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).

I'm super into this idea as a project, can you point me to any design or signal processing info?

A lot of passive radar stuff is ends up export controlled so open source is sparse here

There are a lot of open source projects. And technology (algorithms and receivers) wise it isn't too complicated nor a secret. That stuff is almost 100 years old.

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.

Have a look at this (and other repos/website) https://github.com/30hours/blah2

For room-scale passive radar, search HN for IEEE 802.11bf and WiFi 7 Sensing.

https://github.com/Marsrocky/Awesome-WiFi-CSI-Sensing

> (can amateurs pick up pulsars?)

In theory, yes but it's supposed to be pretty tricky. Since it looks like you're up for making antennas, maybe this is for you:

https://britastro.org/wp-content/uploads/2021/03/SmallApertu...

Vela is the "brightest" pulsar but is only visible in the southern hemisphere. B0329+54 is the "brightest" in the northern hemisphere.

Optical astrophotography has stacking software. Radio astronomy has a counterpart in folding software.

Thank you for the link! A pulsar or something "noisy but distant" in space is my goal, I think. The idea of being able to listen to distant stars is mind blowing to me, just amazing stuff.