Slope-induced nonlinearities on imaging of ocean waves

The MTF (modulation transfer function) is the primary descriptor used to relate the variations of the electromagnetic backscatter from the ocean to the ocean wave parameters. The major assumption involved in the definition of the MTF is a linear relation between the backscattered power and the long-wave slope. An investigation of this assumption is presented showing that the backscattered power and wave-height are nonlinearly related at least for tilt modulation. The nonlinear relation between the backscattered power and the long-wave slope is analyzed for both vertically and horizontally polarized backscatterers. For HH polarization, at maximum slopes of 0.1 rad, the second harmonic contribution is 37% for a pointing angle of 20 degrees and reaches its minimum of roughly 19% at a pointing angle of 50 degrees . For VV polarization, the first-order amplitude and harmonic nonlinearities are always smaller than the corresponding values for HH polarization and are significant only at angles near vertical.<
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