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Objectives: Flood monitoring not only plays an important role in flood prevention and mitigation, but also provides support for understanding the temporal and spatial evolution of floods and forecasting and warning. The global navigation satellite system-reflectometry (GNSS-R) measurement provides a new solution for flood monitoring. Methods: Firstly, we introduce a coherent signal detection method based on CYGNSS (cyclone global navigation satellite system) raw counts delay-Doppler map (DDM), and then analyze the changes of spatial range and time series before and after flood in South Asia are analyzed, based on the sensitivity of spaceborne GNSS - R signals to floods. Results: Research results show that, compared with the flood detection results of soil moisture active passive (SMAP), the monitoring range of the two methods in the dry season is similar. While in the rainy season, the detection range of the surface reflectivity is larger about 6.69%, and the detection range of the power ratio is smaller about 3.6%.Conclusions: The proposed method of using GNSS-R coherent signals to detect floods is more accurate than the surface reflectivity, and this method does not have to consider the influence of surface characteristics on the incoherent scattering power. It is a simple and effective method for flood detection. [ABSTRACT FROM AUTHOR]