Passive radar (also referred to as parasitic radar, passive coherent location, passive surveillance, and passive covert radar) is a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals. It is a specific case of bistatic radar – passive bistatic radar (PBR) – which is a broad type also including the exploitation of cooperative and non-cooperative radar transmitters.^[1][2][3]

Conventional radar systems comprise a colocated transmitter and receiver, which usually share a common antenna to transmit and receive. A pulsed signal is transmitted and the time taken for the pulse to travel to the object and back allows the range of the object to be determined.

In a passive radar system, there is no dedicated transmitter. Instead, the receiver uses third-party transmitters in the environment and measures the time difference of arrival between the signal arriving directly from the transmitter and the signal arriving via reflection from the object. This allows the bistatic range of the object to be determined. In addition to the bistatic range, a passive radar will typically also measure the bistatic Doppler shift of the echo and also its direction of arrival. These allow the location, heading and speed of the object to be calculated. In some cases, multiple transmitters and/or receivers can be employed to make several independent measurements of bistatic range, Doppler and bearing and hence significantly improve the final track accuracy.

The term "passive radar" is sometimes used incorrectly to describe those passive sensors that detect and track aircraft by their RF emissions (such as radar, communications, or transponder emissions). However, these systems do not exploit reflected energy and hence are more accurately described as Electronic Support Measure or anti-radiation systems. Well known examples include the Czech TAMARA and VERA systems and the Ukrainian Kolchuga system.

The concept of passive radar detection using reflected ambient radio signals emanating from a distant transmitter is not new. The first radar experiments in the United Kingdom in 1935 by Robert Watson-Watt demonstrated the principle of radar by detecting a Handley Page Heyford bomber at a distance of 12  km using the BBC shortwave transmitter at Daventry.

Early radars were all bistatic because the technology to enable an antenna to be switched from transmit to receive mode had not been developed. Thus many countries were using bistatic systems in air defence networks during the early 1930s. For example, the British deployed the CHAIN HOME system; the French used a bistatic Continuous Wave (CW) radar in a "fence" (or "barrier") system; the Soviet Union deployed a bistatic CW system called the RUS-1, and the Japanese developed a bistatic CW radar called "Type A".

The Germans used a passive bistatic system during World War II. This system, called the Klein Heidelberg Parasit or Heidelberg-Gerät, was deployed at seven sites (Limmen, Oostvoorne, Ostend, Boulogne, Abbeville, Cap d'Antifer and Cherbourg) and operated as bistatic receivers, using the British Chain Home radars as non-cooperative illuminators, to detect aircraft over the southern part of the North Sea.

Bistatic radar systems gave way to monostatic systems with the development of the duplexer in 1936. The monostatic systems were much easier to implement since they eliminated the geometric complexities introduced by the separate transmitter and receiver sites. In addition, aircraft and shipborne applications became possible as smaller components were developed. In the early 1950s, bistatic systems were considered again when some interesting properties of the scattered radar energy were discovered, indeed the term "bistatic" was first used by Siegel in 1955 in his report describing these properties.^[4]

One of the largest and most complex passive radar systems was the UK's RX12874, or "Winkle". Winkle was deployed in the 1960s in response to the introduction of the carcinotron, a radar jammer that was so powerful it appeared to render long-distance radars useless. Winkle was able to home in on carcinotron broadcasts with the same accuracy as conventional radar, allowing the jammer aircraft to be tracked and attacked at hundreds of miles range. Additionally, by indicating the location of the jammer, other radars in the Linesman/Mediator network could reduce the sensitivity of their receivers when pointed in that direction, thereby reducing the amount of jamming received when pointed near the jammer's location.

The rise of cheap computing power and digital receiver technology in the 1980s led to a resurgence of interest in passive radar technology. For the first time, these allowed designers to apply digital signal processing techniques to exploit a variety of broadcast signals and to use cross-correlation techniques to achieve sufficient signal processing gain to detect targets and estimate their bistatic range and Doppler shift. Classified programmes existed in several nations, but the first announcement of a commercial system was by Lockheed-Martin Mission Systems in 1998, with the commercial launch of the Silent Sentry system, that exploited FM radio and analogue television transmitters.^[5]

Passive radar systems have been developed that exploit the following sources of illumination:

• Analog television signals
• FM radio signals
• Cellular phone base stations
• Digital audio broadcasting
• Digital video broadcasting
• Terrestrial High-definition television transmitters in North America
• GPS satellites (GPS reflectometry).

Satellite signals have generally been found more difficult for passive radar use, either because the powers are too low or because the orbits of the satellites are such that illumination is too infrequent. However, there have been significant developments in the area over the past years. The possible exception to this is the exploitation of satellite-based radar and satellite radio systems. In 2011, researchers Barott and Butka from Embry-Riddle Aeronautical University announced results claiming success using XM Radio to detect aircraft with a low-cost ground station.^[6]

In a conventional radar system, the time of transmission of the pulse and the transmitted waveform is exactly known. This allows the object range to be easily calculated and for a matched filter to be used to achieve an optimal signal-to-noise ratio in the receiver. A passive radar does not have this information directly and hence must use a dedicated receiver channel (known as the "reference channel") to monitor each transmitter being exploited, and dynamically sample the transmitted waveform. A passive radar typically employs the following processing steps:

• Reception of the direct signal from the transmitter(s) and the surveillance region on dedicated low-noise, linear, digital receivers
• Digital beamforming to determine the direction of arrival of signals and spatial rejection of strong in-band interference
• Adaptive filtering to cancel any unwanted direct signal returns in the surveillance channel(s)
• Transmitter-specific signal conditioning
• Cross-correlation of the reference channel with the surveillance channels to determine object bistatic range and Doppler
• Detection using constant false alarm rate (CFAR) scheme
• Association and tracking of object returns in range/Doppler space, known as "line tracking"
• Association and fusion of line tracks from each transmitter to form the final estimate of an object's location, heading and speed.

These are described in greater detail in the sections below.

Passive radar performance is comparable to conventional short and medium-range radar systems. The detection range can be determined using the standard radar equation, but ensuring proper account of the processing gain and external noise limitations is taken. Furthermore, unlike conventional radar, the detection range is also a function of the deployment geometry, as the distance of the receiver from the transmitter determines the level of external noise against which the targets must be detected. However, as a rule of thumb, it is reasonable to expect a passive radar using FM radio stations to achieve detection ranges of up to 150 km, for high-power analogue TV and US HDTV stations to achieve detection ranges of over 300 km and for lower power digital signals (such as cell phone and DAB or DVB-T) to achieve detection ranges of a few tens of kilometres.

Passive radar accuracy is a strong function of the deployment geometry and the number of receivers and transmitters being used. Systems using only one transmitter and one receiver will tend to be much less accurate than conventional surveillance radars, whilst multistatic radars are capable of achieving somewhat greater accuracies. Most passive radars are two-dimensional, but height measurements are possible when the deployment is such that there is significant variation in the altitudes of the transmitters, receiver and target, reducing the effects of geometrical dilution of precision (GDOP).

Advocates of the technology cite the following advantages:

• Lower procurement cost
• Lower costs of operation and maintenance, due to the lack of transmitter and moving parts
• Covert operation, including no need for frequency allocations
• Physically small and hence easily deployed in places where conventional radars cannot be
• Rapid updates, typically once a second
• Difficulty of jamming
• Resilience to anti-radiation missiles.

Opponents of the technology cite the following disadvantages:

• Immaturity
• Reliance on third-party illuminators
• Complexity of deployment
• 1D/2D operation, but possible use 2 different systems for 3D (height + range).

Passive radar systems are currently^[when?] under development in several commercial organizations. Of these, the systems that have been publicly announced include:

• Lockheed-Martin Silent Sentry - exploiting FM radio stations ^[8][9]
• BAE Systems CELLDAR - exploiting GSM base stations ^[10][11]
• Selex ES Aulos passive radar ^[12]
• Thales Air Systems Homeland Alerter - FM radio-based system^[13]
• Hensoldt TwInvis [de] multiband passive radar^[14][15]
• Silentium Defence Maverick-M passive radar for tactical surveillance and MAVERICK-S passive radar for surveillance of air and space. ^[16]
• ERA VERA-NG Passive Surveillance ESM Tracker ^[17]
• Daronmont Technologies SECAR-P Passive Radar for Australia^[18]

Several academic passive radar systems exist as well:

• Manastash Ridge Radar^[19]
• Alim radar system

Research on passive radar systems is of growing interest throughout the world, with various open-source publications showing active research and development in the United States (including work at the Air Force Research Labs, Lockheed-Martin Mission Systems, Raytheon, University of Washington, Georgia Tech/Georgia Tech Research Institute and the University of Illinois), in the NATO C3 Agency in The Netherlands, in the United Kingdom (at Roke Manor Research, QinetiQ, University of Birmingham, University College London and BAE Systems), France (including the government labs of ONERA), Germany (including the labs at Fraunhofer-FHR), Poland (including Warsaw University of Technology). There is also active research on this technology in several governments or university laboratories in China, Iran, Russia and South Africa. The low-cost nature of the system makes the technology particularly attractive to university laboratories and other agencies with limited budgets, as the key requirements are less hardware and more algorithmic sophistication and computational power.

Much current research is currently focusing on the exploitation of modern digital broadcast signals. The US HDTV standard is particularly good for passive radar, having an excellent ambiguity function and very high power transmitters. The DVB-T digital TV standard (and related DAB digital audio standard) used throughout most of the rest of the world is more challenging— transmitter powers are lower, and many networks are set up in a "single frequency network" mode, in which all transmitters are synchronised in time and frequency. Without careful processing, the net result for passive radar is like multiple repeaters jammers.

• Anti-radiation missile
• Bistatic radar
• Multistatic radar
• GNU Radio Passive Radar Project
• Multilateration
• Radar tracker
• Semi-active radar homing
• P-18 radar
• YLC-2 Radar
• Stealth aircraft
• VERA passive sensor
• Low-frequency radar
• Signals intelligence
• Kolchuga passive sensor
• HEMPAS-CCIAS Radar
• Elta EL/L-8388 ELINT passive radar