The effects of rainfall backscattering on space-borne rotating fan-beam scatterometer measurements at Ku-band were investigated in this study.A new scattering model based on Integral Equation Method（IEM） for sea surface with precipitation that accounts for the effects of rain column was presented.The relation between rain scattering and incidence was analyzed

and hence the rain effects on a rotating fan-beam scatterometer are obtained.The rain column is modeled as sublayers with different physical characteristics determined by the vertical profile of rainfall.Each sublayer is assumed to be statistically homogeneous

and the gamma distribution is used to model the drop size distribution within each sublayer.Sea surface is modeled as a rough surface

and its backscattering is calculated using the IEM model.Multiple scattering within and interactions between the rain layers and sea surface are accounted for in the matrix doubling formulation.The presented model is validated using published TRMM PR measurements.Results from model analyses show that rain effects are incident angle dependent

which can lead to non-uniform impacts within the footprint of a fan-beam scatterometer;these effects are strongly related to rain column vertical profile and rain rate

as different rain rate corresponds to different physical parameters.