An apparatus for sensing motion having a transmitter for transmitting a carrier signal; a frequency control connected to the transmitter for controlling the frequency of the carrier signal; a first receiver for receiving the reflected transmitted carrier signal; a second receiver for receiving the reflected transmitted carrier signal, the second receiver being placed out of phase by less than a wavelength of the carrier signal from the first receiver; means for subtracting the carrier signal received by the second receiver from the carrier signal received by the first receiver to produce an error signal; wherein when motion is sensed by the apparatus, the error signal moves from zero thereby causing a corrective signal to be generated and sent to the frequency control, the frequency control forcing the error signal to zero.