Passive Coherent Location (PCL)
The Ukranian Topaz "Kolchuga", the Czech "Kopac", "Ramona" and "Tamara MCS-93" systems as well as the Russian "VERA-E" and "BORAP" systems are all examples of "passive radar" which isn't really a radar at all but a listening device. The Tamara radar system was alleged to have been responsible for the loss of an F-117 in Kosovo in 1999. This was also the subject of a panic in 2002 when it was suspected two systems were among the many fiendish weapons that Saddam Hussein had acquired. The Vera surveillance radar, which is said to be "the only system that can detect the "invisible" U.S. Stealth bomber" is the successor to Tamara. Vera uses "passive location," a unique way of identifying a target without sending out a signal. The technology allows the radar system to be unidentifiable and thus unjammable. Vera can reportedly simultaneously monitor up to 200 aircraft, and it is able to precisely determine their distance and altitude. The ERA company had received orders for Vera from many countries, including Pakistan, China, Malaysia and Egypt. US surveillance and flight tracking developer Rannoch Corporation has taken over ERA, the Pardubice-based company that developed Vera.
Passive Coherent Location (PCL) is a passive radar system, where 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 bi-static range of the object to be determined. In addition to bi-static range, passive radar will typically also measure the bi-static 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 bi-static range, Doppler and bearing and hence significantly improve the final track accuracy.
The PCL system includes bi-static radar, which measures the elliptical distance and the Doppler frequency shift. It works with CW transmitters of opportunity, meaning that it uses electromagnetic radiation, primarily assigned for another purpose, for example, radio or television terrestrial broadcasts. It is necessary to detect at least two (in an ideal case three or more) direct signals from transmitters for a proper determination of a target position.
From the principles of PCL methodology all targets interacting with the electromagnetic field (having the wavelengths of interest) are detectable. In practice, all flying objects heavier than air are built from some kind of metal materials, carbon composites or at least include some metal parts or wires thereby meeting this requirement.
A PCL system should be able to detect extremely small signal changes scattered by targets, which are produced by an interaction of the transmitted signal of opportunity and the target. These changes in the scattered signals are about 100 dB or so lower than the direct signal. Moreover, ground clutter is present in the received signals at about 10 to 50 dB higher than the reflected signals, depending on the terrain.
As of December 2006, there are several PCL systems in various stages of development or deployment, as follows:
- Silent Sentry is a Lockheed Martin (USA) PCL system that uses FM radio transmissions. Two different antenna variants are believed to be available providing an antenna that provides 360.degree. azimuthal coverage from 4 different beams (an Adcock array), and a variant that provides 100.degree. azimuthal coverage from six different beams (linear array). It has a range of up to around 100 nautical miles depending on the variant employed and a number of receive nodes at different locations can be combined to provide increased coverage.
- Celldar is a British system developed jointly by Roke Manor and BAE Systems. The system is a PCL sensor that can exploit GSM signals, currently in the 900 MHz band, but may also be able to use the 900 MHz and 1800 MHz bands simultaneously in the future. The sensor can track targets in 2D over a 100.degree. sector at ranges of up to around 60 km. Celldar is a low level/surface surveillance system designed to achieve good coverage below 10,000 ft.
- CORA is a German PCL sensor, developed by FGAN (Die Forschungsgesellschaft fur Angewandte Naturwissenschaften e.V.), that exploits Digital Video Broadcast--Terrestrial (DVB-T) and Digital Audio Broadcast (DAB) transmissions.
- Cristal is a PCL sensor developed by Thales that exploits FM radio transmissions to track targets. In addition to Cristal, it is believed that Thales has a prototype PCL system that uses analogue TV or DAB transmissions.
- One of the PCL systems developed by ERA Systems Corporation uses an eight-element circular antenna array. The system uses an analog VF block (receiver and beam forming network circuits) with high dynamic range and linearity. The digitizer block uses 24-bit high quality A/D converters (up to 100 kHz BW). The DSP engine block uses a parallel multiprocessor cluster.
- It was reported back in 1999 that China was close to deploying an anti-aircraft defence system that uses technology so advanced it can track even stealth-type warplanes. According to a Newsweek article, US analysts expressed worries that the new early-warning defence system could defeat current US Air Force tactics against enemy air defences. The Newsweek article went on to state that current defences use radar to track incoming aircraft, but outgoing signals can be found and jammed or destroyed. The "passive" technology the Chinese are believed to have, detects aircraft by monitoring disruptions in commercial radio and TV signals, and are essentially undetectable, Newsweek reported. The technology, which could detect US stealth aircraft, including the F-117 bomber and perhaps even the futuristic F-22 fighter, has so alarmed the defense community that top military and industry experts have been called to discuss the strategic implications. At the time, the military in Taiwan, however, said it was unlikely the development of PCL technology by China has been taken beyond the theoretical stage.
PCL represents a technological challenge, which must be handled both from a precise antenna and receiver technique point of view, but mainly through the use of sophisticated digital signal processing methods. Consequently, before deploying a system it is important to make detailed simulations to evaluate known effects and processes influencing PCL system functionality. An extremely high sensitivity is allowed by a relatively long signal integration period (0.1-1.0 sec) for coherent processing of CW signals. Detection of these signals, which are lower by 100 dB, is possible due to enhanced antennas and signal processing.
Conventional aircraft stealth technology is not designed for the frequency bands used by PCL. Stealth technology is designed to minimize the radar cross section (RCS) and is based on achieving an anti-reflective coverage of the aircraft surface and the fuselage shaping from a mono-static RCS point of view. These stealth capabilities are lost at the lower frequency bands, such as VHF for FM radio. Compared to conventional primary active radar, PCL relies on a significantly lower EIRP. This handicap is equalized through the use of a long integration interval in the correlation detector in the receiver, because the reference signal is present with a sufficient signal-to-noise ratio. Weather conditions like rain, snow, hailstones, fog and the like, have only a negligible influence in the frequency bands used by PCL.
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