency is transmitted. It carries a sub-carrier of 9960Hz which is frequency modulated (FM) to 30hz. When plotted on time axis it gives a normal sine curve pattern.

Power relationship between variable and reference signal is so adjusted that when these two are combined a rotating LIMACON shape is produced. The resulting relative field strength is in the ratio of 4:1:4:7 on four cardinal magnetic directions.

Let us assume that the LIMACON pattern radiated by ground station is rotated in anti-clockwise direction. The phase received from Reference and  Variable signal at North is same ie. there is zero phase difference. But aircraft at E,S and W will receive phase from Variable signal on time axis as shown in the Fig   . Compared to Reference signal the Variable signal phase difference will be 90, 180 and 270 degrees respectively. Aircraft at any other position around the VOR station will receive phase difference between Reference and Variable signal equal to the magnetic bearing of the aircraft, reciprocal of which will give the QDM. The Reference signal and LIMACON signal are said to be phase locked at Magnetic North hence magnetic directions are indicated.

Radial:  The magnetic direction radiating from the station are called radials.  Though there are infinite number of radials available but only 360 radial spokes of 1º each from the ground station are in practical use.

Indication: The indication is converted to give QDM in the airborne equipment.

Airborne Equipment:  Consists of

  1. Aerial: It is a di-pole VHF aerial, available in the following


  1. a) V-shaped flexible steel antenna ( cats whiskers )
  2. b) Blade antenna
  3. c) Towel bar antenna

Type (b) and (c) give better performance with R-NAV.

  1. Receiver: Schematic diagram shows the working of the receiver.
  1. Indicator:
  1. a) RMI: The QDM is indicated by a needle which moves over the RMI card.
  2. b) Omni-Bearing Indicator (OBI): It consists of

(i)  Omni-Bearing Selector(OBS): It is a knob by which the required magnetic radial is set. The set radial appears on the OBI window. For tracking in/out, the required radial is set which also affects L/R and TO/FROM indications as will be seen subsequently.

Basis Of OBS

  • Pair of resolver synchros
  • Rotor linked to course selector knob, desired track (Φ) set on display.
  • Input derived from VOR Rx.
  • Brg to beacon i.e. QDM is (θ).
  • Outpt of resolver synchro is given by cos (Φ-θ) and Sin (Φ-θ),
  • If Cos (Φ-θ) = positive, Then indication is TO
  • If Cos (Φ-θ) = negative, Then indication is From
  • If Sin (Φ-θ) = positive, then indication is turn left.
  • If Sin (Φ-θ)= negative, Then indication is Turn Right

Note: Remember it as CPT SPL (Captain Special)

Cos positive To

Sin positive Left

(ii) TO/FROM Indicator: TO or FROM indication will appear according to whether reciprocal or the radial selected is closer to the aircraft position. If answer to the query “does this setting(OBS) take to the station” is yes, the indication  is  TO.

The indication is independent of the heading. You are either in the radial sector ie. within 90º on either side of the selected radial when it will show FROM or on the anti-radial sector when it will show TO.  10º on either side of a line perpendicular to the radial selected is change over sector called ‘Area of Uncertainty’.


iii) L/R Deviation Indicator:  It is a vertical needle which remains centralised when the aircraft is on the selected radial or its reciprocal. Any deviation from the radial is indicated as angular displacement over a five dot or four dot scale on either side of the centre on instrument face. Total deflection takes place when the aircraft is 10º or more away from the radial. Therefore using 5 dot or 4 dot deviation scale, each dot corresponds to 2º or 2.5º respectively.

The needle indicates the deviation. The needle indication to the left or right is followed if steering a course within 90º on the either side of the selected radial ie. if the selected radial and heading are in general agreement then follow the needle. If the heading is within 90º on either side of the reciprocal to the OBS setting then fly opposite to the needle indication.

This follows that for “tracking in”, set the anti-radial on OBS and follow the needle and for tracking out, set the radial and follow the needle.

By rotating OBS, when  vertical bar is centralized, OBS reading indicates  that  aircraft is either on the radial or anti-radial. The ambiguity is resolved by seeing the TO/FROM indicator.

  1. iv) Failure Warning Flag:  An OFF flag appears on the face of the indicator in the event of

–    receiver failure

–    ground station equipment failure

–    signals weak or aircraft out of range

–    failure of the indicator

VOR Frequencies: 108 to 117.95 MHz (A9W emission).

Band 108.0 to 112.0 MHz: Every ‘even’ first decimals (eg 108.20, 108.25, 108.40 etc) with 50Hz separation are available. (As Odd decimals are with ILS Localiser)

Band 112 to 117.95:  All odd and even decimals with frequency separation of 50 KHz (0.05MHz) are available.

CONE of CONFUSION:  In modern equipment ground transmission covers maximum up to 80ºin elevationleaving a gap right overhead the beacon, which spreads as a cone with altitude. Closer to or overflying a VOR beacon, indicators may give confusing indications on OBI.  This cone above the ground VOR beacon is called cone of confusion.


1) Site Error: Irregular terrain, obstacles, power lines etc in the vicinity of the VOR transmitter may affect directional propagation. VORs are ground monitored to accuracy of 1º.  Use of Doppler VOR reduces this error to a great extent.

  1. Propagation Error: Siting errors may get enlarged on the propagated wave in its travel forward.
  1. Airborne Equipment Error: These may be error in converting 1º of phase difference to 1º of change in direction.

Note:  All the above three are added up to give aggregate error.

  1. Interference Error: Due to proximity of channels and locations, VOR transmissions need protection in range and altitude from unwanted signals. Designated Operational Coverage (DOC) is indicated for the VOR stations to provide protection of station transmission from interference.

Range:   The range may be calculated by LOS formula

R = 1.25 √( HT + HR)    HT = Height of transmitter

HR = Height of receiver

Test VOR: Certain stations provide ground testing facility called VOT.  The aircraft equipment is switched on at any position in the airfield. The vertical bar is centralized by rotating OBS knob. The indication for serviceability should be

  1. a) 176 to 184 TO
  2. 356 to 004 FROM
  3. ± 4ºof 180 for TO or ± 4ºof 000 for FM.

In other words as if the aircraft is at North of the Station, on 000 radial sector.

Doppler VOR:  Doppler VOR uses an aerial array of 50 aerials spread on 44ft diameter compared to 10 ft high and 6 ft in diameter cylindrical structure for conventional VOR. It transmits reference signal of 30 Hz AM and Variable signal of 30 Hz FM on the station carrier frequency, just the reverse of conventional VOR. Rest of the operation and equipment being similar, it reduces the effect of terrain and eliminates siting error to a very great extent.

Uses:  VOR is used for

1)   Homing or Tracking In

2)   Tracking out

3)   Position line

4)   Route navigation

Question: VOR station ‘A’ shows ‘TO’ on 000 setting. Another VOR station B left of the route shows FM and vertical needle to the left on 090 setting on OBS. Has the aircraft

  1. i) Crossed 090 radial of stn B
  2. ii) Not yet crossed 090 radial of stn B

iii) Position of the aircraft with respect to 090 radial can not be found

  1. A VOT Test can be performed (a) Anywhere on ground (b) At a fixed point on ground  (c) Anywhere in air  (d) None of the above. A.  (a).
  1. A VOR Test can be performed (a) Anywhere on ground (b) At a fixed point on ground  (c) Anywhere in air  (d) None of the above. A.  (b).

       An aircraft flying from VOR “A” to VOR “B” on magnetic track 120° halfway the CDI is showing 2 dot fly right indication. At this stage pilot tunes Stn “B”.  The new indication will be

(a) CDI 2 dot fly right & TO-FROM indicates FROM. (b) CDI 2 dot fly left & TO-FROM indicates TO. (c) CDI 2 dot fly right & TO-FROM indicates TO. (d) CDI 2 dot fly left & TO-FROM indicates FROM. A.  (c)


  1. An aircraft’s VOR is tuned to a station. When 025° is selected on the OBS the deviation bar becomes central and the TO/FROM indicator indicates TO. What radial is the aircraft on?


(a) O25°(T)      (b) O25°(M)    (c) 205°(T)       (d) 205°(M)


  1. In question 1, if the ADF is tuned to an NDB on the same site as the VOR, and the RBI indicates a constant 011o on ADF, what drift is the aircraft experiencing?


(a)        11° port                                  RBI 011°

(b)       11° starboard      Ac Hdg         11° Stbd Drift

(c)        14° port                  014°

(d)       14° starboard


  1. In question 2, what is the aircraft’s heading?


(a) 0110            (b) 0140 (025-011)      (c) 0360            (d) 0390


  1. If a VOR station in the AlP has 100/30000 written beside it, what does that mean?


(a)        An aircraft can receive its signal at 100 km if it flies at 30000 ft

(b)        An aircraft can receive its signal at 100 nm if it flies at 30000 ft

(c)        Aircraft should not use the beacon on the 1000 radial below 30000 ft

(d)       Aircraft should not use the beacon outside 100 nm or above 30000 ft


  1. To minimise quadrantal error from a VOR station, a pilot should:

(a)        Choose a VOR station on the aircraft’s beam

(b)        Choose a VOR station on the aircraft’s nose or tail

(c)        Both (a) and (b)

(d)       Neither (a) nor (b)


  1. Which of the following are all factors affecting VOR range?

(a)        Transmission power, aircraft altitude, night effect

(b)        Station elevation, site error, static

(c)        Station elevation, aircraft altitude, transmission power

(d)       Night effect, station elevation, coastal effect



  1. Which of the following are the factors affecting VOR accuracy?


(a)        Beacon alignment, site error, propagation error

(b)        Airborne equipment error, propagation error, coastal effect

(c)        Pilotage, night effect, airborne equipment error

(d)       Site error, night effect, beacon alignment


  1. What is the emission pattern of a VOR signal?


(a) A3E           (b) A8W          (c) A9W          (d) J3E


  1. Which of the following is the expected accuracy of a VOR signal?

(a)        ±5°, all the time

(b)        ±3°, 95% of the time

(c)        ±3°, all the time

(d)       ±5°, 95% of the time


  1. Which of the following indications would be acceptable if you tuned the VOR to a station marked ‘VOT’ during the pre-takeoff checks?

(a)        180°, FROM

(b)        004°, FROM

(c)        359°, TO

(d)       175°, TO


  1. The modulation of the fixed signal of a Doppler VOR station is:

(a)        Frequency modulated at 30 Hz

(b)        Frequency modulated at 30 MHz

(c)        Amplitude modulated at 30 Hz

(d)       Amplitude modulated at 30 MHz

VHF Omni Direction Radio Range

Doppler VOR











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  • Arunaksha Nandy Post author

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