Sir - Unlike aircraft, VHF radio systems, which use a single-unit "transceiver", air-traffic-control (ATC) ground/air radio systems use a semi-duplex arrangement. With semi-duplex, the transmitter and receiver are independent items, which, ideally (in the case of ATC), are physically separated.

The rationale behind this independence and separation is that, when transmitting, a controller is assured by his reception of "off-air sidetone" in his headset that his switch, microphone, transmitter, receiver and headset are all operating satisfactorily. A further important benefit, is that a pilot's transmission, which might coincide with a controller's, may be heard by the controller.

It appears that "live" sidetone has been eliminated from systems' specifications. One reason put forward is that these digital systems, far from possessing the rapid-reaction times which the name implies, introduce lengthy delays which create an echo between the controller's speech and received sidetone.

These new systems, therefore, probably use synthetic sidetone, which is merely audio, from the controller's microphone fed directly to the headset. This may well eliminate any echoes, but the losses - in terms of operational safety and disturbance to the controller - could be significant.

Firstly, reception of synthesized sidetone tells a controller only that his or her microphone and headset are working - the transmitter could unserviceable.

Secondly, any pilot transmission coincident with a controller's is probably muted, depriving the controller of any warning, of the simultaneous transmission.

Analogue systems have served the aviation industry well for years, because there was a sound logic in the design concept. The introduction of "advanced" systems may well be a backward step, in that they could introduce a degree of unreliability and a lack of controller confidence in their communications equipment.


Sandgate, Kent, UK


Source: Flight International