Director of the National UFO Reporting Center(1), Peter Davenport, discussed his proposal for using "passive" radar technology for detecting UFOs in the near-Earth environment. He also brought along a witness who shared a UFO incident that occurred on June 20, 2004 in Seattle, WA.
The witness, identified only as Jim, reported seeing a fast-moving, triangular-shaped craft in the clear skies above the I-5 corridor. Jim described the UFO as having a black leading edge with a gray interior section containing three lights (view his notes). Davenport also mentioned that a second sighting allegedly occurred over Seattle a few minutes earlier that same evening.
Davenport went on to suggest that passive radar technology could be used to detect any object that reflects radio waves, including UFOs. Passive radar "listens" for signals (from television, radio, and cell phone transmissions) that have been reflected off of something in the atmosphere. Using multiple (three or more) time-synchronized receivers and high-speed computers, he says it is possible to calculate an object's velocity and to discriminate between airplanes, migratory birds... and UFOs. Davenport said the Navy can detect objects the size of grapefruit from 15,000 nautical miles above earth using passive radar, and concluded that their system has very likely tracked UFOs (though the Navy has no official comment about this assertion).
Traditional radar works by sending out radio signals, listening for those signals to bounce off an object (such as a plane) and using the time taken for the round trip to calculate the object's distance. A new type of detection system, called passive radar, does not transmit signals, it only listens for them. By utilizing television, radio, and cell phone transmitters, along with high-speed computers to sort through the clutter, a passive radar system can detect the way moving objects change the surrounding signals.
Some experts believe these systems could be the death knell for stealth aircraft, which can evade conventional radar but show up as "shadows" on passive radar systems. Others worry this technology could be used for tracking people. According to Roke Manor Research(1), developers of CELLDAR (Cellphone Radar System), passive radar technology "can detect vehicles and even human beings at militarily useful ranges." Read more at New Scientist(2).