Tracks in the Airwaves

01 Dec 2005

We live in an invisible fog of radio waves pouring from TV transmitters, mobile phone masts, airport radars and many other sources.

Just like ships at sea, vehicles moving in this electronic ocean create disturbances that can be ‘seen’ if you know where and how to look. The ATC has been investigating a cunning form of ‘passive sensing’ that can track moving objects, covertly, by the way they perturb the airwaves.

It’s been known for a long time that objects moving near radio transmitters affect the way the signals radiate. The first revelation of wartime radar was a blip in radio reception as a Heyford bomber passed close to a BBC transmitter tower. In modern times, the massive increase in the number of transmitters for communications has left almost all corners of the land awash with radio and microwave signals which should allow objects such as aircraft to be tracked over wide areas.

To test this idea out, ATC scientists have been investigating how to take advantage of the numerous commercial transmitters on our shores to track aircraft.

“Passive sensing (radar) using ‘transmitters of opportunity’ is not dissimilar to conventional radar in that the scattered energy from the target is used for target detection.” says Robert Pollard, radar expert at the ATC. “You need to receive two or three different signals such as Digital Audio Broadcast, TV (digital or analogue) and FM radio. These are the best for power and range. For a demonstration, you need a small array of antenna elements that are easily transported, say on the top of a van.” he says.

‘Hitching a ride’ on the output from commercial transmitters means that tracker systems with low power and high efficiency can be made; and there are some other enticing advantages: “Using passive sensing, you don’t reveal your presence as you do with conventional active radar (see the accompanying item in this issue on networked emitter location). You can be completely covert. Also, from a military perspective, because so many transmitters are available across such a wide radio spectrum, it’s difficult for the target aircraft to maintain its stealth. Oh, yes….and you don’t need a transmitting license either!” says Robert.

Once fully developed, this system would allow mobile military forces in enemy territory to form a picture of local air vehicle movements without giving themselves away.

Other, non-military uses being explored are monitoring for air traffic control and tracking movements of vessels in coastal waters.

The ATC has already experimented with signals from a TV transmitter about 35km away to detect light aircraft passing near a TV transmitter near Chelmsford.

Further trials are planned in the south east of England using local Digital Audio Broadcast and analogue signal transmitters. Results from the ATC Passive sensing research have been fed into the Insyte Celldar passive radar demonstrator system. The Celldar concept was developed by Roke Manor Research and uses mobile telephone transmissions for passive radar. Further development with Roke Manor Research is now being led by Insyste.