Research on the effect of the ground clutter on the measurement of the precipitation by the spaceborne millimeter wave precipitation radar

在未来毫米波星载降水雷达的设计中，如全球降水测量(GPM)任务上的35.5 GHz雷达，为了精确测量天线主瓣区域的降水回波，必须评估天线旁瓣区域的地面杂波的不利影响。因此，测量地面基本归一化雷达散射截面σ^0的精确值是必不可少的。设计并研制了35.5 GHz步进频率极化散射计，用于测量地面基本归一化雷达散射截面。该散射计采用矢量网络分析仪作为发射机和接收机，发射机和接收机天线采用两个标准喇叭天线。散射计安装在塔的顶部，塔的高度从1米到3米不等。土壤的散射截面σ^0随土壤表面粗糙度和土壤含水率的变化而变化。复介电常数是土壤含水率的函数，是确定σ^0值的基本参数。为了测量土壤表面的σ^0随表面粗糙度和土壤水分的变化，制造了转台来安装土壤表面的样品。转台步进式旋转360度。研制了激光粗糙度仪和复介电常数测量仪，分别用于测量土壤粗糙度和土壤复介电常数。通过改变土壤粗糙度和土壤含水率作为参数，测量了土壤归一化雷达散射截面σ^0随入射角的变化。入射角度约为10度时，σ^0值与土壤粗糙度关系不大，且随土壤含水率的增加而增大。

For the design of the future millimeter wave spaceborne precipitation radar, such as the 35.5GHz radar onboard the GPM(Global Precipitation Measurement) mission, the adverse effects of the ground clutter in the antenna sidelobe area must be evaluated in order to precisely measure the precipitation echo from the antenna mainlobe area. Therfore it is indispensable to measure the precise values of the basic normalized radar scattering cross section σ^0 of the ground surface. The 35.5GHz step frequency polarimetric scatterometer is designed and developed to measure basic normalized radar scattering cross section of the ground surface. The scatterometer uses the vector network analyzer as the transmitter and receiver and two standard horn antennas are used for the transmitter and receiver antennas. The scatterometer is mounted on the top of the tower whose height is variable from 1 meter to 3 meters. The scattering cross section σ^0 of the soil varies as the soil surface roughness and soil moisture content changes. The complex dielectric constant which is the basic parameter to determine the value of σ^0 is the function of the soil moisture content. In order to measure the σ^0 of the soil surface as functions of the surface roughness and soil moisture content, the turn table is manufactured to mount a sample of the soil surface. The turn table rotates stepwisely by 360 degrees. The laser roughness meter and the instrument to measure complex dielectric constant are also prepared to measure soil roughness and soil complex dielectric constant, respectively. The normalized radar scattering cross section σ^0 of the soil is measured as a function of the incidence angle by varying the soil roughness and soil moisture content as parameters. The σ^0 values does not depend on soil roughness so much when the incidence angle is about 10 degrees and it increases as the soil moisture content increases.