TY - JOUR A1 - Asgarimehr, Milad A1 - Wickert, Jens A1 - Reich, Sebastian T1 - TDS-1 GNSS Reflectometry BT - Development and Validation of Forward Scattering Winds JF - IEEE journal of selected topics in applied earth observations and remote sensing N2 - This study presents the development and a systematic evaluation study of GNSS reflectometry wind speeds. After establishing a wind speed retrieval algorithm, UK TechDemoSat-1 (TDS-1) derived winds, from May 2015 to July 2017, are compared to the Advanced Scatterometer (ASCAT). ERA-Interim wind fields of the European Centre for Medium-range Weather Forecasts (ECMWF) and in situ observation from Tropical Atmosphere Ocean buoy array in the Pacific are taken as reference. One-year averaged TDS-1 global winds demonstrate small differences with ECMWF in a majority of areas as well as discuss under- and overestimations. The pioneering TDS-1 winds demonstrate a root-mean-squared error (RMSE) and bias of 2.77 and -0.33 m/s, which are comparable to the RMSE and bias derived by ASCAT winds, as large as 2.31 and 0.25 m/s, respectively. Using buoys measurements as reference, RMSE and bias of 2.23 and -0.03 m/s for TDS-1 as well as 1.40 and -0.68 m/s for ASCAT are obtained. Utilizing rain microwave-infrared estimates of the Tropical Rainfall Measuring Mission, rain-affected observation of both ASCAT and TDS-1 are collected and evaluated. Although ASCAT winds show a significant performance degradation resulting in an RMSE and bias of 3.16 and 1.03 m/s, respectively, during rain condition, TDS-1 shows a more reliable performance with an RMSE and bias of 2.94 and -0.21 m/s, respectively, which indicates the promising capability of GNSS forward scattering for wind retrievals during rain. A decrease in TDS-1-derived bistatic radar cross sections during rain events, at weak winds, is also demonstrated. KW - Advanced scatterometer (ASCAT) KW - European Centre for Medium-Range Weather Forecasts (ECMWF) KW - GNSS forward scatterometry KW - GNSS reflectometry KW - TechDemoSat-1 (TDS-1) KW - wind speed Y1 - 2018 U6 - https://doi.org/10.1109/JSTARS.2018.2873241 SN - 1939-1404 SN - 2151-1535 VL - 11 IS - 11 SP - 4534 EP - 4541 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Asgarimehr, Milad A1 - Zavorotny, Valery A1 - Wickert, Jens A1 - Reich, Sebastian T1 - Can GNSS Reflectometry Detect Precipitation Over Oceans? JF - Geophysical research letters N2 - 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. KW - GNSS Reflectometry KW - rain detection KW - rain splash KW - TDS-1 KW - ocean surface KW - electromagnetic scattering Y1 - 2018 U6 - https://doi.org/10.1029/2018GL079708 SN - 0094-8276 SN - 1944-8007 VL - 45 IS - 22 SP - 12585 EP - 12592 PB - American Geophysical Union CY - Washington ER -