Start detecting signal reflections from airplanes.
With multiple synchronized receivers you can build a passive radar.
Single reflections can easily be spotted by just staring at the spectrum (Doppler).
With multiple synchronized receivers you can build a passive radar.
Single reflections can easily be spotted by just staring at the spectrum (Doppler).
An easy start is always looking at VHF reflection of strong transmitters that are not creating a lot of noise (like FM stations). ILS or VORs stations are classics.
There are also a lot of meteor and space radars ( https://en.wikipedia.org/wiki/GRAVES_(system) ). Universities are super happy in publishing very very detailed (https://www.iap-kborn.de/en/research/department-radar-remote...) specs of their sky radars. And frequencies are well known for ages.