How do motion detectors determine the distance and velocity of a moving object?
Motion detectors use either sound or radio (or light) waves to detect the motion of an object. The waves are reflected from the object and the reflected wave detected. If the waves are sent in pulses, the time between emitting and receiving a pulse depends on the distance. If the velocity of the wave is c and the detected time interval is t, the distance to the object is c*t/2. The speed of the object is most often determined by doppler effect on a continuous wave. The reflected wave will be detected at a different frequency than the transmitted wave. If the object is approaching, the detected wave frequency will be higher than that emitted, and if going away, lower. Detectors for both speed and distance can be done with pulse modulation of a continuous wave.
