Airplane scattering (or most often reflection) is observed on radio waves from high frequency to VHF through to microwaves and, besides back-scattering, yields momentary propagation up to 800 kilometers (500 miles) even in mountainous terrain. The most common back-scatter applications are air-traffic radar, bistatic forward-scatter guided-missile and airplane-detecting trip-wire radar, and the US space radar. It is also a less common propagation mode used by radio amateurs.