Satellite Scatterometry

Scatterometers send a radar pulse from the satellite to the sea surface.  Part of this pulse is reflected from the sea surface and returned to the satellite.  The amount of backscattered energy is measured on board the satellite and relayed to a data collection station on the ground.  The incidence angle of the measurement is the angle between the radar beam emitted by the satellite and the sea surface.

The fraction of energy returned to the satellite is related to wind speed.
 

For weak winds (top):
The sea surface is relatively smooth and most of the radar pulse is reflected at an angle away from the satellite.
 

For strong winds (bottom):
The sea surface is much rougher.  Small ocean waves, with scales of 1-3 cm, reflect the radar pulse in many directions.  Some of this reflected energy is returned to the satellite.
 

Note that wind speed is not measured directly by the satellite.  Scatterometry assumes that there is a well-defined relation between wind speed and the amount of reflected radar energy.  Wind speed is computed based on this reflected energy using a scientifically determined relationship referred to as the wind speed model function.

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