This paper describes a method how to classify targets - especially helicopters - by evaluating their vibration signatures. The mechanical vibrations of the targets are mainly caused by movements of engines, turbines, gears, etc.. Different types of targets have different vibration spectra which were measured in fieldtrials by an experimental heterodyne laser radar. The signals show the type characteristic vibration signatures which were evaluated by mathematical algorithms. Therefore classification of non-cooperative targets is feasible.
"Target Classification by Laser Vibration Sensing", Proc. SPIE 1181, 5th Conf on Coherent Laser Radar: Technology and Applications, (12 December 1989); doi: 10.1117/12.963785; https://doi.org/10.1117/12.963785