This complicates and slightly delays the routing of voice messages, leading to an echo in the headphones of controllers.
To assure themselves that their voices are being transmitted correctly, controllers speak into their headset microphone which sends the voices through a range of multiplex communications equipment and back into the headphones of controllers - a facility known as "on-air sidetone."
When controllers hear their own voices in their headphones, they have an added level of confidence in knowing that their equipment is functioning and that their voices are reaching pilots. Without the sidetone facility controllers might shout, as if speaking into a telephone without hearing your own voice.
For controllers at West Drayton there has, for a long time, been a slight but almost imperceptible delay in the time taken between speaking into the headset microphone and hearing their voices in the headphones. The delay is due in part to the complex digital processing which routes the voice messages to their destinations, ensuring that the controller and pilots can communicate without interruption.
The new problem with echo in the headsets of controllers arises when Swanwick is added to the communications loop. A voice message from a controller at Swanwick goes into the site's internal communications equipment, then to the equipment at the London Air Traffic Control Centre, then to a remote radio site where it is transmitted into the ether for reception by pilots. For on-air sidetone to work, the message is then re-captured by equipment at the remote radio site, routed back to the London Air Traffic Control Centre, passed to Swanwick's communication suite and ends up in the headphones of the controller.
With Swanwick added to the chain, the extra digital processing involves contributes to a slight delay in the voice of the controller being returned to the headset. The echo resembles that of some satellite telephone calls.
The easiest solution is to bypass the communications loop altogether and provide sidetone directly from the controller's microphone to the headphones. So as the controllers speak, they hear their voices, as in a conventional telephone.
But this does not give controllers an added assurance that their voices are being transmitted across the communications network. Normally this would not matter. Pilots acknowledge receipt of voice messages. Even so, the criticality of voice communications is such that controllers feel the need to be assured that the voice communications systems are working properly. Misunderstood communications between pilots and air traffic controllers have been a contributory cause of some of the world's worst air crashes.
It is thought by some controllers and technical staff that a direct connection between the microphone and headset which bypasses the communications may not be approved by the Civil Aviation Authority's Safety Regulation Group.
One proposed solution has been to provide a direct microphone to headphones link, known as off-air sidetone, and have a separate light, an audible signal or other device which proves to the controller that their voice messages are travelling correctly around the communications loop. However there are doubts among some whether this would satisfy safety regulators. Also it could be seen as a short-cut.
No obvious, acceptable and unequivocally safe solution has yet presented itself. However NATS is confident that the problem will soon be resolved. Some officials say it is a temporary and minor blip, the sort of problem that testing is designed to identify.
A NATS spokesman told Computer Weekly: "It is correct to say that when controller speaks into headset they hear their own voice played back. The real safety check is to get a response. The pilot must read that back. This gives an added reassurance that they know their message has been delivered, received and understood. The problem with the echo is somewhat marginal in safety terms. We are going to fix it. There's a slight delay, as there is on some international calls, whether by satellite or cable. We will find a technical solution to it before the Operational date. We are certain we can do that and indeed we have a solution in sight. It is an issue but we are confident we can crack it."