Normally, to detect an airplane, a radar system sends out radiation, radio waves of a certain frequency, which then reflect off the plane and back to a receiver.


I'm not an expert on stealth technology but the basic idea for a stealth aircraft is to reflect as little radar radiation as possible back to the receiver of the radar system. This is done by using materials on the outside of the plane that absorb the radar radiation and by choosing a shape of the plane that also scatters radiation so it doesn't get back to the radar system.


Traveling at supersonic (greater than the speed of sound) should
not effect stealth technology because we are more concerned about the speed of radar (light), which is much greater than the speed of sound.


Check out this web page for more detail:


http://lycos.howstuffworks.com/question69.htm