| Popis: |
It is well known that the bistatic radar altimeter have some differences with respect to a monostatic one: indeed the spatial resolution is achieved over elliptical areas. These ellipses are in general tilted with respect to the isodoppler line, as these depend on the relative direction of motion of both transmitter and receiver. We can notice that also in the bistatic altimeter the spatial resolution can be improved at least in along track direction, by synthetic aperture approach: the instruments based on this concept are called Doppler Beam Sharpening (DBS) altimeters. Moreover under usual sea conditions, the geometric optics backscattering model appears applicable and the first point of our task appears the slope "m" measurement. We can estimate the slope by considering the backscattering in the specular or reflection central point, or/and by estimation of the decreasing of the backscattering versus observation angle. The second approach appears the best (in order to leave aside from sea surface pollution), which entails the backscattering estimation in the same region from many observation angle (multilook), with uncoherent processing . Therefore a new analytical closed-form model is derived for the average power echo received by a bistatic altimeter from oceanic surface, taking into account the previous requirement not usual in Altimetry. We can notice that, if the aim of our investigation is the wind measurement, due to the models available in literature, the accuracy required in the slope measurement is the order of 10%. Because of the nonlinear interaction at the interface between the water and the air, the wave crests are more peaked than the wave troughs and this effect can be taken into account by given to the surface height probability density function a non symmetric behaviour that is a non- zero skewness coefficient. This has been also done applying an exponential non linearity to a bivariate Gaussian distribution: an update version of this model is also proposed. Moreover the backscattering model, recently developed for fractal Brownian surfaces, and applied to wind-roughened water surfaces, is also taken into account. A Software Simulator was implemented and the simulation results have validate this model. |
