Abstract: An apparatus for sensing motion of a surface comprises a stabilized platform comprising a support structure and a moveable mass resiliently suspended with respect to the support structure; a transmitter transducer and a receiver transducer mounted on the moveable mass, the transmitter transducer arranged to transmit an electromagnetic wave towards the surface, and the receiver transducer arranged to receive a reflected wave from the surface, and detection means for measuring motion of the surface based on a Doppler shift in the reflected wave. The detection means may comprise an amplifier arranged to receive a Doppler modulated signal from the receiver transducer, and a phase detector arranged to receive an amplified signal from the amplifier and to provide a demodulated output signal indicative of the motion of the surface.