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For the first time, a rain signature in Global Navigation Satellite System Reflectometry (GNSS-R) observations is demonstrated. Based on the argument that the forward quasi-specular scattering relies upon surface gravity waves with lengths larger than several wavelengths of the reflected signal, a commonly made conclusion is that the scatterometric GNSS-R measurements are not sensitive to the surface small-scale roughness generated by raindrops impinging on the ocean surface. On the contrary, this study presents an evidence that the bistatic radar cross section sigma(0) derived from TechDemoSat-1 data is reduced due to rain at weak winds, lower than approximate to 6 m/s. The decrease is as large as approximate to 0.7 dB at the wind speed of 3 m/s due to a precipitation of 0-2 mm/hr. The simulations based on the recently published scattering theory provide a plausible explanation for this phenomenon which potentially enables the GNSS-R technique to detect precipitation over oceans at low winds.

Plain Language Summary Using Global Navigation Satellite System (GNSS) signals, reflected off the Earth's surface (GNSS Reflectometry), is an innovative remote sensing technique with a broad spectrum of geophysical applications. Currently, recent satellite missions, such as the U.K. TechDemoSat-1 and U.S. Cyclone Global Navigation Satellite System (CYGNSS), pioneer GNSS Reflectometry as a new space observation technology on a global scale. Despite a wide variety of monitored geophysical parameters, the reflected signals have never been used to obtain rain information. For the first time, this study demonstrates a signature in the received signals, due to the modified ocean surface waves by rain splashes, enabling the technique to detect precipitation over oceans induced by weak winds. A plausible physical explanation for this phenomenon is provided based on the recent scattering theory. This study can serve as a starting point for developing a new GNSS Reflectometry application, rain detection over oceans, which might be also implemented for future low-cost GNSS remote sensing missions. The presented findings also provide a better physical understanding of L band forward scattering mechanism which is directly relevant to the main objective of the currently operational GNSS Reflectometry satellite missions.