Microwave sensor

A microwave Doppler intruder sensor transmits on two frequencies and homodynes the transmitted and received signals to obtain two Doppler IF signals. The relative phase of the signals indicates the range of the intruder. In order to discriminate against moving foliage, sensor vibrations etc., only signals from a target that moves by more than a threshold in a predetermined time are recognised as being from intruders. The threshold is adjusted up or down depending on the number of target recognitions in a further predetermined time.