Passive radar
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Passive radar uses one or more receivers, but does not have its own transmitter. The receivers are either bistatic or multistatic, as they are not co-located with the transmitter, instead they detect ambient radio signals emanating from nearby radio transmitters.
Introduction
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.The term "passive radar" is sometimes used incorrectly to describe passive Radio Frequency (RF) sensors that detect and track aircraft by the aircraft's own RF emissions (such as radar, communications, or transponder emissions). These systems are more accurately described as ESM systems using time difference of arrival (multilateration) or triangulation processing to locate targets. Well known examples include the Czech TAMARA and VERA systems, as well as the Ukrainian Kolchuga system.
History
A bistatic radar system is one in which there are separate and widely spaced antennas for the transmission and reception of a radar signal. 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 defense 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".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. 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.
Passive multistatic radar systems use the signals generated by non-cooperative transmitters to detect and track targets. The Germans used a passive bistatic system during the Second World War. This system, called KLEINE HEIDELBERG, 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.
Benefits of passive radar
- Cheaper purchase and operations & maintenance costs.
- Covert operation.
- Detect targets continuously, typically once a second.
- May detect some types of stealth aircraft better than conventional radar systems.
- Non-intrusive - No frequency allocation - allowing deployment in areas where normal radars cannot be deployed.
- Physically small and hence easily deployed.
Drawbacks of passive multistatic radar
- A reliance on third-party transmitters, giving the operator little control over the availability of the illuminator.
- Low effective radiated power on many types of transmitters.
- Line of sight is required between the:
- transmitter and the target.
- the target and the receiver.
- the receiver and the transmitter (or a network connection) .
Types
There are many different ways to design passive radars:
- TV broadcasts.
- [Radio - usually] [FM]
- [Cellular].
- Enemy radar systems.
- Space platforms (communications and navigation satellite signals).
The basic principal of operation is to cross-correlate signals from the antenna with a copy of the broadcast signal (which is usually received on a separate, dedicated receiver channel). Because the target is moving, it is actually necessary to cross-correlate the signals with several hundred frequency-shifted replicas of the reference signal, to take account of every potential Doppler shift. This is, in essence, a matched filter bank. Systems must also cancel out unwanted direct signal in the echo channels to prevent the masking of small echo signals. This is normally achieved through appropriate beamforming and adaptive filtering techniques. Having detected targets in range-Doppler space by cross-correlation, sophisticated tracking algorithms are then used for plot-to-target association and to estimate the target location, heading and speed from the measurements.
Unlike conventional radars, passive radars typically have superb Doppler measurements, reasonable range measurements and poor bearing measurements. Targets are primarily resolved in range and Doppler. After signal processing and tracking, radar accuracy is comparable to a microwave surveillance radar.
Radar range is a function of the illuminator being used. Systems exploiting GSM transmitters may only have ranges of the order of 20 km, FM radio stations give around 100–150 km, whereas high power television broadcast stations may have ranges several times that. Passive systems exploiting other radar transmitters will have ranges comparable to the radar being exploited. Systems are typically externally noise limited, rather than internally noise limited. Sources of external noise limitation include the transmitter being exploited itself, other transmitters on the same frequency and cross-channel interference. Clutter and specular returns from large targets can also limit performance.
Passive radar systems can be ground-based and fixed, or deployed on mobile platforms including submarines, ships and aircraft.
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 Labs and other agencies with limited budgets, as the key requirements are less hardware and more algorithmic sophistication.
Much current research is currently focussing 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 through 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!
A recording of the 2004 Watson-Watt Lecture at the UK Institution of Electrical Engineers (IEE) can be [viewed] 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.
The effects of radar cross section reduction
- Purposeful shaping of targets is generally intended to create a blind region in the forward sector within which echoes are not directed back to monostatic radars. The radar energy is deflected in some other direction where a bistatic receiver may have increased signal. This effect would predominate in the bistatic region that may exist between bistatic angles of 50 to 150 degrees. The use of RAM is not effective in the forward scatter region of bistatic angles approaching 180 degrees. Any target, even one that is completely radar absorbent to monostatic radars, produces a strong RCS in the forward scatter region which varies only with the physical size of the target. Thus the forward scatter region at bistatic angles up to 180 degrees may offer significant advantages for bistatic systems. Experiments using generic targets have shown that the bistatic RCS is generally 3–5 dB lower than monostatic RCS due to the loss of corner reflectors and large reflecting surfaces such as turbine blades, but advantages can be seen at specific angles due to shaping. Also, the very large RCS in the forward scatter region is readily apparent. [Measurement and Calculation of the Bistatic Radar Cross Section]. The principal difficulty of exploiting forward scatter in passive radar systems is the very demanding dynamic range requirements on the receiver system, together with the low information content in the range, bearing and Doppler measurements.
- In addition to the tactical advantages of a passive system, the use of low frequency broadcast TV and FM radio signals as the illuminating source produces a much higher RCS than high frequency monostatic radars as the long wavelengths cause whole structural portions of the targets to resonate. Target tracking, in three-dimensional position and velocity should be more accurate with a multistatic system than with a monostatic system, using either triangulation or hyperbolic (or both) target location strategies.
Radar target imaging
- Researchers at the University of Illinois at Urbana-Champaign, with the support of DARPA, have shown that it is possible to build a synthetic aperture image of an aircraft target using passive multistatic radar. Previous researchers had shown that, “under certain assumptions, the data collected at transmitting frequency f is a sample of the Fourier transform of the target reflectivity and is equivalent to a monostatic measurement taken at the bisector of the bistatic angle (β/2) and at a frequency of f cos (β/2).” 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).
- In work supported by DARPA and NATO C3 Agency, researchers at the University of Illinois and Georgia Institute of Technology have been investigating Automatic Target Recognition with passive radars. 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. [ATR]
See also
External links
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