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7.6.5 Radio Navigation Aids

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VHF Omni Range (VOR)

The VHF Omni-directional Radio Range, the abbreviations for which are ‘VOR’ and ‘Omni’, enables a pilot to determine the direction of his aircraft from any position to or from a VOR beacon, and, if necessary, track to or from the beacon on a selected bearing.

VOR is a Very High Frequency (VHF) navigation aid which operates, in Australia, in the 112.1 to 117.9 megacycles (mcs) frequency band. Because it is a VHF aid, its ground to air range is limited to ‘line of sight’ reception which is typical of VHF transmission.

The range achieved is dependent, therefore, on the siting of the VOR beacon with relation to surrounding terrain, and on the height at which the aircraft is flying.

VHF Omni-directional Radio Range

As a VHF navigation aid, the VOR is static-free, and the information given by it is displayed visually on easily read and interpreted cockpit instruments. An infinite number of bearings can be obtained and they may be visualized as radiating from the beacon like spokes from the hub of a wheel.

However, for practical purposes the number of bearings can be considered to be limited to 360, one degree apart, and these 360 bearings are known as radials. A Radial is identified by its magnetic bearing outbound from the VOR beacon.

VOR Errors

Errors are likely to exist in any VOR system due to a number of causes. These include ground station error, site effect error, error due to vertical polarisation effects and airborne equipment error. The algebraic sum of all these errors is known as the aggregate error. Each of these errors will be explained individually to give a better understanding of the limitations of the system, although, from an operational view point, the pilot is concerned primarily with the aggregate error.

  • Ground Station Error: This is a systematic error associated with the transmitter, aerial and earth systems and power supply that is with the actual ground equipment. Error on a particular bearing is very small, and is plotted accurately on commissioning. It is similar to quadrantal error in an airborne ADF system. Ground station error, in practice, is usually less than ± 2°.

  • Site Effect Error: There are, superimposed on the ground station error, site effects which are due to topographical features near the ground station. These site effects modify the ground station errors. The combined effect of these errors are determined at various altitudes at the time of commissioning and the overall error must be less than ± 3°.

    Terrain effects caused by the VOR radials being distorted by signals reflected from rough terrain may be experienced under certain circumstances. These effects are evidenced in the form of slow or quick oscillations of the deviation indicator. The VOR track is said to band or scallop depending on the rate of oscillation. Bends on VOR radials normally do not exceed 2° from the average alignment of the track and the scalloping amplitude must be less than ± 2°.

  • Vertical Polarisation (Attitude Effects): If vertical polarisation effects are presented they are detected in manoeuvres which tilt the aircraft aerial, that is in a turn by an aircraft. When in a banked attitude, the receiving antenna on the aircraft, instead of sampling only the horizontally polarised VOR signals, may pick up large sloping obstructions. Under these circumstances, the deviation indicator is seen to move abnormally in either direction, thus giving rise to incorrect guidance information.

    In the type of VOR ground equipment installed in Australia, the vertical component actually radiated by the beacon is very small, hence this form of interference will be rarely encountered in practice.

  • Airborne Equipment Error: As the name implies, airborne equipment error is the error attributable to the various components of the VOR equipment in the aircraft. In well designed and built equipment it is generally less than ± 2°.

  • Aggregate Error: The algebraic sum of all the errors mentioned above is known as the aggregate error. It should be appreciated that procurement of precise figures on aggregate error, even on a probability basis, is extremely difficult if not impossible. However, the purpose in explaining the presence of these errors is to illustrate that the VOR is not a precision aid. It will be found that, in practice, aggregate errors greater than ± 5° are seldom encountered. It should be appreciated, too, that factors for aggregate error and for pilot error are used in the determination of standards used for aircraft separation purposes.