Someone presented a passive radar system using an RTLSDR receiving TV broadcasts reflected off of nearby planes at a 2017 GNURadio conference. If amateurs can do it, you can bet the military has already designed, built, tested, deployed, and retired a similar system.<p>This isn't exactly a new subject, the same concept powered the first radar guided missiles in the 1940s (enemy is illuminated by a ground or airborne radar, missile tracks the reflection).<p>I'm fairly certain I've already read an article in an IEEE magazine talking about this topic (passive radar via cell towers) a few years ago.