System and method for detecting the presence of an object in an aperture in which for modulated electromagnetic waves are directed from an edge of a frame defining the aperture and electromagnetic waves reflected from an opposite edge of the frame are received at substantially the same location. The phase change between the modulated electromagnetic waves and the reflected electromagnetic waves is measured wherein the presence of an obstacle in the aperture causes a variation in the phase change from a situation where an obstacle is not present. The system and method can be used in vehicles to determine the presence of obstacles in the path of a closing window, door, trunk lid, convertible top, or sunroof.