For the last 30 years stealth technology on planes and ships has brought battleground advantage to the world's leading military powers. Countries like the United States have been able to fly undetected on missions into Iraq, Serbia, Afghanistan, and Iraq (again) to deliver precision strikes to enemy targets. However, new radar technology could render stealth technology obsolete.

Cassidian, part of the massive Europe-wide European Aeronautic Defence and Space Company (EADS), has developed a kind of "passive radar" which it claims can detect stealth aircraft. Passive radar detects radiation signals emitted by other sources -- be they radio broadcasts or mobile phone networks -- and analyses distortions to figure out where objects are located.

Stealth technology conventionally works by minimising the reflective profile of an object, with perhaps the most famous example being the iconic triangular-ish Northrop Grumman B-2 Spirit bomber. Conventional radar systems emit signals which then "bounce" off objects, giving away their location (exactly how a bat "see" in the dark using noise) -- the B-2's shape, wide and shallow with perpendicular surfaces, absorbs those signals (or reflects them away from any detector) to avoid revealing its presence.


The system Cassidian proposes, however, wouldn't be fooled by standard stealth cloaking techniques because it takes advantage of a range of signals which surround us constantly. There's no need to fire out signals and look for their reflections -- instead, the detector system looks at a host of signals floating in the atmosphere already (like aforementioned radio and mobile phone signals) and looks for how they're blocked or altered by having to pass through or around objects. Triangulating several different sources can build up a picture of a landscape or airspace, with stealth planes and ships just as visible as everything else.

Even more worrying for commanders, too, is that because passive radar stations don't emit anything, there's no way to track them down. The tactic of sending in a stealth bomber to take out enemy radar capabilities before sending in the conventional planes wouldn't work -- passive radar detectors can be small and spread out over a large area.

Cassidian also claims that passive radar has applications for civilian air traffic control. Simpler, smaller passive radar stations will be cheaper and easier to set up and maintain than the current systems used at airports around the world. A prototype of the system is currently being tested at Stuttgart Airport, and if it works as well as hoped then we could see it begin to appear at other airports within a couple of years.


Passive radar has started to look like an effective tool in smaller-scale situations -- Wired.co.uk reported on the development of tech that can detect people through walls using the Wi-Fi and other signals that permeate human settlements.

It could have a use in a hostage situation (where the police need to know where the bad guys are inside a sealed room), or the fire service could use it to locate people within buildings filled with smoke.

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