[bunabhucan]

You could use a focused receiver to look for TV signals being emitted from an object thousands of feet in the air over (say) the ocean off the coast of California. Since you know there is no TV station antenna floating in the sky out there you know you are getting reflected rf from a plane.

Your receiver is "passive" - it doesn't emit a signal - so the attacking enemy has to find it visually. If the enemy bomb all the TV antennas then you need to emit your own signal to generate the reflection.