A radar altimeter is an active, non-imaging remote sensing device. It measures the height of the terrain along the track beneath an air- or spaceborne platform using electromagnetic radiation from the microwave region of the electromagnetic spectrum. Radar altimeters operate similar to laser profilers. Both emit a short pulse of electromagnetic radiation towards the Earth’s surface and detect the time delayed echo. By measuring the time delay and knowing the speed of propagation of the pulse, the range (distance) from the instrument to the surface can be determined. By using the forward motion of the altimeter platform and transmitting a continuous stream of pulses a profile can be built up. If the exact location of the platform as a function of time is known, a surface profile can be generated.
For a high accuracy of the range resolution, a narrow antenna beam is required, which can be achieved either by using large antennas or short radar beams. In the first case, the radar altimeter is beam-limited; in the second case it is pulse-limited. As large antennas are not practical in space, pulse-limited systems are used for space-borne platforms. Pulse-limited altimeters use frequency modulated (chirp) pulses generated by a chirp generator. The accuracy of the measurements also depends on atmospheric transmission effects, as the speed of the electromagnetic radiation traveling at the speed of light will be delayed when passing through the ionosphere and the atmosphere twice. In general, the range resolution of radar altimeters is in the order of a few centimetres.
In the beginning, radar altimeters were used for measurements of surface profiles of the ocean topography to get information about currents, ocean circulation, wind and waves. Another basic application of altimetry were measurements over ice sheets and glaciers, e.g. for mass balance determination. Further application domains are geoid measurements also revealing deep sea trenches and the precise monitoring of satellite orbits.