general

VHF Omni-directional Radio Ranges [VORs] operate in the Very High Frequency aviation navigation [NAV] band between 112.1 and 117.9 MHz. As VHF transmissions are line-of-sight the ground to air range depends on the elevation of the beacon site, the height of the aircraft and the power output. The VOR beacons are usually located at airfields but as they serve to define designated air routes [ airways] some are located away from airfields, often on high ground.

A simplified concept of the ground beacon is that it simultaneously transmits two signals, a constant omni-directional signal called the reference phase and a directional signal which rotates through 360°, during a 0.03 second system cycle, and consistently varies in phase through each rotation. The two signals are only exactly in phase once during each rotation – when the directional signal is aligned to magnetic north.

Imagine a wheel with 360 spokes, at one degree azimuth spacing, with the VOR beacon being the hub. The spokes are numbered clockwise from one to 360 and each spoke or radial represents a magnetic bearing from the VOR beacon. The airborne navigation circuitry measures the phase angle difference between the directional signal phase received and the reference signal phase and interprets that as the angular, or 'radial', indication currently being received. Radials are identified by magnetic bearing – e.g. the 30° radial – and thus form the basis for VOR, and designated air route, navigation. Essentially the system indicates a line of position, from the selected VOR, on which the aircraft is located at any time.

The beacon also transmits a Morse code aural identification signal at about 10 second intervals.

The airborne system utilising the VOR beacon transmissions usually consists of an antenna (probably a V - type dipole mounted horizontally on the fin or fuselage but could be the more expensive 'blade' or 'towel rail' types), a conventional VHF receiver (if combined with the VHF communications transceiver it is then called a NAV / COMM unit), navigation circuitry and the separate panel mounted navigation indicator or 'Omni Bearing Indicator' [OBI].

Some hand held aviation COMMS transceivers can also receive the NAV band VOR transmissions and appear to have some navigation circuitry but, from all reports, their VOR navigation capability, if it exists at all, is limited.

Basic Omni Bearing Indicator, like this Bendix-King model, has a manually operated radial or 'omni bearing' selector [OBS] which rotates an azimuth ring marked from 0° to 355°. The OBS selected radial is indicated by the arrow at top dead centre and the reciprocal bearing is indicated by the bottom arrow. The other features of a basic OBI are the TO–FROM indicators, a deviation bar, a deviation indicator needle and a NAV/OFF alarm flag.

The TO–FROM indications on the OBI are dependent on the aircraft's position relative to a notional ground baseline, formed perpendicular to the selected radial and passing through the beacon site. Unlike the NDB the indication is completely independent of the aircraft's heading. The navigation circuitry compares the difference between the radial being received and the radial selected. If the aircraft is located anywhere within range on the radial side of the baseline the 'FROM' indication will be displayed on the OBI and, if located within range on the reciprocal side, the 'TO' indication will be displayed. For example if the 030° radial is selected on the OBI, the ground baseline is established between 300° and 120°. If the radial received indicates the aircraft is anywhere in the blue shaded area of the diagram and no matter whether it is headed towards or away from the VOR, or in any direction whatsoever, the OBI will display 'FROM'. Similarly if it is in the yellow area the OBI will display 'TO' no matter which direction the aircraft is headed. There are two areas of ambiguity – near bearings at right angles to the radial (e.g. shown at 120° and 300°) – where the OBI will give fluctuating indications, or display the 'OFF' flag.

The deviation bar and the deviation indicator needle together form the Course Deviation Indicator or CDI. If the needle is over the centre point the aircraft is then located at some position along the selected radial – or its reciprocal. The five division marks or dots either side of the centre point are spaced at two degree intervals, thus if the needle is over the third mark, left or right of centre, the aircraft is positioned at a radial six degrees in azimuth from the selected radial, or its reciprocal. (Actually the aircraft is at the centre mark and the needle indicates the position of the selected radial). Full travel of the needle from the centre to either side represents 10° – or more – of azimuth. The ambiguity of whether the OBS selection is the radial or the reciprocal is determined by the TO / FROM indication; in the diagram at left 030 must be the radial as the aircraft is in the FROM area. When the aircraft passes overhead the beacon the needle will swing from side to side, the alarm flag may temporarily indicate that navigation is 'OFF' and the TO/FROM indication will reverse.

A difficulty for a non IFR trained pilot using the VOR is a lack of perception of which way to turn the aircraft to fly to a selected radial, using the CDI indications. However, for VFR purposes, this is easily ascertained if the pilot follows two simple rules:
1. To track FROM a VOR select the radial required and ensure FROM is indicated.
2. To track TO a VOR rotate the OBS until the CDI is centred and TO is indicated.
In both cases as wind effect drifts the aircraft off track the deviation indicator needle will move to one side and that movement indicates the direction to turn to regain track. i.e. turn towards the needle.

VOR applications

Like the NDB / ADF there are several applications for the VOR in light aircraft cross country VMC navigation. The applications briefly described below will be detailed in the 'Using the VOR' module.

Homing & tracking to a VOR. Even with a crosswind component tracking toward a VOR is quite simple, rotate the OBS until the CDI is centred and TO is indicated, turn onto that magnetic heading and then just keep the CDI centred and you will track more or less direct to the VOR.

Tracking from a VOR. Rotate the OBS to the required track [radial], ensure FROM is indicated, turn onto that magnetic heading and just keep the CDI centred and you will maintain the track.

Position fixes. If two VORs are in range then the bearing from each can be ascertained, roughly plotted on the chart [after converting to true bearings] and the aircraft position will be close to the intersection point of the LOPs. Alternatively a VOR bearing and a NDB bearing can be used or a VOR bearing and a line feature on the chart, the latter technique being the most frequently used.

Running fix / distance from VOR. The 1-in-60 rule can be applied when the aircraft is within range of a transmitter by turning the aircraft so that the station is abeam and then measuring the degrees traversed against time, as in the NDB running fix application above. The advantage with the VOR is that the CDI needle indicates the degrees traversed. As in the NDB application the position fix is the distance along the second radial from the beacon.

VOR errors

Standard VOR systems are more accurate than NDB / ADF but are still subject to errors at the ground station, bending distortion of signals caused by terrain effect and avionics errors. The aggregation of all errors is very unlikely to exceed 5°.

Though very thin on the ground in the outback areas of Australia, NDB and VOR can be very useful, provided the aircraft is within range, but not the best value for money. That distinction now belongs to another, and more advanced, supplementary navigation tool – the Global Positioning System.

Navigating directly to a VOR is the easiest way to use this kind of navaid.

• Dial in the frequency of the VOR into your NAV1 radio

• Turn the OBS (course selector knob) for the instrument(OBI) until you see the word "TO" and the CDI needle centres

• Note the heading shown at the top of the instrument; fly that course to go directly to the VOR

That's basically all there is to it! There are complications such as crosswinds that will affect your ability to get to the VOR. If you have winds you will have to adjust for them and fly a heading that will allow you to follow the proper course.

In an ideal situation you could fly the heading you just determined right to the VOR station. In that case the needle on the OBI would remain centred right up until the time when you passed over the VOR. More likely, though, your course will drift off to the side. By watching the needle on the OBI you can see this happen and also tell how to adjust your heading to get back on course:

try this excellent programme by Tim Carlson