Radar Altimeters
type: Concept

ALTIMETERS
type: Concept
change note: 2021-08-04 17:01:28.0 [sritz] insert AltLabel (id: null category: primary text: ALTIMETERS language code: en);
definition: The altimeter is a 13.5 GHz near-nadir pointing radar which measures the range to the surface of the ocean. The height measurement from the altimeter to the ocean surface is useful in various oceanographic applications. Determination of sea surface topography is useful for marine gravity determinations, seafloor bathymetry estimations, and dynamic circulation of the oceans. In addition, wave height and wind speed can be measured by altimeters. The characteristics of the radar return signal are determined by the ocean surface. The wave height is found from the sharply rising leading edge slope of the return wave form, and the amplitude of the ocean return signal is a measure of the backscatter coefficient which is related to wind speed. The earliest returns from the transmitted radar pulse come from the wave crests with contributions increasing with time from reflectors deeper in the waves. The plateau region of the wave form, which is at a fairly constant level is reached at about the time that the first reflectors from the wave troughs are received. As a result the return signal has a steep slope in low sea states and a relatively gentle slope in high wave heights. The mathematical model relating the shape of the leading edge to significant wave height assumes that the sea surface is Gaussian and linear. Amplitude of the ocean return signal is normalised by an automatic gain control (AGC) loop. This AGC setting is a measure of the backscatter coefficient (radar cross-section, s0) at the surface which depends on wind speed.
in scheme: instruments