Surface Simpli fi cation Inside a Tolerance Volume

An improvement of a method for measuring distance to target using transmitted and reflected frequency-modulated continuous waves is provided. A low frequency wave produced by mixing a transmitted wave and a wave reflected from the target is separated into frequency components such as a Doppler signal component, a fundamental wave component and its harmonic components. Then, Doppler signals are obtained from the fundamental wave and harmonic components. The ratio in intensity between at least two Doppler signals is calculated to measure the distance to the target. Since the phase of Doppler signal is reversed depending upon whether the target is moving toward or away from the distance measuring apparatus, the direction of the relative movement between the target and the apparatus may be detected. Furthermore, the relative speed between the target and the distance measuring apparatus may be detected from the frequency of each Doppler signal. When the ratio of the distance to the target to the relative speed reaches a predetermined value, a command signal to actuate a device is produced.