Earth Remote Sensing Instruments
Instrument Class: Active Remote Sensing
Instrument Type: Altimeters
Instrument Subtype: Radar Altimeters
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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.