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))|where dt1 and dt2 are the intervals of time, determined by the sensor, taken by a pulse-echo to transit the distances R1 and R2 respectively. This apparatus may also be used to determine sonic velocity, Sv from: Sv=|2D/(dt2-dt1)|.