Over on a Finnish aircraft spotting forum, one poster OH7HJ has been using the “Aircraft Scatter” technique to fingerprint individual aircraft (in Finnish, use Google Translate to read in English). Aircraft scatter is a method that can be used to detect aircraft via strong radio signals that are reflected by the aircraft body. OH7HJ shows that each different type of aircraft will present a different reflection intensity at different points of the reflection, allowing each aircraft to be uniquely identified.
In the thread the original poster used a standard hardware radio, but an RTL-SDR dongle or other software defined radio could also be used. He tuned to a strong analogue TV carrier and plotted the audio spectrograph in Spectrum Lab. If analogue TV is no longer available in your country other strong signals such as amateur radio beacons or radar signal carriers could also be used for aircraft scatter.
Below we show a small selection of some of the interesting images from page 9 of the thread, please see the actual thread for the rest. There is also more information and images contained in the other pages of the discussion thread too.