The communications problem centres on the need, initially at least,
for voice messaging systems at the Swanwick site to link into
legacy equipment at the existing London Air Traffic Control Centre
at West Drayton near Heathrow
An explanation:
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."