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GROUND BASED NAVIGATION AIDS



Beginner pilots should become familiar with both VFR ground-based navigation systems and ground reference points when learning to fly in case of a radio failure or failure of the navigation systems.


VHF Omnidirectional Range (VOR) are designed in three different types of navigation aids (NAVAIDs).

  • VOR - Is a VOR by itself.

  • VORDME - VOR/DME (Distance Measuring Equipment)

  • VORTAC - VOR/TAC (TAC short for TACAN -Tactical Air Navigation for military use)

These VOR station symbols are shown on sectional charts as is, with in a compass rose.

Whereas, if a VOR station is in conjunction with an airport it is shown as an open dot within hard-surfaced runways, that indicates the approximate location of either a VOR, VOR-DME or VORTAC station.


On all types of aeronautical sectional charts, such as VFR sectional or IFR sectionals, every VOR will have a radio communication information box.

The Omnidirectional means a VHF radio transmitting a straight line courses, known as radials from a ground based station and the "Omni" means all VOR stations.

VOR stations transmit two signals:

  1. The omnidirectional pulsates 30 times per second, and the rotating signal rotates 30 revolutions per second. The omnidirectional is timed so that it pulsates at the same exact time the rotating signal passes through magnetic north.

  2. Each radial transmission from the station is projected as a magnetic bearing from 001 or 1° east in a circle to 360° representing a compass rose on an aeronautical sectional chart.


VOR's transmit a VHF frequency that has a line-of-sight restriction. The reception range is generally about 40 - 45 miles depending on the type of VOR, starting at an altitude of 1,000 feet AGL.


There are three different types of VOR and VORTAC stations, they are:

  • T - (Terminal) with a range of 25 miles and 12,000ft and below.

  • L - (Low altitude) with a range of 40 miles and 18,000ft and below.

  • H - (High altitude) with a range from 130 - 40 miles and 60,000ft to 14,500ft.

The accuracy of VOR radials is excellent, within +-1°, but there are a few internal parts of VOR instruments that can affect the accuracy. To be sure that the VOR instruments in the aircraft are reading accurate pilots should do a VOR check, even though the check is not required for VFR flight. There are three types of checks that can be done, they are:

  • FAA VOR test facility (VOT)

  • Certified airborne checkpoints

  • Certified ground checkpoints, located on the airport surface

Note: that ​VOR receiver checks are not required for VFR flight, it is good practice.


To test the VOR receiver identify the VOT on the VOR receiver. Frequencies are found in the Chart Supplement either in the paper version or online.


Turn the omnibearing selector (OBS) until the course deviation indicator (CDI) needle is centered.

  • The indicated course should be either 0 or 180, regardless of your position on the airport.

  • If 0 the TO/FROM indicator should show FROM.

  • If 180 the TO/FROM indicator should show TO.

  • When the NAV Flag appears, it indicates no reliable signal being received.

VOR test facility (VOT) are available on a specific frequency at certain airports. This allows you to check the accuracy of the VOR receiver on the ground.

  • The testing frequency is found in the Chart Supplement.

  • Accuracy of the VOR receiver should be + - 4°

Airborne and ground checkpoints consist of certified radials that should be received over a specific point or landmark while airborne in the immediate vicinity of an airport or specific point over the airport surface.

  • Accuracy of the airborne check is + - 6°

  • Accuracy of the ground check is + - 4°


If the aircraft has two VOR instruments installed a dual VOR check can be accomplished. To check this simple tune both receivers to the VOR frequency. Maximum variation between both VOR receivers is 4°.

Identify the VOR inflight:


If a VOR station is out of service or undergoing maintenance, the Mores code identification is removed from the transmission. Pilots should not use the station for navigation purposes.

Tips:

  • Identify the station first by listening for the Morse code identifier.

  • Remember, The VOR signal is line-of-sight.

  • When using a VOR for navigation, determine the inbound radial.



References:


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