radioguy22

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).

The RTLSDR is a great tool. But signals really be bandpassed (what the guy in the paper did) because its dynamic range is limit to only 8 Bit.

Yes. But nowing a bit of signal theory, hf electronics and embedded is super helpful.

With 2 antennas you can start playing around with beam forming. This will enable you to scan the sky from one position without moving the antenna. Then you can map the signal strength to a sky projection.

Have a look at the low-frequency antennas from LOFAR. They look like tents for chickens. A lot of them.

It sounds complex and yes.. it's also a bit complex. But still in the range of a project for the home lab.