— 1. Introduction

In order to facilitate radiotelephony communication with aircraft flying at low altitudes over north-eastern and south-western near-border regions of Hungary, new radio stations will be installed in these regions. The existing and the new stations will operate in offset carrier mode (CLIMAX mode) with 5 kHz upward and downward shift of the transmitted carrier frequency from the nominal value. This technique will be applied for the radio frequencies of FIS service (125.500MHz FIS WEST and 133.000MHz FIS EAST) after 31 May 2013.

The offset carrier technique is in accordance with the standards laid down in ICAO Annex 10. In order to receive these offset carrier transmissions, the airborne VHF receivers must be compatible with the offset carrier technique.
This AIC addresses the compatibility issues of airborne VHF receivers with offset carrier systems.

 — 2. Effect of mixing offset carrier signal

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

 — 3. Squelch/muting circuits

Airborne receivers that are fitted with muting or squelch circuits, commonly operate on the basis of a received noise measurement obtained by sampling part of the audio band above 4kHz. In areas where offset 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 desired signal is present. In general airborne receiver manufacturers recognise this possibility and provide additional circuitry which detects the presence of heterodynes or operates directly on the level of the received carrier signal (carrier override) and lifts the mute.

Some commonly used aircraft receivers which employ carrier override techniques are set to operate at carrier input levels of typically 40μ�V EMF (-81dBm). This level is far in excess of that required to provide an adequate audio output and effectively causes the receiver to be desensitised when operating in an offset carrier environment and hence for ground-to-air communication to be lost.

In any cases of doubts regarding the squelch circuit operation, aircraft crew is advised to open the squelch manually and check the traffic of the radio channel.

 — 4. Compatibility

It is essential for the safe operation of aircraft that reliable radio communications are provided. Operators and aircraft owners are therefore requested to note that offset carrier systems will be used for provision of radio communication of the FIS service within the Budapest FIR and aircraft receivers must be compatible with these systems. In choosing aircraft receivers, preference should be given to those designs which maintain high sensitivity when operating with offset carrier transmissions. Where a carrier over-ride is provided within the mute circuitry (to overcome the failure of noise operated mute circuits to function satisfactory in presence of audio heterodynes), it is recommended that the carrier override level be set as low as possible with an upper limit of 24μ�V EMF (-85dBm).This level is compatible with the certification requirements contained within EUROCAE minimum performance specification ED23C. It is also recommended that in choosing new radio equipment, preference should be given to multi class (classes C, E, H1 and H2) receivers. These receivers are compatible to the offset carrier transmissions and are able to operate on 8.33 kHz spaced radio channels as well.