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VHF Omni-directional Range VOR

General Information

Various types of air navigation aids are in use today, each serving a special purpose. These aids have varied owners and operators, namely: the Federal Aviation Administration (FAA), the military services, private organizations, individual states and foreign governments. The FAA has the statutory authority to establish, operate and maintain air navigation facilities and to prescribe standards for the operation of any of these aids, which are used for instrument flight in federally controlled airspace. These aids are tabulated in the Airport/Facility Directory (A/FD). In this handbook, we are talking about on of the most important types of navigation aids, which is the VOR that is mostly collocated with DME or TACAN (VORTAC).


VHF Omni-directional Range/Tactical Air Navigation (VORTAC)

a. A VORTAC is a facility consisting of two components, VOR and TACAN, which provides three individual services: VOR azimuth, TACAN azimuth and TACAN distance (DME) at one site. Although consisting of more than one component, incorporating more than one operating frequency, and using more than one antenna system, a VORTAC is considered to be a unified navigational aid. Both components of a VORTAC are envisioned as operating simultaneously and providing the three services at all times.

b. Transmitted signals of VOR and TACAN are each identified by three-letter code transmission and are interlocked so that pilots using VOR azimuth with TACAN distance can be assured that both signals being received are definitely from the same ground station. The frequency channels of the VOR and the TACAN at each VORTAC facility are "paired" in accordance with a national plan to simplify airborne operation.


VOR Receiver Check

a. The FAA VOR test facility (VOT) transmits a test signal that provides users a convenient means to determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located. The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically authorized in the A/FD or appropriate supplement.

b. To use the VOT service, tune in the VOT frequency on your VOR receiver. With the Course Deviation Indicator (CDI) centered, the omni-bearing selector should read 0 degrees with the to/from indication showing "from" or the omni-bearing selector should read 180 degrees with the to/from indication showing "to." Should the VOR receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180 degrees on any omni-bearing selector (OBS) setting. Two means of identification are used. One is a series of dots and the other is a continuous tone. Information concerning an individual test signal can be obtained from the local FSS.

c. Periodic VOR receiver calibration is most important. If a receiver's Automatic Gain Control or modulation circuit deteriorates, it is possible for it to display acceptable accuracy and sensitivity close into the VOR or VOT and display out-of-tolerance readings when located at greater distances where weaker signal areas exist. The likelihood of this deterioration varies between receivers, and is having an accurate receiver is periodic calibration.
Yearly intervals are recommended at which time an authorized repair facility should recalibrate the receiver to the manufacturer's specifications.

d. Federal Aviation Regulations (14 CFR Section 91.171) provides for certain VOR equipment accuracy checks prior to flight under instrument flight rules. To comply with this requirement and to ensure satisfactory operation of the airborne system, the FAA has provided pilots with the following means of checking VOR receiver accuracy:


1. VOT or a radiated test signal from an appropriately rated radio repair station.

2. Certified airborne checkpoints.

3. Certified check points on the airport surface.

e. A radiated VOT from an appropriately rated radio repair station serves the same purpose as an FAA VOR signal and the check is made in much the same manner as a VOT with the following differences:

1. The frequency normally approved by the Federal Communications Commission is 108.0 MHz.

2- Repair stations are not permitted to radiate the VOR test signal continuously; consequently, the owner or operator must make arrangements with the repair station to have the test signal transmitted. This service is not provided by all radio repair stations. The aircraft owner or operator must determine which repair station in the local area provides this service. A representative of the repair station must make an entry into the aircraft logbook or other permanent record certifying to the radial accuracy and the date of transmission. The owner, operator or representative of the repair station may accomplish the necessary checks in the aircraft and make a logbook entry stating the results. It is necessary to verify which test radial is being transmitted and whether you should get a "to" or "from" indication.

f. Airborne and ground checkpoints consist of certified radials that should be received at specific points on the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport.

1. Should an error in excess of plus or minus 4 degrees be indicated through use of a ground check, or plus or minus 6 degrees using the airborne check, Instrument Flight Rules (IFR) flight shall not be attempted without first correcting the source of the error.

2. Locations of airborne check points, ground check points and VOT's are published in the A/FD and are depicted on the A/G voice communications panels on the National Ocean Service (NOS) IFR area chart and IFR enroute low altitude chart.

3. If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, one system may be checked against the other. Turn both systems to the same VOR ground facility and note the indicated bearing to that station. The maximum permissible variations between the two indicated bearings are 4 degrees.


Limitations of VOR Navigation

Being able to find your position accurately using the static free signals form VOR stations along a route and to follow a course to a VOR station without having to manually calculate course corrections for wind drift, are some of the advantages of the omnirange navigation

system. It is very important, however, to understand the limitations of the system so that you can recognize errors and possible ambiguities when they occur. Remember that, because the omnirange signals are in the VHF range, your airplane must be in a direct line of sight to the VOR station to receive its signals.

This means that far away from the station, you must have enough altitude to receive the signals. Mountainous terrain may also break the line of sight. It might be possible, under certain conditions, to receive signals from two different VOR stations transmitting on the same frequency if you are flying at very high altitudes.

Be aware of this possibility and use the navigation radio's identification button to ensure that you are tracking the correct VOR station.

VOR Classes:

Navigational requirements depend upon the altitude, speed and flight segment you are flying. To meet these differing needs, VOR stations fall into three classifications. Tune in terminal

(T) VORs when you are flying at or below 12000 ft and 25 nm (nautical miles) from the
station. These (T) VORs are used near airports in the approach segment of a flight. Further away from airports, low altitude (L) VORs has a range of about 40 nm and you can receive their signals up to an altitude of 18000 ft. For flight levels of 14500 ft and above, use the high altitude (H) VORs whose signals have a range of about 100 nm up to 18000 ft and 130 nm between 18000 ft