cosmic background radiation from the big bang is spread across the entire sky.when a stealth aircraft flies through it, it blocks out the radiation. this makes a moving hole in the radiation.with the proper-receiving equipment, it is posible to detect, track, and target an aircraft.
This method of detecting aircraft is by the missing cosmic background radiation that they block out. This raditation is normaly spread across the entire sky; except when something blocks it out such as a aircraft flying in front of it. It works like a radar but it does not send out a signal.