Passive radar

Gizmodo links to an interesting Business Week article on passive radar systems. These radar systems can detect and monitor moving objects (cars, planes, etc.) over hundreds of miles using the reflections of cellular, FM and TV broadcasts off their sides.

Passive or transmission-less radar has the following advantages over conventional radar systems: (1) it is impossible to detect, (2) the radar device can be very cheap since it does not need to incorporate a transmitter, (3) it does not require a license to transmit, (4) it can leverage the high-power, wide-coverage radio transmissions that are already occuring (FM, TV, cellular) as well as the network of already-deployed base-stations and towers.

Applications include surveillance, air- and ground-traffic monitoring, ground-based air defence and airborne applications. The article cites Celldar and Lockheed Martin's Silent Sentry, among others.

[continued below]

Three quotes from the article:

Despite Celldar's military potential, Lloyd predicts the first applications will come in the civilian sector. He says transportation officials are eager to use Celldar to monitor road traffic because it would avoid the expense of installing either sensors in roads or TV cameras overhead. And police cars equipped with Celldar could follow a car driven by a suspected crook or terrorist from a safe distance, without danger of being seen.

Lockheed-Martin's system is dubbed Silent Sentry. Last fall, in a demo for the U.S. Air Force, a third-generation Silent Sentry radar tracked all the air traffic over Washington, D.C., by picking up FM and TV echoes. Because FM and TV transmissions are more powerful than their cell-phone cousins, Silent Sentry can detect planes as far away as 135 miles, roughly 10 times the reach of an individual cell-phone tower.

John D. Sahr, a University of Washington electrical engineer, [...] has operated a passive-radar system unshrouded by military secrecy. It harnesses an FM station's signals to study particles in the ionosphere -- the top layer of the atmosphere, over 300 miles up. Sahr decided to go with passive radar, he says, "because it's incredibly cheap" -- $20,000 vs. $25 million for a comparable active system. "You could probably do an amateur system for under $5,000," Sahr adds. A system for small airports might cost as little as $15,000. That's important because of the 5,280 public airports in the U.S., only about 300 currently have radar.