|In this cases, image was acquired in the morning over Ike hurricane. We observe that it is much easier to see the hurricane eye as well as the arms of the hurricane and the rain cells impacts on the sea surface when the incidence angle effect is removed.|
SAR images over the ocean can be considered as images of the sea surface roughness at small centimetric scales.
Indeed, the SAR is an active radar which emits electromagnetic waves and receives them after interactions with the small scales of the surface. Before inverting the backscattered signal to get a geophysical quantity, the first step is generally to convert the complex received signal (phase and amplitude of the electromagnetic wave) into the normalized radar cross section (NRCS). This is the radiometric and geometric calibration step.
As for photography, the NRCS derived from SAR depends on the observed scene as well as the conditions of observation. That is the reason why the NRCS depends on the sensor configuration (incidence angle, polarization, frequency) but also on the small roughnesses of the observed surface (that is why one can derive information related to the sea surface from the NRCS).
For a given SAR image, the polarization and frequency are constant but the incidence angle variation is significative across the image and impacts NRCS across the image. To limit this instrumental signature and increase the geophysical ones, we use the normalized roughness quantity. It is obtained by dividing the image amplitude by the amplitude corresponding to a roughness observed under an arbitrary constant wind speed and direction as follows :
This correction is particularly interesting as the calibration issues that can happen in products become obvious after transformation to normalized roughness. Indeed, as indicated in the previous equation the correction is applied to an assumed absolutely calibrated product.