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Hey! You're the resident Forum Space guy.
I have a question... Radio Telescopes act almost exactly like Passive Radar, right? Except that they're pointed towards space, not Earth, and they detect radiation from stars, not from ambient signals within the atmosphere.
So what makes them distinct? The range at which they operate?
Passive radar picks up objects reflecting signals from things such as commercial broadcasts and communications signals.
A Radio Telescope specifically looks for natural emissions. Although you could say they work on a similar principle, a telescope is vastly more sensitive and has pin-point accuracy, while a radar system is a broad-sweep sorta deal.
A radio telescope CAN detect signals from Earth, but it comes through as annoying interference and that's why almost every radio telescope is far from civilization (like the VLA or Arecibo Array), they're also big, REALLY big. The bigger they are, the more signal they can pick up and focus, I dare you to find a passive radar system that's 500m wide~!
("Five-hundred-meter Aperture Spherical radio Telescope", coming soon to a China near you!)