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Very High Frequency (VHF) Omnidirectional Range (VOR) – Part 1

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What is a VOR?

The VOR system is avaiable in three different types of navigational aids (NAVAIDs): VOR, VOR/DME and VORTAC.  A VOR provides magnetic bearing information to and from the station.  When DME is installed with the VOR then it is known as a VOR/DME.  When military tactical air navigation (TACAN) equipment is installed with the VOR then it is known as a VORTAC.  DME is always integrated into VORTAC.

An omnidirectional range is a VHF radio transmitting ground station that projects straight line courses (radials) from the station in all directions.  The distance VOR radials are projected depends upon the power output of the transmitter.

The radials projected from the station are referenced to magnetic north (not the north pole).  Radials are identified by numbers beginning with 001, which is 1° east of magnetic north, and progress in sequence through all the degrees of a circle until reaching 360°.  To aid in navigation, a compass rose is superimposed on navigational charts at the location of the station.

VOR stations transmit on a VHF frequency band of 108.0 to 117.95 MHz.  Since the equipment is VHF, the signals transmitted are limited to line of sight.  Therefore, it’s range depends on the altitude op the recieving equipment.  Generally, the reception range of the signals at an altitude of 1,000 feet AGL is about 40 to 45 miles.  This distance increases with altitude as shown in the chart below.  VHF omnidirectional Range.

VORs and VORTACs are classed according to operational use.  The three classes are as follows:

T (Terminal)

L (Low altitude)

H (High altitude)

The normal useful range for the various classes is shown in the following table:

VOR/VORTAC NAVAIDS

Normal Usable Altitudes and Radius Distances

Class/Altitudes/Distance (Miles)

  • T/12,000′ and below/25
  • L/Below 18,000’/40
  • H/Below 14,500’/40
  • H/Within the conterminous 48 States only between 14,500 and 17,999’/100
  • H/18,000′ to FL 450/130
  • H/FL 450 to 60,000’/100

The useful range of some facilities may be less than 50 miles.  For more detailed information about specific VORs, refer to the Comm/NAVAID remarks in the Airport/Facility Directory.

The accuracy of course alignment of VOR radials are considered to be excellent, generally within +/- 1°.  However, parts of the VOR reciever equipment deteriorate, and affect its accuracy.  This is particularly true at great distances from the station.  Periodic checks and calibrations should be made of the equipment to maintain accuracy.

VOR stations can be identified by its morse code ID or by a recorded voice ID which states the name of the station followed by the word VOR.  Some flight service stations (FSS) transmit voice messages on the same frequency that the VOR operates so voice transmissions shouldn’t be relied upon to identify stations.  Some FSSs remotely transmit over several VORs which have different names than the transmitting FSS.  If the VOR is out of service for maintenance, the coded ID is removed and not transmitted.  This alerts pilots that this station shouldn’t be used for navigation.  VOR recievers are designed with an alarm flag to indicate when signal strength is inadequate to operate the navigational equipment.  The happens when you are either too far from the VOR or are flying too low and out of sight of the signals.  VHF omnidirectional range.

 

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