VHF Omni-directional Range (VOR) 8

Figure 8.3. shows one revolution of a limaçon with phase differences corresponding to four cardinal points. The blue sine wave is the reference signal. Hence, for example:

•A phase diff. of 227° measured at the aircraft = 227° Radial.

•A phase diff. of 314° measured at the aircraft = 314° Radial.

Thus a VOR beacon transmits bearing information continuously. This information is supplied even during the identification period.

Terminology

A Radial (QDR) is a magnetic bearing FROM a VOR beacon.

VHF Omni-directional Range (VOR) 8

Figure 8.4 A Radial is a Magnetic Bearing from the VOR (i.e. QDR)

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8 VHF Omni-directional Range (VOR)

(VOR) Range directional-Omni VHF 8

Figure 8.5 Tracking Between Two VORs

VOR

N

Figure 8.6 RMI Usage

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VHF Omni-directional Range (VOR) 8

Transmission Details

VOR beacons operate within the VHF band (30-300 MHz) between 108.0 - 117.95 MHz as follows:

•40 channels, 108-112 MHz:

This is primarily an ILS band but ICAO has allowed it to be shared with short range VORs and Terminal VORs (TVOR): 108.0, 108.05, 108.20, 108.25, 108.40, 108.45 ….. 111.85 MHz (even decimals and even decimals plus 0.05 MHz)

•120 channels, 112 - 117.95 MHz (a channel every 0.05 MHz):

The emission characteristics are A9W:

A = main carrier amplitude modulated double side-band.

9 = composite system.

W = combination of telemetry, (telephony) and telegraphy.

Identification

UK VORs use 3 letter aural Morse sent at approximately 7 groups/minute, at least every 10 seconds. The ‘ident’ may also be in voice form e.g. “This is Miami Omni etc” immediately followed by the Morse ident. The voice channel is used to pass airfield information via ATIS. This information uses AM (amplitude modulation) and is transmitted at the same time as the bearing information. A continuous tone or a series of dots identifies a TEST VOR (VOT).

Monitoring

All VOR beacons are monitored by an automatic site monitor. The monitor will warn the control point and remove either the identification and the navigational signals or switch off the beacon in the event of the following:

•Bearing information change exceeding 1°.

•A reduction of >15% in signal strength, of both or either of the 30 Hz modulations, or of the RF carrier frequency.

•A failure of the monitor.

When the main transmitter is switched off the standby transmitter is brought on-line and takes time to stabilise. During this period the bearing information can be incorrect and no identification is transmitted until the changeover is completed.

Hence, do not use the facility when no identification is heard. It is vital to monitor a terminal VOR let down into an airfield. If a VOR is transmitting the identification TST it indicates that the VOR is on test and the bearing information should not be used.

VHF Omni-directional Range (VOR) 8

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8 VHF Omni-directional Range (VOR)

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Types of VOR

CVOR	Conventional VOR is used to define airways and for en-route navigation.
BVOR	A broadcast VOR which gives weather and airfield information between beacon
	identification.
DVOR	A Doppler VOR - this overcomes siting errors.
TVOR	Terminal VOR which has only low power; and is used at major airfields.
VOT	This is found at certain airfields and broadcasts a fixed omni-directional signal
	for a 360° test radial. This is not for navigation use but is used to test an aircraft’s
	equipment accuracy before IFR flight. More than +/-4° indicates that equipment
	needs servicing.
VORTAC	Co-located VOR and TACAN (DME) beacons.
DBVORTAC	Combination.

The Factors Affecting Operational Range of VOR

The higher the transmitter power, the greater the range. Thus en route VORs with a 200 watt transmitter will have about a 200 NM range, and a TVOR will normally transmit at 50 watts.

The transmitter and receiver height will also have an effect on the operational range of VOR as the transmissions give line of sight ranges, plus a slight increase due to atmospheric refraction. This can be assessed by using the formula:

Maximum theoretical reception range (NM) = 1.23 × (√h1 + √h2)

where: h1 = Receiver height in feet AMSL, and

h2 = Transmitter height in feet AMSL.

Uneven terrain, intervening high ground, mountains, man-made structures etc., cause VOR bearings to be stopped (screened), reflected, or bent (scalloping), all of which give rise to bearing errors.

Where such bearing errors are known, AIPS will publish details: e.g. “Errors up to 5.5° may be experienced in sector 315° - 345° to 40 NM”.

Designated Operational Coverage - (DOC)

To guarantee no co-frequency interference between the 160 frequencies available worldwide, it would be necessary to separate co-frequency beacons by at least twice their anticipated line of sight range, e.g. an aircraft at a height of 25 000 ft and the VOR situated at MSL.

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Reception range (NM) = 1.23 × √25 000

	=	194.5 NM
Separation	=	389 NM

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