Scatterometry

A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves scattered by diffusion in a medium such as air. Diffusionmeter using visible light are found in airport or along roads to measure the horizontal visibility. Radar scattometers use radio or microwaves to determine the normalized radar cross section (sigma-0) of a surface. They are often mounted on weather satellites to find wind speed and direction, and in industries to analyze surface roughness of surfaces.

Optical diffusionmeters are devices used in meteorology to find the optical range or the horizontal visibility. They consist of a light source, usually a laser, and a receiver. Both are placed at a 35° angle downward, aimed at a common area. Lateral scattering by the air along the light beam cause an attenuation coefficient. Any departure of the clear air extinction coefficient (e.g. in foggy situation) is measured and is proportional to the visibility.

These devices are found in automatic weather stations for general visibility, along airport runways for runway visual range, or along roads for visual conditions. Their main drawback is that the measurement is done over the very small volume of air between the transmitter and the receiver. The visibility is therefore only representative of the general condition around the instrument in generalized conditions (synoptic fog for instance). This is not always the case (e.g. patchy fog).

A radar scatterometer operates by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and subtracted from the signal+noise measurement to determine the backscatter signal power. Sigma-0 is computed from the signal power measurement using the distributed target radar equation. Scatterometer instruments are very precisely calibrated in order to make accurate backscatter measurements.

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