"SENSORS: Radar hides signal, penetrates concrete"

Two problems with conventional radar make it unsuitable for many applications: Anyone with a radar receiver can tell when you activate it, and it can't image objects closer than about 100 feet. Granted, radar automatically opens the door for you at the grocery store, and Stealth bombers are supposedly transparent to radar. But the grocery store radar uses a Doppler algorithm that can only sense movement, not make images, and an aircraft can only be made invisible to radar directed at it from the ground. Now Ohio State University electrical engineer Eric Walton claims to solve both problems with $100 worth of parts. Walton's "noise radar" hides its signal in wideband noise, making it undetectable by the enemy, and it can image objects right through concrete walls. By spreading low-level noise across gigahertz of radio spectrum, the noise radar signal becomes undetectable to normal radar receivers, which are designed to look for high-level signals and to filter out weak signals assumed to be noise. Spread-spectrum transmitters and receivers are widely used today, but spread-spectrum receivers cannot decode noise radar signals, because the signals are spread across gigahertz of bandwidth--simply too much territory for the receivers to cover when searching for correlations. In fact, even two of Walton's own noise radar transceivers, sitting side by side, cannot detect each other, because the signals they send out are random and unique. The only receiver that can detect the signal from a noise radar, Walton said, is the very device that sent out the signal in the first place, its unique code being the random signal itself.
Text: http://www.eetimes.com/showArticle.jhtml?articleID=190301115