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Passive Radar

February 22, 2013

Before broadcast digital television and cable television, everyone received analog television signals from just a few broadcast channels. Most of my generation will remember what would happen to the video signal when an aircraft flew nearby. NTSC video was amplitude modulated, so reflected signals from the aircraft would lead to multipath reception that would cause the image to ghost or roll. The audio was usually unaffected, since it was a frequency modulated signal.

Detection of an aircraft through the reflection of signals from a remote transmitter is one type of radar.[1-4] Such bistatic radar systems are unlike conventional, monostatic radar, which has the transmitter and receiver sited at the same place, usually using the same antenna system.[1] Conventional radar systems transmit pulses of electromagnetic radiation, and the transit time of these pulses gives the range to the aircraft.

ALQ-71 Electronic Countermeasures Pod

The Vietnam War era AN/ALQ-71 electronic countermeasures pod was succeeded by the AN/ALQ-99, soon to be replaced by the "Next Generation Jammer."

(U.S. Air Force photo, via Wikimedia Commons.)

Electronic countermeasure (ECM) systems, attached as "pods" on military aircraft, transmit random pulses to confuse such rangefinding (see photo). There are escalating tricks, such as coded pulses and frequency-hopping, that make ECM systems harder and harder to design. As a consequence, money has been spent on stealth technology, instead. Stealth aircraft are designed with a surface topography that reflects very little radiation back to a radar set.

The first stealth fighter, the F-117, was a strange looking bird with flat surfaces at all angles in what would seem to be the Frank Gehry interpretation of what an aircraft should look like. The design was selected to bounce monostatic radar signals away from the aircraft. As a result, the F-117 had only a hundredth the radar cross-section of a conventional fighter aircraft. Advanced manufacturing techniques allowed subsequent stealth aircraft, such as the F-22, to have a more contoured shape.[1]

Lockheed F-117 Nighthawk Stealth Fighter

The first stealth technology fighter, the Lockheed F-117.

(U.S. Air Force photo, via Wikimedia Commons.)

Stealth technology did come with some inherent limitations. You still need engines to fly, and these emit heat, which can be detected. The craft are still optically visible, so missions are limited to night; and, stealth pilots can't use their own radar or radio systems in combat.[1] Bi-static passive radar appears to be another way to defeat stealth technology.[1-3]

As often happens with military technology, an expensive technology such as stealth is often trumped by an inexpensive countermeasure. In the case of stealth aircraft, it's passive radar, and not because passive radar is new. Robert Watson-Watt, who demonstrated the essential radar idea in 1935, detected a bomber aircraft at twelve kilometers using reflected signals from a BBC shortwave transmitter at Daventry.[4] Massachusetts Institute of Technology Aeronautics and Astronautics professor, John Hansman, was quoted in a 2001 MIT publication as saying,
"Some stealth aircraft, like the F-117, are specifically designed to have a low radar cross section to monostatic, or conventional, radars. They are not stealthy to some bi-static configurations."[1]
In 2001, Roke Manor Research, at that time a division of Siemens, announced that it had developed a system for aircraft detection using mobile phone base stations as both transmitters and receivers.[1] This is significant, since cellphone towers generally blanket populated areas; and, since they are already deployed, you wouldn't need to pay for most of your passive radar system. Signal strength is not high, but any aircraft would be quite close to at least some cellphone towers, although the signals could be jammed like any others.[1]

Going back to the original Watson-Watt idea, Lockheed experimented with its Silent Sentry system that used signals from radio and television broadcast stations on VHF frequencies.[1] Such stations broadcast powerful signals, often with effective radiated powers approaching a megawatt. The smaller number of transmitters makes signal analysis easier, and the high powers make effective jamming much less likely. Such a system is also contemplated by Cassidian, a division of the European Aeronautic Defence and Space Company (EADS).[3]

Lest we always focus on the dark side, there are also civilian applications of passive radar technology. The Cassidian system has been proposed as an air traffic control radar system, and it's being demonstrated at the Stuttgart Airport.[3] Such bi-static systems are easier to deploy and maintain.[3]

Antenna for ASR-9 Airport Surveillance Radar

The antenna portion of the ASR-9 Airport Surveillance Radar system.

Note the horn feeds connected by waveguides.

(US Federal Aviation Administration photograph via Wikimedia Commons.)

The passive radar approach has been proposed also by Thales for the air traffic control application in Britain. The idea there is to release the radar spectrum for other uses, such as mobile communications, now up to "5G" performance.[4] The Thales two year feasibility study, which also involves Roke Manor, will use signals from television transmitters.[4]

Enabling technologies for this approach are computer signal processing systems, often reducible to single-purpose chips, and inexpensive electronically-steerable antennas. The system should be able to measure location and speed, the latter through Doppler shift, perhaps more reliably than the present monostatic radar system.[4] Perhaps this would be one reason to keep broadcasting alive in an Internet world.


  1. Tao Yue, "Detection of the B-2 Stealth Bomber And a Brief History on 'Stealth'," The Tech, vol. 121, no. 63 (November 30, 2001).
  2. Arend G. Westra, "Radar versus Stealth - Passive Radar and the Future of U.S. Military Power," Joint Force Quarterly, Issue 55 (Quarter 4, 2009), National Defense University (PDF File). Text only version, from The Free Library.
  3. Ian Steadman, "'Passive radar' could render stealth planes obsolete," Wired (UK), October 1, 2012.
  4. Steve McCaskill and Peter Judge, "Air Traffic Control Systems Could Ditch Radar And Release 5G Spectrum," Tech Week Europe, February 13, 2013.
  5. Airport Surveillance Radar (ASR-11), US Federal Aviation Administration Web Site.

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