I worked on a downstream consumer of a passive radar system years ago and was amazed at its capabilities. We had it set up on a tennis court outside of Farnborough UK during a bi-annual air show and it was able to track all the activities of the various planes going through their routines. And I recall seeing commercial aircraft out in the English channel, 50 miles or so away. I don't know what became of the system, but there's an old marketing brochure online here: http://www.mobileradar.org/Documents/Silent_Sentry.pdf
