Calculations of surface clutter interference with precipitation measurement from space by 35.5 GHz radar for Global Precipitation Measurement Mission

Surface clutter interference with precipitation measurement from space using 35.5 GHz radar was evaluated for the Global Precipitation Measurement (GPM) Mission. The GPM Mission is unique in that it consists of a core satellite with dual-frequency precipitation radar (13.6 GHz and 35.5 GHz) and eight small companion satellites that are equipped with microwave radiometers. The 35.5 GHz precipitation radar has very high sensitivity; its designed minimum detectable rain rate at the rain top is 0.3 mm/h. In this study, a Taylor distribution with random errors in the excitation current is considered in calculating the radiation pattern of a 35.5 GHz slotted waveguide planar phased array antenna. The signal-to-clutter power ratio (S/C) was evaluated for the antenna pattern given by the Taylor distribution (peak side lobe level=-35dB, n=6; the same values as for the TRMM PR), where S is the received power from the rain scattering volume and C is the backscattered power from sea surface. Uniform rain rates of 0.3 and 1.0 mm/h were assumed in the calculation of S. We show that the interference of surface clutter with precipitation measurement can be suppressed more at 35.5 GHz than at 13.6 GHz because of the short wavelength. The calculated S/C ratio distribution showed that the effect of the side lobe clutter is not negligible, especially for low rain rates (less than 1.0 mm/h), but it is negligible for heavier rain (over 1.0 mm/h). The calculations also show that the effect of the main lobe clutter is severe and not negligible for either light or heavy rain. The conclusion is that 35.5 GHz precipitation radar can accurately observe rain with a planar phased array antenna fed with a Taylor distribution (n=6, peak side lobe level=-35 dB).