An instrument that uses radar to determine a vehicle’s (e.g. a satellite) height above the surface and for measuring the height of small objects (e.g. waves, hills) on a planetary surface. In 352 oceanography, the former capability is used to obtain the absolute sea surface height in relation to the geoid, and the latter to gather information about oceanic wave fields. An altimeter works by transmitting an electronic pulse in the microwave frequency to the Earth’s surface. The pulse reflects off the surface and returns to the sensor, with altitude determined from the pulse travel time and from the waveform of the returned pulse.