An autodyne motion sensor apparatus and method intended for use with automatic doors comprising, in a preferred embodiment, a phase coherent transceiver (11) incorporating dual demodulators in phase quadrature, means for amplification of the demodulated signals (12 and 13), means for converting the amplified demodulated signals to digital form (14 and 15) for input to a microprocessor, a microprocessor (16), provision for externally generated microprocessor interrupts corresponding to the instantaneous position of the door (17), provision for a uniform series of timing interrupts (18) to be applied to the microprocessor, provision for other microprocessor inputs a) characterizing the position and sense of motion of the door and b) parameter values utilized by the motion detection algorithm executed by the microprocessor, and a microprocessor detection signal output used to actuate the controlled door. The post-demodulation functionality of the sensor is determined essentially entirely by a stored program executed by the microprocessor. The signature of the door is recorded by the program when the sensor is powered up or when recalibration is otherwise initiated. Thereafter, the program subtracts the recorded signature of the door from the digital representation of the raw sensor input, thereby rendering the sensor selectively insensitive to the motion of the door while it remains fully capable of detecting the motion of other objects within the field of the sensor.