[OldSchool]

I've often wondered is to what extent existing RF noise as "illumination" could be leveraged to effect some kind of passively acquired representation of objects. Resolution would of course be limited by wavelength but improved if the antenna array was physically moving in a describable manner. We had useful SAR during the cold war and orders of magnitude more compute power today. If you know of more efforts along these lines from hacker to research level, please post a link.