The satellite-borne pulsed radar altimeter is in current use in providing practically continuous records of the height of certain satellites above the ocean surface, with a precision approaching 10-20 cm. This precision is largely wasted unless the geocentric position of the satellite is also monitored to a similar accuracy by ground-based laser tracking facilities. The combination will enable us to monitor the variations of the geocentric ocean surface in space and time within the range of the tracking centres. Much of the variation is due to the shape of the geoid, of great interest in itself, but detection of dynamic departures from the geoid, possible at decimetre accuracy, opens up new possibilities in ocean exploration. The greatest benefits will be to the determination of ocean (and Earth) tides, the open-sea behaviour of storm surges, and changes in topography due to varying patterns of circulation.