Passive radar
Encyclopedia
Passive radar systems 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.
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 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 ESM
systems. Well known examples include the Czech
TAMARA
and VERA
systems and the Ukrainian
Kolchuga
system.
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 simply called "Type A".
The Germans used a passive bistatic system during World War II
. This system, called the Kleine 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 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 Siegel in 1955 in his report describing these properties.
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.
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.
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. 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.
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:
These are described in greater detail in the sections below.
, 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.
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).
or even complete reconstruction of the reference signal from the received digital signal.
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.
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.
. 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.
(CFAR) algorithm is typically used.
is typically used. Most false alarms are rejected during this stage of the processing.
. 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
.
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.
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.
, 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.
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).
Opponents of the technology cite the following disadvantages:
Several academic passive radar systems exist as well
, 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), Poland (including Warsaw University of Technology). There is also active research on this technology in several government 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 a passive radar is like multiple repeater jammers
!
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.
operate a distributed passive radar exploiting FM broadcasts to study ionospheric turbulence at altitudes of 100 km and ranges out to 1200 km . Meyer and Sahr have demonstrated interferometric images of ionospheric turbulence with angular resolution of 0.1 degree, while also resolving the full, unaliased Doppler Power Spectrum of the turbulence .
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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
Bistatic radar
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. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
, the latter also including the exploitation of cooperative and non-cooperative radar transmitters.
Introduction
Conventional radar systems comprise a collocated transmitterTransmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
, which usually share a common antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
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
Bistatic range
Bistatic range refers to the basic measurement of range made by a radar or sonar system with separated transmitter and receiver. The receiver measures the time difference of arrival of the signal from the transmitter directly, and via reflection from the target...
of the object to be determined. In addition to bistatic range, a passive radar will typically also measure the bistatic Doppler shift
Bistatic Doppler shift
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 Doppler shift is due to the component of motion of the object in the direction of the transmitter, plus the component of motion of the...
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 ESM
Electronic warfare support measures
In military telecommunications, the terms Electronic Support or Electronic Support Measures describe the division of electronic warfare involving actions taken under direct control of an operational commander to detect, intercept, identify, locate, record, and/or analyze sources of radiated...
systems. Well known examples include the Czech
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
TAMARA
Tamara passive sensor
Tamara was the third generation Czechoslovak electronic support measures system that used measurements of time difference of arrival of pulses at three or four sites to accurately detect and track airborne emitters by multilateration. Tamara's serial numbers were KRTP-86 and KRTP-91 and it...
and VERA
VERA passive sensor
VERA -VERA passive radiolocator is an electronic support measures system that uses measurements of time difference of arrival of pulses at three or four sites to accurately detect and track airborne emitters....
systems and the Ukrainian
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
Kolchuga
Kolchuga passive sensor
The Kolchuga passive sensor is an ESM system developed in Ukraine. Its detection range is limited by line-of-sight but may be up to for very high altitude, very powerful emitters...
system.
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 KingdomUnited Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
in 1935 by Robert Watson-Watt
Robert Watson-Watt
Sir Robert Alexander Watson-Watt, KCB, FRS, FRAeS is considered by many to be the "inventor of radar". Development of radar, initially nameless, was first started elsewhere but greatly expanded on 1 September 1936 when Watson-Watt became...
demonstrated the principle of radar by detecting a Handley Page Heyford
Handley Page Heyford
The Handley Page Heyford was a twin-engine British biplane bomber of the 1930s. Although it had a short service life, it equipped several squadrons of the RAF as one of the most important British bombers of the mid-1930s, and was the last biplane heavy bomber to serve with the RAF.-Design and...
bomber at a distance of 12 km using the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
transmitter at Daventry
Daventry
Daventry is a market town in Northamptonshire, England, with a population of 22,367 .-Geography:The town is also the administrative centre of the larger Daventry district, which has a population of 71,838. The town is 77 miles north-northwest of London, 13.9 miles west of Northampton and 10.2...
.
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
Chain Home
Chain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. The system otherwise known as AMES Type 1 consisted of radar fixed on top of a radio tower mast, called a 'station' to provide long-range detection of...
system; the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
used a bistatic Continuous Wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
(CW) radar in a "fence" (or "barrier") system; the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
deployed a bistatic CW system called the RUS-1; and the Japanese developed a bistatic CW radar simply called "Type A".
The Germans used a passive bistatic system during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. This system, called the Kleine 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
Chain Home
Chain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. The system otherwise known as AMES Type 1 consisted of radar fixed on top of a radio tower mast, called a 'station' to provide long-range detection of...
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 synchronizer
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
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.
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
Distant Early Warning Line
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 addition to the Faroe Islands, Greenland, and Iceland...
. 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
MPQ-64 Sentinel
The AN/MPQ-64 Sentinel is a three-dimensional radar used to alert and queue Short Range Air Defense weapons to the locations of hostile targets approaching their front line forces. The Sentinel radar is deployed with forward area air defense units of the U.S. Army and USMC. The radar uses an...
monostatic surveillance radars. Fluttar radars were deployed on the DEW line for approximately five years.
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.
Typical illuminators
Passive radar systems have been developed that exploit the following sources of illumination:- Analog televisionAnalog televisionAnalog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
signals - FM radio signals
- Cellular phone base stations
- Digital audio broadcastingDigital audio broadcastingDigital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
- Digital video broadcasting
- Terrestrial High-definition televisionHigh-definition televisionHigh-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
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
Satellite radio
Satellite radio is an analogue or digital radio signal that is relayed through one or more satellites and thus can be received in a much wider geographical area than terrestrial FM radio stations...
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.
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 filterMatched filter
In telecommunications, 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. This is equivalent to convolving the unknown signal with a conjugated time-reversed version of the template...
to be used to achieve an optimal signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
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
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
the transmitted waveform. A passive radar typically employs the following processing steps:
- Reception of the direct signal from the transmitter(s) and from the surveillance region on dedicated low-noise, linear, digital receivers
- Digital beamformingBeamformingBeamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
to determine the direction of arrival of signals and spatial rejection of strong in-band interference - Adaptive filterAdaptive filterAn adaptive filter is a filter that self-adjusts its transfer function according to an optimization algorithm driven by an error signal. Because of the complexity of the optimization algorithms, most adaptive filters are digital filters. By way of contrast, a non-adaptive filter has a static...
ing to cancel any unwanted direct signal returns in the surveillance channel(s) - Transmitter-specific signal conditioning
- Cross-correlationCross-correlationIn signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long-duration signal for a shorter, known feature...
of the reference channel with the surveillance channels to determine object bistatic range and Doppler - Detection using constant false alarm rateConstant false alarm rateConstant false alarm rate 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.Other detection algorithms are not adaptive...
(CFAR) scheme - Association and trackingRadar trackerA radar tracker is a component of a radar system, or an associated command and control system, that associates consecutive radar observations of the same target into tracks...
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.
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 figureNoise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
, high dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
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
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
signal.
Digital beamforming
Most passive radar systems use simple antenna arraysPhased array
In wave theory, a phased array is an array 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 suppressed in undesired directions.An antenna array...
with several antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
elements and element-level digitisation
Digitizing
Digitizing or digitization is the representation of an object, image, sound, document or a signal by a discrete set of its points or samples. The result is called digital representation or, more specifically, a digital image, for the object, and digital form, for the signal...
. This allows the direction of arrival of echoes to be calculated using standard radar beamforming
Beamforming
Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
techniques, such as amplitude monopulse using a series of fixed, overlapping beams or more sophisticated adaptive beamforming
Adaptive beamformer
An adaptive beamformer is a beamforming system which performs adaptive spatial signal processing with an array of radar antennas in order to transmit or receive signals in different directions without having to mechanically steer the array....
. 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
Very Long Baseline Interferometry
Very Long Baseline Interferometry is a type of astronomical interferometry used in radio astronomy. It allows observations of an object that are made simultaneously by many telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.Data...
used in astronomy).
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 functionRadar ambiguity function
In pulsed radar and sonar signal processing, an ambiguity function is a two-dimensional function of time delay and Doppler frequency\chi showing the distortion of a returned pulse due to the receiver matched filter due to the Doppler shift of the return from a moving target...
or even complete reconstruction of the reference signal from the received digital signal.
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 filterAdaptive filter
An adaptive filter is a filter that self-adjusts its transfer function according to an optimization algorithm driven by an error signal. Because of the complexity of the optimization algorithms, most adaptive filters are digital filters. By way of contrast, a non-adaptive filter has a static...
can be used to remove the direct signal in a process similar to active noise control
Active noise control
Active noise control is a method for reducing unwanted sound.- Explanation :...
. 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.
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
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
of the receiver to ensure good low-level coverage.
Cross-correlation processing
The key processing step in a passive radar is cross-correlationCross-correlation
In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long-duration signal for a shorter, known feature...
. This step acts as the matched filter
Matched filter
In telecommunications, 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. This is equivalent to convolving the unknown signal with a conjugated time-reversed version of the template...
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
Discrete Fourier transform
In mathematics, the discrete Fourier transform is a specific kind of discrete transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function...
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.
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 rateConstant false alarm rate
Constant false alarm rate 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.Other detection algorithms are not adaptive...
(CFAR) algorithm is typically used.
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 filterKalman filter
In statistics, the Kalman filter is a mathematical method named after Rudolf E. Kálmán. Its purpose is to use measurements observed over time, containing noise and other inaccuracies, and produce values that tend to be closer to the true values of the measurements and their associated calculated...
is typically used. Most false alarms are rejected during this stage of the processing.
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 ellipseEllipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
. 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
Non-linear filter
A nonlinear filter is a signal-processing device whose output is not a linear function of its input. Terminology concerning the filtering problem may refer to the time domain showing of the signal or to the frequency domain representation of the signal. When referring to filters with adjectives...
, such as the extended or unscented Kalman filter
Kalman filter
In statistics, the Kalman filter is a mathematical method named after Rudolf E. Kálmán. Its purpose is to use measurements observed over time, containing noise and other inaccuracies, and produce values that tend to be closer to the true values of the measurements and their associated calculated...
.
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 functionRadar ambiguity function
In pulsed radar and sonar signal processing, an ambiguity function is a two-dimensional function of time delay and Doppler frequency\chi showing the distortion of a returned pulse due to the receiver matched filter due to the Doppler shift of the return from a moving target...
and hence cross-correlation yields a useful result. Some broadcast signals, such as analogue television, contain a structure in the time domain
Time domain
Time domain is a term used to describe the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the case of continuous time, or at various...
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
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
(CW) component, however, such as a strong carrier tone
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
, 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
Estimator
In statistics, an estimator is a rule for calculating an estimate of a given quantity based on observed data: thus the rule and its result are distinguished....
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.
Performance
Passive radar performance is comparable to conventional short and medium range radar systems. Detection range can be determined using the standard radar equationRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, 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.
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).
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 aircraftStealth aircraftStealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...
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:
- Immaturity
- Reliance on third-party illuminators
- Complexity of deployment
- 2D operation
Commercial & Academic 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 http://www.lockheedmartin.com/data/assets/10644.pdfhttp://www.dtic.mil/ndia/jaws/sentry.pdfhttp://www.lockheedmartin.com/wms/findPage.do?dsp=fec&ci=17983&rsbci=22&fti=0&ti=0&sc=400
- BAE Systems' CELLDAR - exploiting GSM base stations http://www.roke.co.uk/skills/radar/http://web.archive.org/web/20060308181747/http://www.roke.co.uk/sensors/stealth/celldar.asp
- Thales Air Systems' Homeland Alerter - FM radio based system
Several academic passive radar systems exist as well
- Manastash Ridge RadarManastash Ridge RadarThe Manastash Ridge Radar is a passive radar which uses commercial FM broadcasts to study ionospheric turbulence as well as meteor trails, and aircraft...
http://rrsl.ee.washington.edu/Projects/Manastash/ - International Project for Radio Meteor Observation http://www.amro-net.jp/radio.htm
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, RaytheonRaytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...
, University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
, Georgia Tech
Georgia Institute of Technology
The Georgia Institute of Technology is a public research university in Atlanta, Georgia, in the United States...
/Georgia Tech Research Institute
Georgia Tech Research Institute
The Georgia Tech Research Institute is the nonprofit applied research arm of the Georgia Institute of Technology in Atlanta, Georgia, United States...
and the University of Illinois), in the NATO C3 Agency in The Netherlands, in the United Kingdom (at Roke Manor Research
Roke Manor Research Limited
Founded in 1956, Roke Manor Research Limited is a UK company based at Roke Manor in Romsey, Hampshire. It is a contract research and development business for communications, networks, and electronic sensors. In addition to supporting Chemring, work is also conducted in both the public and private...
, QinetiQ
QinetiQ
Qinetiq is a British global defence technology company, formed from the greater part of the former UK government agency, Defence Evaluation and Research Agency , when it was split up in June 2001...
, University of Birmingham, University College London and BAE Systems
BAE Systems
BAE Systems plc is a British multinational defence, security and aerospace company headquartered in London, United Kingdom, that has global interests, particularly in North America through its subsidiary BAE Systems Inc. BAE is among the world's largest military contractors; in 2009 it was the...
, France (including the government labs of ONERA), Germany (including the labs at FGAN-FHR), Poland (including Warsaw University of Technology). There is also active research on this technology in several government or university laboratories in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. 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
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
digital TV standard (and related DAB
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
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
Radar jamming and deception
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-ChampaignUniversity of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...
and Georgia Institute of Technology
Georgia Institute of Technology
The Georgia Institute of Technology is a public research university in Atlanta, Georgia, in the United States...
, 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
Fast Fourier transform
A fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
(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.
Ionospheric Turbulence Studies
Researchers at the University of WashingtonUniversity of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
operate a distributed passive radar exploiting FM broadcasts to study ionospheric turbulence at altitudes of 100 km and ranges out to 1200 km . Meyer and Sahr have demonstrated interferometric images of ionospheric turbulence with angular resolution of 0.1 degree, while also resolving the full, unaliased Doppler Power Spectrum of the turbulence .
See also
- Bistatic radarBistatic radarBistatic 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. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
- MultilaterationMultilaterationMultilateration is a navigation technique based on the measurement of the difference in distance to two or more stations at known locations that broadcast signals at known times. Unlike measurements of absolute distance or angle, measuring the difference in distance results in an infinite number of...
- Radar trackerRadar trackerA radar tracker is a component of a radar system, or an associated command and control system, that associates consecutive radar observations of the same target into tracks...
- Semi-active radar homingSemi-active radar homingSemi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...
- P-18 radarP-18 radarThe P-18 or 1RL131 is a 2D VHF radar developed and operated by the former Soviet Union.- Development :...
- YLC-2 RadarYLC-2 RadarThe YLC-2 radar is a three-dimensional main guidance and surveillance radar developed by the Nanjing Research Institute of Electronics Technology....
- Stealth aircraftStealth aircraftStealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...
- VERA passive sensorVERA passive sensorVERA -VERA passive radiolocator is an electronic support measures system that uses measurements of time difference of arrival of pulses at three or four sites to accurately detect and track airborne emitters....
- Low-frequency radarLow-frequency radarLow-frequency radar uses frequencies lower than 1 GHz, as opposed to the usual radar bands, which range from 2 Ghz and up,and the maximum is 40 Ghz.The radar cross section of any target depends on the radar transmitted frequency...
- Signals intelligence
- Kolchuga passive sensorKolchuga passive sensorThe Kolchuga passive sensor is an ESM system developed in Ukraine. Its detection range is limited by line-of-sight but may be up to for very high altitude, very powerful emitters...
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.
- Researchers at Embry-Riddle claiming use of XM-Radio to detect aircraft can be found here http://daytonabeach.erau.edu/coe/electrical-computer-software-engineering/research/bistatic-radar.html