Sleeping Receivers?
AGC BN3 - Loss of Communication
and .........
'Sleeping' Receivers
A series of incidents has been reported where ATC was unable to contact an aircraft that had previously established two-way communication with the ground controller. In almost every case, satisfactory reception was only restored after a transmission from the affected aircraft. In these cases the aircrew have used the phrase, "receiver gone to sleep" or "suspected sleeping receiver" in their reports. The likelihood of a loss of separation and increased risk of collision arising from a prolonged loss of communication (PLOC) was highlighted by the UK Airprox Board in 1999 when two aircraft, on opposing tracks, were both "out of communication" for a period of 5 minutes. The Airprox report (150/99) mentioned that one of the operating companies had experienced several incidents when their aircraft radio was "neither receiving nor transmitting". The CAA is aware of more than 250 incidents of missed calls since 1999. CAA Air Traffic specialists led a team with representatives from NATS, EUROCONTROL, Thales and British Airways to investigate this issue and recommend actions to CAA to address 'sleeping receivers' causing prolonged loss of communication.
The investigation revealed that on a small but critical percentage of occasions, the aircraft communications transceiver failed to return from the transmitting to the receiving state. To mitigate this problem, one transceiver manufacturer has devised and published a non-mandatory service bulletin. The recent incorporation of this service bulletin into the ATC transceivers carried by a major UK airline has proved to be completely successful, but the CAA believes that this problem is very likely to be replicated in other transceivers. The CAA is now investigating whether high power ground transmitters at frequencies close to the civil and military aeronautical frequencies are likely to adversely affect the performance of an airborne receiver and if so, what measures are necessary to improve the immunity from strong signals and third order inter-modulation (IP3).
To progress this work the CAA will lead a team to investigate 'sleeping receivers' causing prolonged loss of communication (PLOC) between pilots and ATC.