EVALUATION OF SURFACE CLUTTER FOR THE DESIGN OF THE TRMM SPACEBORNE RADAR

Surface clutter interference through antenna sidelobes on rainfall measurements by spaceborne rain radar is quantitatively examined in order to clarify design criteria for the antenna, the development of which is one of the most important technological issues in realizing a spaceborne rain radar. The received intensities of both rain echo and sea clutter are evaluated numerically, assuming appropriate precipitation and sea surface scattering models and a realistic antenna pattern for a phased array antenna which is fed with the Taylor distribution. The results indicate that a region exists just above the sea surface where the sea surface clutter masks the backscattered power from the rain. The region extends to an altitude of approximately 1.2 km for an incidence angle of 17-degrees. The results also demonstrate the feasibility of quantitative measurements of rainfall in regions above this interference zone if a planar array antenna fed with the Taylor distribution of peak sidelobe level -30 dB is used. It is also found that interference through sidelobe-sea clutter coupling is larger in the inner part of radar swath than in the outer part. This means that the specular component of sea surface scattering, especially in calm sea surface conditions, has a significant impact on rainfall measurements from space.