| The electromagnetic scattering from the sea surface is not only playing an important role in radar guidance and target exploration in the field of military defense, but also valuable in the domain of civil. Sea surface radar echo(sea clutter) contains the echo information of the target, thus studying the scattering characteristics of sea surface under complex sea situation is of great importance. In this paper, the accelerating algorithm of Iterative Physical Optics(IPO) in optimal design was discussed in the electromagnetic scattering properties of 3-D sea surface with the overturning waves at low grazing angles. The model of sea surface with cavity was established and high frequency algorithm was applied to solve this problem.Firstly, in this paper, the basic characteristic parameters of the sea surface were introduced, and the basic models of sea spectrum were also discussed. A description of virtual reality scene 1-D overturning wave model adding influence of wind speed was given. With the same wind speed, overturning waves’ height and wavelength were unified with the real sea surface. Then we gave the simulation result of overturning waves.Secondly, a high frequency approximation algorithm-Physical Optics(PO) was discussed, under the condition of the conductor type and the medium type were deduced. Then discussion of numerical examples and verification were given. The Iterative Physical Optical(IPO) method based on the method of Physical Optics was also presented. After that the IPO method was extended to deal with the scattering problem of rough surface, and the optimization process was carried out. The Jacobi iteration was improved to the Forward and Backward(FB) iteration, and relaxation algorithm was quoted. And then we got the formula of Iterative Physical Optics using Forward and Backward iteration with relaxation factor(FBIPO). In order to improve the operation efficiency, the Open MP technique was taken into account, and the CPU parallel program was accelerated. The parallel scheme with high performance was constructed to deal with the iteration problem.Finally, we discussed the simulation of 3-D PM sea surface with overturning waves. With a specific wind speed, front-end and back-end of the 3-D overturning wave model were added PM spectrum sea surface to simulate a real sea surface. The optimized Iterative Physical Optics method mentioned above was applied to calculate this model. Then we discussed the scattering characteristics of overturning wave from formation stage to collapse stage at low grazing angle incidence. When the shape of overturning wave fully formed, it is similar to a cavity structure which can be equivalent to a resonant cavity. So IPO method was applied to deal with the cavity part of sea surface with overturning wave. When the grazing angle is gradually reduced, the echo signal from resonant cavity will strengthen increasingly, and the scattering characteristics are quite different from the beginning stage of overturning wave formation. In addition, backward echo signals enhance obviously, and HH polarization of backward scattering is significantly greater than the VV polarization. After the collapse of overturning waves, the wave broken region will be crowned with the white foam. Because of the wave breaking, air is involved in the water which forms the bubble mixed sea structure, causing the water dielectric constant changed. Therefore, scattering characteristics of the breaking wave area using the modified equivalent dielectric constant was presented. |
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