Passive Covert Radar
Human activities generate a great deal of radio energy, as in communications, navigation, and entertainment applications. Some of these sources provide enough energy such that their reflection or transillumination can enable passive covert radar (PSR) MASINT, which is also called passive coherent location (PCL).
A foreign transmitter, preferably a purpose-built radar transmittersuch as used in air traffic control, but really any powerful transmitted such as TV or FM, potentially can produce reflected signals that do not return to the designated receiver of the foreign radar operator. A signal may reflect such that it can be intercepted and fed into a friendly radar receiver, giving at least information on the presence of a radar target illuminated by the foreign transmitter. This is the simple case with the unintended reflection going to a single radar support receiver.
Interferometry is also possible with such systems. This is especially attractive for naval vessels, which, since they often travel in groups, will have different times difference of arrival (TDOA) of the reflections from the foreign receiver. To restate an important difference, basic PCR works with a single radar receiver and conventional display format, from a single reflection. TDOA works with a set of reflections, from the same target, arriving at multiple points. "Passive sensors are shown to make a valuable contribution to the air defence mission."
Another group evaluated the PCR technology in an environment like that of a naval task group Ships have more space, and thus the equipment and power are less limited than for airborne or man-portable systems. This British study tested illumination with a Watchman air traffic control pulse doppler radar, and a Bridgemaster marine radar, against experimental receiver types. The researchers also developed simulations of the system.
Against the marine transmitter, the receiver combined a square-law: Power-level detector with cross-collation of a local copy of the pulse against the received signal. This method improved sensitivity for poorer time resolution, because correlated peaks are twice the width of uncorrelated peaks.
Using the air traffic control illuminator, the receiver used pulse compression filtering of a chirp signal, which provided processing gain along with the ability to separate closely spaced targets. This also implemented a moving target indicator that suppressed clutter, but it was recognized that an MTI signal would not be available in a noncooperative environment. They concluded their work demonstrated feasible convergence of PCR and TDOA, using a shipborne R-ESM system with communications among the receivers, such that the processed signal is an interferometric process.
Read more about this topic: Radar MASINT
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