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Scatterometer Principles

The scatterometer sends microwave pulses to the earth's surface and measure the backscattered power from the surface roughness. The roughness may describe characteristics of polar ice or vegetation over land. Over the ocean, which covers over three-quarters of the earth's surface, the backscatter is largely due the small centimeter waves on the surface. The idea of remote sensing of ocean surface winds was based on the belief that these surface ripples are in equilibrium with the local wind stress. The backscatter depends not only on the magnitude of the wind stress but also the wind direction relative the direction of the radar beam. The capability of measuring both wind speed and direction is the major uniqueness of the scatterometer. Spacebased wind retrieval is largely dependent on an empirical relation between the backscatter and the equivalent neutral winds.

All wind retrieval from past and present scatterometers suffers, at various degrees, ambiguities in wind direction because of the sinusoidal relationship between the backscatter and wind direction. The problem is mitigated by measuring at the same location in different directions. Wind fields from operational numerical weather forecast have usually been used as initial field for the iterative direction choosing procedures. Due the insufficient data, the relation between backscatter and wind vector under extreme conditions (high and low winds) and under heavy precipitation is less well established. Strong winds and high precipitation conditions are prevalent in marine storms, but such conditions constitute only a small portion of the global wind data set.

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