A NOVEL radar will soon be helping to keep an eye on aircraft flying over London. Conventional radar uses a rotating antenna to sweep the sky, sending out radio pulses and detecting those which are reflected back from aircraft. The experimental system that will begin operation in June does not send out any signals of its own, but instead relies on a network of receivers to pick up television programmes.

It is hard to imagine British TV staples like “Cash in the Attic”, “Downton Abbey” and endless repeats of “Top Gear” being used to detect aircraft. But that, in effect, is what will be happening. By measuring the slight differences between the original broadcast signal and the signals reflected from aircraft flying in the vicinity, it is possible to plot the position of aircraft on a screen, just as is done with conventional radar. The difference is that a system that relies on signals already in the air can be simpler, cheaper and use a lot less power.

The London trial is being run by Thales UK, an engineering group, Roke Manor Research, an R&D consultancy, and NATS, which manages Britain’s air-traffic control. It is backed by the British government’s Technology Strategy Board. The group call their system multi-static primary surveillance radar (MSPSR). It is a form of “passive” radar, which goes back a long time.

Although it had been known since the late 19th century that radio waves can be reflected by objects, it was not until the years leading up to the second world war that a number of countries began secretly developing ways to detect aircraft. Many of the early tests relied on ordinary broadcasts to provide the signals. In an experiment in 1935 a Handley Page Heyford bomber flew between a receiving station and a BBC transmission tower to see how well it could be detected. Well enough, it seems, because the Air Ministry immediately ordered a full demonstration system.

Blip, blip

In 1940 the US Navy coined the term radar to describe “radio detection and ranging”. Modern radar is highly sophisticated but it largely relies on emitting a signal. By measuring the time taken for the reflected signal to return it is possible to work out the position of an aircraft. A slight shift in the frequency, because of the Doppler effect, means its speed and direction can be measured too. (An everyday example of the Doppler effect is how an ambulance siren changes pitch as it passes by.)

Passive radar can take similar measurements and is already used in limited ways, mostly by the armed forces. Relying on background signals to detect objects has an advantage in stealth operations: no pulse is sent out to alert anyone to what you are doing, or attract an anti-radar missile.

The growth in radio and television broadcasts—especially with digital and high-definition TV—now provides an enormous amount of high-frequency radio waves which are ideally suitable for passive radar systems. Moreover, the availability of cheap and powerful computing makes it feasible to analyse the data required to build a system like MSPSR. Thales and its partners expect to be able to produce results as good as conventional radar.

The trials are designed to see how passive radar could support Britain’s air-traffic management. It could help small airports that lack radar or fill gaps in areas where coverage is currently patchy. MSPSR might also reduce the interference caused in some places by wind turbines. And because it is a networked system it could be more reliable than the present set-up, which typically depends on using just one radar at each airport.

The aviation industry is cautious about adopting new technologies wholesale, so there is a long way to go before conventional radar is turned off in favour of passive systems. But governments may be tempted to think about doing so, for reasons that go beyond passive radar’s lower operating costs. With growing demand for wireless devices, passive radar would allow the radio spectrum currently used by conventional radar to be freed up and auctioned off to mobile operators.

One difficulty is that passive radar relies on a third party for its signals. The continued availability of transmissions is considered to be part of Britain’s critical national infrastructure, says John Smith, the project leader for Thales. Nevertheless, he adds, the study will look at how agreements can be made with broadcasters to ensure reliability and performance, and how to deal with routine maintenance when TV transmission towers are turned off. Air-traffic controllers and pilots would not take kindly to being confronted with the equivalent of a TV test card.