View Full Version : VHF offset carrier

Golden Rivet
13th Nov 2003, 21:22
Hi all

Anyone know of any web sites dealing with theory and operation of vhf offset carrier transmissions.

Many thanks


13th Nov 2003, 23:17
Here’s something that may be of interest. AIC 106/1989.

UK Civil Aviation Authority


Aeronautical Information Service (AIS 1c)
Control Tower Building, LONDON/Heathrow Airport
Hounslow, Middlesex TW6 1JJ
Editorial: 01-745 3456
Distribution: 0242-235151
Content: 01-832 5166 P & E Comms 2


1. There have been a number of recent incidents, some with
potentially dangerous consequences, in which ground to air VHF
radio-telephony communication between ATC and an aircraft has
been lost. No faults have been found in NATS ground equipment
and it has been concluded that the most likely cause of the failure is
the inability of some types of airborne receiver to function
satisfactorily when receiving off-set carrier transmissions.

2. Within the United Kingdom, virtually all VHF RTF channels
Provided for en-route air traffic control and those used for emergency, flight information services and VOLMET are operated from between two and four radio stations using the off-set carrier technique in accordance with the standards set down in ICAO Annex 10. The off-sets employed are; ± 5 kHz for a two station system, ± 7.5 kHz and 0Hz for a three station system and ± 7.5 kHz and ± 2.5 kHz for a four station system.

3. When an aircraft is operating within the range of two or more stations, the individual transmissions combine in the airborne receiver to cause one or more audio heterodynes having a minimum frequency of approximately 5 kHz. These heterodynes appear above the normal audio pass-band of the receiver and are not heard by the flight crew.

4. Where airborne receivers are fitted with muting or squelch circuits it is common practice for these to operate on the basis of a received noise measurement obtained by sampling part of the audio band above 4 kHz. In areas where off-set carriers are used, the heterodynes will therefore be measured as noise and may cause the audio output of the receiver to be muted even though a perfectly adequate wanted signal is present. In general, airborne receiver manufacturers recognize this possibility and provide additional circuitry which either detects the presence of heterodynes or operates directly on the level of the received carrier and hence lifts the mute.

5 An examination of some commonly used aircraft receivers which employ carrier over-ride techniques has revealed that they are set to operate at carrier input levels of typically 20 micro-volts PD (-81dBm). This level is far in excess of that required to provide an adequate audio output and effectively causes the receiver to be de-sensitized when operating in an off-set carrier environment and hence for ground-to-air communications to be lost.

6 With the high density of traffic now being experienced in UK airspace it is essential for the safe operation of aircraft that reliable air-ground communications are provided. Operators are therefore requested to note that oft-sat carrier systems are used extensively for the provision of VHF RTF services within the UK and that aircraft receivers must be compatible with these systems. In choosing aircraft receivers preference should be given to those designs which maintain a high sensitivity when operating with off-set carrier transmissions. Where a carrier over-ride is provided within the mute circuitry to overcome the failure of noise operated mute circuits to function satisfactorily in the present of audio heterodynes, it is recommended that the carrier over-ride level be set as low as possible with an upper limit of 12 micro-volts PD (-85dBm). This level is compatible with the certification requirements contained within CAP 208 Volume 1 Part 4 and EUROCAE Minimum Performance Specification ED23A.

7 AIC 8/1988 (Pink 102) is cancelled.

Golden Rivet
13th Nov 2003, 23:53
many thanks - an interesting article.

specifically interested in whether offset carrier can be used with 8.33khz spacing.



14th Nov 2003, 00:11
In a word. No. Not possible.

14th Nov 2003, 00:34
I can't see that it is possible.

Given that the new spacing is 8.33kHz and the max offset for climax ops is 7.5kHz the transmission would bleed over into the adjacent channel.

14th Nov 2003, 00:58
See Eurocontrol's user guide.