Germany-based company Airbus Defence and Space (part of the Airbus Group), has made major progress in the development of passive radar for tracking aircraft. This is the third and most recent category of radar to be conceived of. Active or primary radars are the original category of radar – simply put, they emit energy (radio waves), in directional beams, which is reflected off “targets” and then detected by the radar system, allowing the target to be located and tracked. Secondary radars send interrogatory signals, which are answered in a predefined frequency by transponders on board military or civil aircraft. And now there is passive radar.

“In the last 15 or so years, some very clever ideas have been developed concerning the concept of bi-static radars,” points out Airbus DS Electronics and Border Security passive radar programme head Frank Bernhardt. While classic primary radars are mono-static – that is, the transmitter and receiver are co-located (in fact, today, can be on the same antenna), passive radars are bi-static: the transmitters and receivers are in quite separate locations.

There are, in fact, two different concepts of passive radar. One involves using bi-static geometry with cooperative transmitters (that is, the transmitters are being consciously used as part of a bi-static radar system); and second, the use of bi-static geometry with “transmitters of opportunity” (that is, any transmitter available, including those for cellphones, radio stations and TV broadcasts). It is on the second type that the company is focusing its efforts.

One of the future applications of this passive radar research is civilian air traffic control/air traffic management (ATC/ATM) missions. Passive radar can provide 360˚ coverage, all the time, and, hence, can cover a larger volume of sky within a period of time than is usual with rotating primary ATC/ATM radars. It promises to provide much better coverage at lower cost than current systems, as it does away with the need to build additional dedicated active radar antennas.

“Bi-static radar is really different to mono-static radar,” he explains. “We need massive computing power for bi-static. That’s no problem. We now have the computers. Fifteen years ago, it would not have been possible to have passive radar providing real-time tracking at all. You need a combination of receiver/transmitter pairs – more than three such pairs – to make it work.” Over the past 15 years, not only has computing power significantly increased (while costs have come down) but the number of transmitters (most obviously, cell/mobile phone towers but also digital audio and video broadcasting) has also been greatly augmented.

With passive radar, an aircraft is illuminated by radio waves from the various transmitters at the same time and from different angles, which is why more than one receiver is required and why so much computing power is needed to correlate the data from the different receivers. The company now has a fully functioning technology demonstrator system. This has successfully made use of digital audio broadcast and digital video broadcast transmitters (which highly support passive radar), among others. This still needs lots of engineer support, but the intent is to develop a practical, fully autonomous, prototype system by 2018. That will then require certification by the German authorities. Industrialisation, to bring the sys-tem to production status, will take a bit longer.

“Passive radar will never be alone in the world,” notes Bernhardt. “There will always be some active radars. As far as we know, we, at Airbus are pretty much the most advanced in the world, regarding real-time, multiband and multisensor capability.” The company is already receiving expressions of interest from potential customers.

Keith Campbell visited Airbus DS Electronics and Border Security in Ulm, Germany, as a guest of the company.