Passive radar
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.
This system, called the Klein Heidelberg Parasit or Heidelberg-Gerät, was deployed at seven sites (Limmen, Oostvoorne, Ostend, Boulogne, Abbeville, Cap d'Antifer and Cherbourg) and operated as bistatic receivers, using the British Chain Home radars as non-cooperative illuminators, to detect aircraft over the southern part of the North Sea.
The monostatic systems were much easier to implement since they eliminated the geometric complexities introduced by the separate transmitter and receiver sites.
Winkle was able to home in on carcinotron broadcasts with the same accuracy as conventional radar, allowing the jammer aircraft to be tracked and attacked at hundreds of miles range.
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.
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.
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.
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.
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.
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.
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.
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.
Work has been published that has demonstrated the feasibility of this approach for tracking aircraft using the vision carrier of analogue television signals.
The detection range can be determined using the standard radar equation, but ensuring proper account of the processing gain and external noise limitations is taken.
Most passive radars are two-dimensional, but height measurements are possible when the deployment is such that there is significant variation in the altitudes of the transmitters, receiver and target, reducing the effects of geometrical dilution of precision (GDOP).
Of these, the systems that have been publicly announced include: Several academic passive radar systems exist as well: Research on passive radar systems is of growing interest throughout the world, with various open-source publications showing active research and development in the United States (including work at the Air Force Research Labs, Lockheed-Martin Mission Systems, Raytheon, University of Washington, Georgia Tech/Georgia Tech Research Institute and the University of Illinois), in the NATO C3 Agency in The Netherlands, in the United Kingdom (at Roke Manor Research, QinetiQ, University of Birmingham, University College London and BAE Systems), France (including the government labs of ONERA), Germany (including the labs at Fraunhofer-FHR), Poland (including Warsaw University of Technology).
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.
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.
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.
The University of Strathclyde is developing an in-orbit system to detect and track space debris from small fragments to inactive satellites.
Clemente and Vasile have demonstrated the technical feasibility of the detection of small pieces of debris using a range of existing illuminators and a receiver in Low Earth Orbit.
