Passive radar systems (also referred to a passive coherent location and passive covert radar) encompass 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, the latter also including the exploitation of cooperative and non-cooperative radar transmitters. This long range RADAR antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll[1]. RADAR is a system that uses radio waves to determine and map the location, direction, and/or speed... Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. ...

Introduction

Conventional radar systems comprise a collocated 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. Antenna tower of Crystal Palace transmitter, London A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications. ... In radio terminology, a receiver is an electronic circuit that receives a radio signal from an antenna and decodes the signal for use as sound, pictures, navigational-position information, etc. ... A Yagi-Uda antenna An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. ...

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 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. Bistatic range refers to the basic measurement of range made by a radar or sonar system with separated transmitter and receiver. ... Bistatic Doppler shift is a specific example of the Doppler effect that is observed by a radar or sonar system with a separated transmitter and receiver. ...

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 ESM systems. Well known examples include the Czech TAMARA and VERA systems and the Ukrainian Kolchuga system. French ship Monge, specialised in SIGINT In telecommunication, the term electronic warfare support measures (ESM) is the division of electronic warfare involving actions taken under direct control of an operational commander to search for, intercept, identify, and locate sources of radiated electromagnetic energy for the purpose of immediate threat recognition. ... Tamara was the third generation electronic support measures (ESM) system that uses measurements of time difference of arrival (TDOA) of pulses at three or four sites to accurately detect and track airborne emitters by multilateration. ... VERA (in Czech known as radar Věra, Věra being a female first name) is an electronic support measures (ESM) system that uses measurements of time difference of arrival (TDOA) of pulses at three or four sites to accurately detect and track airborne emitters. ... The Kolchuga passive sensor is an ESM system developed in Ukraine. ...

Typical illuminators

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

• Analog television signals
• FM radio signals
• GSM basestations
• Digital audio broadcasting
• Digital video broadcasting
• Terrestrial High-definition television transmitters in North America

Satellite signals have generally been found to be inadequate 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. The possible exception to this is the exploitation of satellite-based radar and satellite radio systems. Analog television (or analogue television) encodes television picture and sound information as an analog signal, that is, by varying the amplitude and/or frequencies of the signal. ... FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity sound over broadcast radio. ... Not to be confused with Get Some Mates The Global System for Mobile Communications (GSM) is the most popular standard for mobile phones in the world. ... Digital Audio Broadcasting or DAB is a technology for broadcasting audio programming in digital form that was designed in the late 1980s but which is now being introduced in many countries. ... Official DVB logo, found on compliant devices DVB, short for Digital Video Broadcasting, is a suite of internationally accepted, open standards for digital television maintained by the DVB Project, an industry consortium with more than 270 members, and published by a Joint Technical Committee (JTC) of European Telecommunications Standards Institute... Projection screen in a home theater, displaying a high-definition television image. ... A satellite radio or subscription radio (SR) is a digital radio that receives signals broadcast by communications satellite, which covers a much wider geographical range than terrestrial radio signals. ...

Principle

In a conventional radar system, the time of transmission of the pulse and the transmitted waveform are 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: A matched filter is obtained by correlating a known signal, or template, with an unknown signal to detect the presence of the template in the unknown signal. ... Signal-to-noise ratio (often abbreviated SNR or S/N) is meaningful both in the context of Electrical engineering and, informally, for Usenet or other newsgroup-like services. ... In signal processing, sampling is the reduction of a continuous signal to a discrete signal. ...

• Reception of the direct signal from the transmitter(s) and from 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 objects location, heading and speed

These are described in greater detail in the sections below. Beamforming is a signal processing technique used to make a collection of fixed simple antennas act like a single, highly focused, movable antenna. ... An adaptive filter is a digital filter that performs digital signal processing and can adapt its performance based on the input signal. ... In statistics, the term cross-correlation is sometimes used to refer to the covariance cov(X, Y) between two random vectors X and Y, in order to distinguish that concept from the covariance of a random vector X, which is understood to be the matrix of covariances between the scalar... Constant false alarm rate (CFAR) detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference. ... A radar tracker is a component of a radar system that aggregates individual radar observations into tracks. ...

Generic passive radar signal processing scheme

Image File history File links Download high-resolution version (496x829, 44 KB) By Paul Howland. ...

Receiver system

A passive radar system must detect very small target returns in the presence of very strong, continuous interference. This contrasts with a conventional radar, which listens for echoes during the periods of silence in between each pulse transmission. As a result, it is essential that the receiver should have a low noise figure, high dynamic range and high linearity. Despite this, the received echoes are normally well below the noise floor and the system tends to be externally noise limited (due to reception of the transmitted signal itself, plus reception of other distant in-band transmitters). Passive radar systems use digital receiver systems which output a digitized, sampled signal. In telecommunication, noise figure (NF) is the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature (usually 290 K). ... Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity. ... The word linear comes from the Latin word linearis, which means created by lines. ... Digitized is a method of creating sprites to games using live video footage. ... In signal processing, sampling is the reduction of a continuous signal to a discrete signal. ...

Digital beamforming

Most passive radar systems use simple antenna arrays with several antenna elements and element-level digitisation. This allows the direction of arrival of echoes to be calculated using standard radar beamforming techniques, such as amplitude monopulse using a series of fixed, overlapping beams or more sophisticated adaptive beamforming. Alternatively, some research systems have used only a pair of antenna elements and the phase-difference of arrival to calculate the direction of arrival of the echoes (known as phase interferometry and similar in concept to Very Long Baseline Interferometry used in astronomy). A giant phased-array radar in Alaska In telecommunication, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and... A Yagi-Uda antenna An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. ... Digitizing, or digitization, is the process of turning an analog signal into a digital representation of that signal. ... Beamforming is a signal processing technique used to make a collection of fixed simple antennas act like a single, highly focused, movable antenna. ... Amplitude monopulse direction finding refers to a common technique employed in radar systems to improve the accuracy with which the direction of arrival of a pulse can be estimated. ... An adaptive beamformer is signal processing system often used with an array of radar antennae (or phased array) in order to transmit or receive signals in different directions without having to mechanically steer the array. ... Phase-interferometry describes a technique that can be used in radar and direction finding applications to accurately estimate the direction of arrival of a signal from the phase difference of the signal measured on two (or more) separated antennas. ... Very Long Baseline Interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy, in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. ...

Signal conditioning

With some transmitter types, it is necessary to perform some transmitter-specific conditioning of the signal before cross-correlation processing. This may include high quality analogue bandpass filtering of the signal, channel equalization to improve the quality of the reference signal, removal of unwanted structures in digital signals to improve the radar ambiguity function or even complete reconstruction of the reference signal from the received digital signal. This article or section is in need of attention from an expert on the subject. ...

Adaptive filtering

The principal limitation in detection range for most passive radar systems is the signal-to-interference ratio, due to the large and constant direct signal received from the transmitter. To remove this, an adaptive filter can be used to remove the direct signal in a process similar to active noise control. This step is essential to ensure that the range/Doppler sidelobes of the direct signal do not mask the smaller echoes in the subsequent cross-correlation stage. An adaptive filter is a digital filter that performs digital signal processing and can adapt its performance based on the input signal. ... Active noise control (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for preventing unwanted sound. ...

In a few specific cases, the direct interference is not a limiting factor, due to the transmitter being beyond the horizon or obscured by terrain (such as with the Manastash Ridge Radar), but this is the exception rather than the rule, as the transmitter must normally be within line-of-sight of the receiver to ensure good low-level coverage. Radio signals, like all electromagnetic radiation, usually travel in straight lines. ...

Cross-correlation processing

The key processing step in a passive radar is cross-correlation. This step acts as the matched filter and also provides the estimates of the bistatic range and bistatic Doppler shift of each target echo. Most analogue and digital broadcast signals are noise-like in nature, and as a consequence they tend to only correlate with themselves. This presents a problem with moving targets, as the Doppler shift imposed on the echo means that it will not correlate with the direct signal from the transmitter. As a result, the cross-correlation processing must implement a bank of matched filters, each matched to a different target Doppler shift. Efficient implementations of the cross-correlation processing based on the discrete Fourier transform are usually used. The signal processing gain is typically equal to the time-bandwidth product, BT, where B is the waveform bandwidth and T is the length of the signal sequence being integrated. A gain of 50dB is not uncommon. Extended integration times are limited by the motion of the target and its smearing in range and Doppler during the integration period. In statistics, the term cross-correlation is sometimes used to refer to the covariance cov(X, Y) between two random vectors X and Y, in order to distinguish that concept from the covariance of a random vector X, which is understood to be the matrix of covariances between the scalar... A matched filter is obtained by correlating a known signal, or template, with an unknown signal to detect the presence of the template in the unknown signal. ... The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... In mathematics, the discrete Fourier transform (DFT), sometimes called the finite Fourier transform, is a Fourier transform widely employed in signal processing and related fields to analyze the frequencies contained in a sampled signal, solve partial differential equations, and to perform other operations such as convolutions. ...

Target detection

Targets are detected on the cross-correlation surface by applying an adaptive threshold, and declaring all returns above this surface to be targets. A standard cell-averaging constant false alarm rate (CFAR) algorithm is typically used. Constant false alarm rate (CFAR) detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference. ...

Line tracking

The line-tracking step refers to the tracking of target returns from individual targets, over time, in the range-Doppler space produced by the cross-correlation processing. A standard Kalman filter is typically used. Most false alarms are rejected during this stage of the processing. The Kalman filter is an efficient recursive filter which estimates the state of a dynamic system from a series of incomplete and noisy measurements. ...

Track association and state estimation

In a simple bistatic configuration (one transmitter and one receiver) it is possible to determine the location of the target by simply calculating the point of intersection of the bearing with the bistatic-range ellipse. However, errors in bearing and range tend to make this approach fairly inaccurate. A better approach is to estimate the target state (location, heading and speed) from the full measurement set of bistatic range, bearing and Doppler using a non-linear filter, such as the extended or unscented Kalman filter. The ellipse and some of its mathematical properties. ... A non-linear filter is a signal-processing device whose output is not a linear function of its input. ... The Kalman filter is an efficient recursive filter which estimates the state of a dynamic system from a series of incomplete and noisy measurements. ...

When multiple transmitters are used, a target can be potentially detected by every transmitter. The return from this target will appear at a different bistatic range and Doppler shift with each transmitter and so it is necessary to determine which target returns from one transmitter correspond with those on the other transmitters. Having associated these returns, the point at which the bistatic range ellipses from each transmitter intersect is the location of the target. The target can be located much more accurately in this way, than by relying on the intersection of the (inaccurate) bearing measurement with a single range ellipse. Again the optimum approach is to combine the measurements from each transmitter using a non-linear filter, such as the extended or unscented Kalman filter.

Narrow band and CW illumination sources

The above description assumes that the waveform of the transmitter being exploited possesses a usable radar ambiguity function and hence cross-correlation yields a useful result. Some broadcast signals, such as analogue television, contain a structure in the time domain that yields a highly ambiguous or inaccurate result when cross-correlated. In this case, the processing described above is ineffective. If the signal contains a continuous wave (CW) component, however, such as a strong carrier tone, then it is possible to detect and track targets in an alternative way. Over time, moving targets will impose a changing Doppler shift and direction of arrival on the CW tone that is characteristic of the location, speed and heading of the target. It is therefore possible to use a non-linear estimator to estimate the state the of the target from the time history of the Doppler and bearing measurements. Work has been published that has demonstrated the feasibility of this approach for tracking aircraft using the vision carrier of analogue television signals. However, track initiation is slow and difficult, and so the use of narrow band signals is probably best considered as an adjunct to the use of illuminators with better ambiguity surfaces. This article or section is in need of attention from an expert on the subject. ... Time-domain is a term used to describe the analysis of mathematical functions, or real-life signals, with respect to time. ... A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ... A carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) to represent the information to be transmitted. ... It has been suggested that this article or section be merged with estimation theory. ... Analog television encodes picture information by varying the voltages and/or frequency of the signal. ...

Performance

Passive radar performance is comparable to conventional short and medium range radar systems. 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, 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 kilometers. This long range RADAR antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll[1]. RADAR is a system that uses radio waves to determine and map the location, direction, and/or speed...

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 systems are capable of achieving somewhat greater accuracies. Most passive radars are two-dimensional, but height measurements are possible when the deployment is such there is significant variation in the altitudes of the transmitters, receiver and target, reducing the effects of geometrical dilution of precision (GDOP). Dilution of precision (DOP) or Geometric Dilution of Precision (GDOP) is a GPS term used in geomatics engineering to describe the geometric strength of satellite configuration. ...

Advantages and disadvantages

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
• Capabilities against stealth aircraft due to the frequency bands and multistatic geometries employed
• Rapid updates, typically once a second
• Difficulty of jamming
• Resilience to anti-radiation missiles

Opponents of the technology cite the following disadvantages: B-2 Spirit stealth bomber. ... An anti-radiation missile is a missile which is designed to detect and home in on the emissions of an enemy radar installation. ...

• Immaturity
• Reliance on third-party illuminators
• Complexity of deployment
• 2D operation

Commercial systems

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

• Lockheed-Martin's Silent Sentry - exploiting FM radio stations [1][2][3]
• BAe System's CELLDAR - exploiting GSM basestations [4][5]
• Thales Air Systems' Homeland Alerter - FM radio based system

Current research

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 FGAN-FHR). There is also active research on this technology in several laboratories in China and Russia. The low cost nature of the system make the technology particularly attractive to university laboratories and other agencies with limited budgets, as the key requirements are less hardware and more algorithmic sophistication. Raytheon Company (NYSE: RTN) is a major United States military contractor based in Waltham, Massachusetts. ... Georgia Institute of Technology The Georgia Institute of Technology, or Georgia Tech, is located in Atlanta, Georgia, U.S.A. With over 16,000 students, Georgia Tech is one of four public research universities in the University System of Georgia. ... The Georgia Tech Research Institute (GTRI) is the nonprofit applied research arm of the Georgia Institute of Technology in Atlanta, Georgia. ... QinetiQ (LSE: QQ.) (pronounced kÄ­-nÄ•tÄ­k, as in kinetic energy) is a British defence technology company, formed from the greater part of the former government agency DERA when it was split up in June 2001 (with the smaller part becoming Dstl). ... BAE Systems plc is the worlds fourth largest defence contractor [2] and a commercial aerospace manufacturer. ... The Office National dÉtudes et de Recherches Aerospatiales (National Office of Aerospatial Studies and Research, ONERA) is a French national research institution dedicated to the study of aerospace problems (flow mechanics, materials and structures, information processing, physics). ...

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 a passive radar is like multiple repeater jammers! High-definition television (HDTV) means broadcast of television signals with a higher resolution than traditional formats (NTSC, SECAM, PAL) allow. ... DVB-T stands for Digital Video Broadcasting - Terrestrial and it is the DVB European consortium standard for the broadcast transmission of digital terrestrial television. ... DAB may refer to: Digital Audio Broadcasting Dave & Busters, a chain of restaurants in the United States Daytona Beach International Airport (IATA airport code: DAB) in Daytona Beach, Florida, United States Defense Acquisition Board, a Pentagon group that reviews large proposed purchases by the U.S. Department of Defense... Radar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information. ...

Target imaging

Researchers at the University of Illinois at Urbana-Champaign and Georgia Institute of Technology, with the support of DARPA and NATO C3 Agency, have shown that it is possible to build a synthetic aperture image of an aircraft target using passive multistatic radar. Using multiple transmitters at different frequencies and locations, a dense data set in Fourier space can be built for a given target. Reconstructing the image of the target can be accomplished through an inverse fast Fourier transform (IFFT). Herman, Moulin, Ehrman and Lanterman have published reports based on simulated data, which suggest that low frequency passive radars (using FM radio transmissions) could provide target classification in addition to tracking information. These Automatic Target Recognition systems use the power received to estimate the RCS of the target. The RCS estimate at various aspect angles as the target traverses the multistatic system are compared to a library of RCS models of likely targets in order to determine target classification. In the latest work, Ehrman and Lanterman implemented a coordinated flight model to further refine the RCS estimate. - see [6]. The University of Illinois at Urbana-Champaign, also known as UIUC and the U of I (the officially preferred abbreviation), is the flagship campus in the University of Illinois system. ... The Georgia Institute of Technology, or Georgia Tech, is a public, coeducational university located in Atlanta, Georgia, USA, and part of the University System of Georgia. ... The Defense Advanced Research Projects Agency (DARPA) is an agency of the United States Department of Defense responsible for the development of new technology for use by the military. ... The NATO Consultation, Command and Control Agency (NC3A) was formed in 1996 from the merging of the previous SHAPE Technical Centre (STC) in The Hague, The Netherlands and the NATO Communications and Information Systems Agency (NACISA) in Brussels, Belgium. ... A fast Fourier transform (FFT) is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse. ...

History

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 a BBC shortwave transmitter. 1935 (MCMXXXV) was a common year starting on Tuesday (link will take you to calendar). ... Sir Robert Alexander Watson-Watt, FRS (April 13, 1892–December 5, 1973), is considered by many to be the inventor of radar. (Radar development was first started elsewhere - see History of radar). ... The Handley Page Heyford was a British biplane bomber of the 1930s. ... The British Broadcasting Corporation, invariably known as the BBC (and also informally known as the Beeb or Auntie) is the largest broadcasting corporation in the world, employing 26,000 staff in the UK alone and with a budget of £4 billion. ... A solid-state, analog shortwave receiver Shortwave radio operates between the frequencies of 2,310 kHz and 30 MHz (30,000 kHz) [1] and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly...

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 simply "Type A". This article or section does not cite its references or sources. ... Chain Home was the codename for the ring of coastal radar stations built by the British during World War II. The system comprised two types of radar: the metre-wave Chain Home stations which provided long-range early warning, and the centimetre-wave Chain Home Low stations, which were shorter... A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ...

The Germans used a passive bistatic system during World War II. This system, called the Kleine Heidelberg device, was located at Ostend and operated as a bistatic receiver, using the British Chain Home radars as non-cooperative illuminators, to detect aircraft over the southern part of the North Sea. Combatants Major Allied powers: United Kingdom Soviet Union United States Republic of China and others Major Axis powers: Nazi Germany Italy Japan and others Commanders Winston Churchill Joseph Stalin Franklin Roosevelt Harry Truman Chiang Kai-Shek Adolf Hitler Benito Mussolini Hideki Tojo Casualties Military dead: 17,000,000 Civilian dead... Chain Home was the codename for the ring of coastal radar stations built by the British during World War II. The system comprised two types of radar: the metre-wave Chain Home stations which provided long-range early warning, and the centimetre-wave Chain Home Low stations, which were shorter...

Bistatic radar systems gave way to monostatic systems with the development of the synchronizer 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 Seigel in 1955 in his report describing these properties. Synchronization is a problem in timekeeping which requires the coordination of events to operate a system in unison. ...

Experiments in the United States led to the deployment of a bistatic system, designated the AN/FPS-23 fluttar radar, in the North American Distant Early Warning (DEW) line. The fluttar radar was a CW fixed-beam bistatic fence radar developed in 1955 to detect penetration of the DEW line by low-flying bombers. The fluttar radars were designed to fill the low-altitude gaps between SENTINEL monostatic surveillance radars. Fluttar radars were deployed on the DEW line for approximately five years. A rough map of the three warning lines The Distant Early Warning Line, also known as the DEW Line or Early Warning Line, was a system of radar stations in the far northern Arctic region of Canada, with additional stations along the North Coast and Aleutian Islands of Alaska, in...

The rise of cheap computing power and digital receiver technology in the 1980's 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.

See also

• Bistatic radar
• Multilateration

Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. ... Multilateration, also known as hyperbolic positioning, is the process of locating an object by accurately computing the time difference of arrival (TDOA) of a signal emitted from the object to three or more receivers. ...

External links

• Simple example of passive radar using analogue TV
• A recording of the 2004 Watson-Watt Lecture at the UK Institution of Electrical Engineers (IEE) can be seen at the IEE website, which was on the subject of "Passive Covert Radar: Watson-Watt's Daventry Experiment Revisited". This includes a summary of the work in this field since World War II.
• A recording of a briefing on "The Role of Passive Radar Sensors for Air Traffic Control" at an IEE seminar from June 2006 can seen here.
• A recording of a briefing on "PCL Radar Tracking" at an IEE seminar from June 2006 can seen here.