By Tom Symonds
Transport Correspondent, BBC News, Heathrow Airport
The radar system is in operation at Heathrow
Heathrow Airport has unveiled a new radar system designed to spot debris on its runways that can delay plane take-offs and disrupt passengers' journeys.
The twin radar towers scan the runway so closely they can spot an object the size of a cotton reel in amongst the jet planes on a two and a half mile stretch of tarmac.
The system, called Tarsier, and developed by the former government research arm QinetiQ, is now fully operational on the airport's southern runway, and will shortly be installed on the northern runway.
So called foreign object debris regularly closes the two runways at Heathrow. Usually it can be quickly removed, but the airport is close to its full capacity, and any hold-up can cause knock-on delays.
The risk is that even small objects can be sucked into jet engines causing serious damage.
Until now keeping the runways clear of rubbish has been the sole responsibility of checker teams, using brightly painted four-wheel drive vehicles.
The runways are currently patrolled by checker teams
They allowed the BBC to join them as they carried out the day's inspection of the runways, which involved driving in the opposite direction to that in which aircraft were taking off.
The teams monitor air traffic control (ATC) radio broadcasts to the aircraft, trying to cover as much of the runway as possible between takeoffs.
As each airliner begins to move, ATC radios "Checker Vacate" and the driver veers sharply off the runway, bouncing over rough grass, to get clear.
Then it is a question of waiting until the plane has taken off before turning back on to the runway.
At one point the giant A380 double-deck airliner took off a few hundred feet above the checker vehicle.
The team showed us a selection of rubbish found in the last week including a luggage strap, a suitcase lock, and a bolt, origin unknown.
But other checkers have even found a fish on the runway, alive, having been dropped by a bird.
The new radar system is not designed to replace checker patrols, but to augment them by allowing an operator in a control room to see on a screen where debris has been spotted and guide the vehicles in to pick it up.
QinetiQ is also working on cameras with high-specification zoom lenses that will enable the operator to see what is on the runway before the teams arrive.
Generally debris causes nothing more than disruption as take-offs and landings are suspended to allow the patrols to remove it safely.
But in 2000 a twisted piece of metal on a runway at Charles de Gaulle Airport punctured the tyre of a Air France Concorde aircraft, causing it to explode.
Pieces of rubber were flung against the fuel tanks in the wings resulting in a rupture.
As fuel leaked it caught light and, trailing flames, the plane plunged to the ground killing all those on board.
This disaster, which led to the supersonic airliner being withdrawn from service, was the catalyst for the development of the Tarsier radar system.
It has also been installed at Vancouver, Dubai International and Providence Rhode Island airports.
HOW TARSIER RADAR WORKS
Two radar towers perform 180 degree scans of the runway surface
The radar works by comparing a baseline image (1) of a clear runway against the image that it is currently 'seeing' (2). It is programmed to ignore aircraft and ground vehicles
If the radar detects a foreign object, the airfield operations centre is alerted and a runway checker vehicle dispatched to investigate
Later versions of the radar will incorporate hi-res cameras to allow operators to see suspect objects