gambit
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It is called the Radar Detection of Agitated Metals (RADAM) effects...Secondary systems: Google "NCTR" - Non Cooperative Target Recognition. Used to be a very deep secret but to my great surprise was published in Aviation Week and other tech journals many years ago.
The story goes something like this...
"An experienced F-15 pilot was visiting Hughes, the guys who make the APG-63 radar. He was talking to a technician, who was looking at the waveform of a radar return. He noticed a tight series of spikes, and asked 'what are these?' 'Oh, those are the harmonics created by the rotating parts of the engine, both compressor and turbine. They vary from engine to engine. We normally filter them out as garbage.' Eureka. They created a software algorithm and database of Russian and also NATO engines. This sort of NCTR was considered a positive ID and allowed for BVR shots.
Just typing this gives me the creeps, but it is open source stuff.
NCTR in the F-15 (for example) is a technical, software-based system that analyses the returns bounced back from fan blades and turbine blades. Granted, in something like the F-22, NCTR may be altogether more sophisticated and involve multiple sensors and techniques, but that has nothing to do with JEM.
JEM, as described by USAF technical manuals, involves: "modulations from moving parts on the target, such as engine turbine blades, which can create higher order doppler returns from front or tail aspect targets which result in detections exhibiting false velocity information". JEM tracks are normally resolved automatically by the radar and data associated with them are discarded. Thus, JEM has nothing to do with EIDing a contact.
Analysis of radar detection of agitated metals (RADAM)
It has been observed that the radar returns from moving multielement metal targets often exhibit an unexpected modulation that has both random (or noise-like) and semicoherent components.
Analysis of Radar Detection of Agitated Metals (RADAM).
In RADAM algorithm, there is ALWAYS a pairing of signals as in one blade has its opposite, if one blade is traveling away from the observer there is one blade traveling towards the same observer -- Doppler shifts. If one element exhibit a phase shift, its opposite will exhibit an opposite phase shift. And so on. The key factor in RADAM algorithm is this pairing such as in wheel spokes, jet engine turbines, and even a walking human with rhythmic moving arms and legs. The higher the metal content the more prominent these patterns. The faster the movements the more prominent these patterns. RADAM algorithms are incorporated into high-end radar systems that can singled out helos even among strong ground clutter.Abstract : The rapid discontinuous fluctuations observed in the scattering from a moving multielement target, known as the intermittent-contact RADAM effect, are produced by variations in the surface currents caused by motion-induced changes in current paths between the target elements. Such a target can be modeled as a loaded scatterer. Tracked vehicles produce very strong and distinctive RADAM modulation at VHF frequencies.