Do stealthed aircraft tend to break Mach 1? If so, it's 100% useless. If not then it's only useful for warning if the plane is slow. I'd be more interested to know if there's a way to set microphones/pressure sensors up on planes in the air to detect changes to the air around them such as the wake of a plane that's passed. Then you could mount them on a fast plane and have it canvass sensitive areas. Again, it probably wouldn't be good for catching/stopping the stealthed plane, but at least you'd know if it'd been there.