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1) One technique uses conventional radar to search for discintive patterns of air turbulence, because it's very hard to supress air turbulence.
2) Another technique looks for enegry-drops or shadows in radar illumination reflections. This is done by merging information from several ground-based radars, none of which is capable of detecting the aircraft on its own. So basically it's piecing infromation from different radars to form a big picture of where the plane should be.
3) A passive radar system listens to low-frequency radio waves and utilizes high-speed computers to distinguish signals that are bouncing off moving objects in the air. Serbians may have used low band radars to track and shoot down the F-117 in 1999.
4) Stealth cannot be created in every frequency.