Ku波段雷达探测波束在经过降雨区域时，由于雨滴对微波能量的吸收或散射，其回波信号会发生衰减，从而改变雷达的后向散射系数，最终使观测结果产生偏差，因此降雨识别对提高雷达观测结果的精度具有重要意义。根据本文研究，2018年发射的中法海洋卫星CFOSAT(China–France Oceanography Satellite)所搭载的海浪波谱仪SWIM(Surface Wave Investigation and Monitoring)对降雨事件的发生频率存在低估。本文对一种基于MP(Matching Pursuit)算法的降雨标识方法进行了改进，以高度计回波估计的误指向角和雷达后向散射系数的沿轨波形作为改进后算法的输入，并增加了滑动窗口对沿轨波形的处理。将改进后的降雨标识与相同时空范围内Jason-3的降雨识别结果以及GPM(Global Precipitation Measurement)的后处理高精度全球降水产品进行了对比，结果显示，相比SWIM 6.0.3版本的2级产品中的降雨标识，本文的降雨标识对降雨具有更高的准确度。

Objective: Rain flag is necessary for Ku-band altimeters, because the presence of rain in the sub-satellite track will cause attenuation of the backscatter signal, which can lead to errors of the altimeter products. The SWIM (Surface Wave Investigation and Monitoring) instrument payload on the CFOSAT (China–France Oceanography Satellite) is a Ku-band (13.575 GHz) real aperture radar which illuminates the surface sequentially with six incidence angles. The nadir beam of the SWIM can be used as an altimeter except for measuring the SSH (Sea Surface Height). The rain events identified by rain flag in SWIM L2 nadir products offered by CNES (Centre National d’Etudes Spatiales) has been underestimated compared to these in Jason-3. Thus, the rain flag in SWIM L2 nadir products need to be improved. Methods: Apparently, the dual-frequency rain flag algorithm used in Jason-3 products cannot be applied in SWIM which only works on Ku-band. To address this issue, a rain flag based on MP (Matching Pursuit) algorithm is introduced and modified to make it applicable to SWIM in this article, which is extremely versatile and can be easily adapted to any altimeter data. The along track waveform of mispointing angles can easily be decomposed by MP algorithm based on wavelet packet decomposition. Then the intervals where the mispointing angles presents short-scale coherent variations can be detected. Except for rain events, the σ^0-blooms can also cause this kind of variations in the waveform of the mispointing angles. In this article, the along track waveform of σ^0 is also used to produce the rain flag. The flag given by MP algorithm where the σ^0 is over 15 dB and lasts for 6 seconds should be removed. Results: The dual-frequency rain flag in Jason-3 products and the products of NASA’s Integrated Multi-satellite Retrievals for GPM(Global Precipitation Measurement) has been used to test the performance of the SWIM rain flag offered by this article. The percent of rain events given by dual-frequency rain flag in Jason-3 is 3.1%, while that in SWIM L2 nadir products offered by CNES is 1.03%. By using the method in this article, the difference between the amounts of rain events in Jason-3 and SWIM is only 0.2%. When rain rate reaches over 3 mm/h, this method performs better than SWIM L2 nadir product. In addition, the consistence between Jason-3 and SWIM nadir rain flag in the method is well in low latitudes, but it will descend when latitudes is larger than 40 degrees. Conclusion: The quantity of rain flags in SWIM L2 nadir products at present has been apparently underestimated. This article provides a new SWIM nadir rain flag based on MP algorithm. Compared to other kinds of rain flag, this new rain flag can be used in altimeters works on single Ku band without radiometers. The difference is that after the waveform comprised by radar mispointing angles is processed by MP algorithm, the backscatter coefficients is also taken into account and a sliding window is added to reduce the influence of the σ^0-bloom. After the collocation with high resolution observation by GPM, the results show that the rain flag defined by this new method performs well when rain rate is larger than 3 mm/h and it is consistent with the dual-frequency rain flag. But when latitudes is larger than 40 degrees, the consistency will decline, which needs further research to confirm the reason.