APPARATUS FOR MEASURING THE VERTICAL MOTION OF A FLOATING PLATFORM

An apparatus and method for measuring the vertical motion of a floating platform e.g. a survey vessel, caused by wave action. The apparatus includes a sensor having three accelerometers (11,12,13) arranged on mutually perpendicular axes (Ax,Ay,Az) so that one accelerometer acts in a vertical plane and the other two act in a horizontal plane. Output signals (S1,S2,S3) from the accelerometers are continually sampled through a multiplexer (14) and are passed through an ADC (19) and a shift register (21) to provide output signals for a data processor (D) which provides a signal A indicative of the vertical position of the platform. In the method, the accelerometer output signals (S1,S2,S3) are corrected for offsets by use of a reference signal (So) and the corrected signals are used to derive the signal (A) which may be double integrated to obtain the final signal (A3) relating to vertical displacement.