Apparatus is described for accurately determining the elevation, E, of a pulse-echo ultrasonic range sensor (2) relative to a datum level (14). The figure shows vertical elevations of the apparatus with plate deployed at level-1 (12) and level-2 (13) above a datum level (14), the levels being separated by a known distance, D. The elevation of the sensor is given by the relationship:R1=D|(dt1/(dt2−dt1))|