Impact of specular point location inaccuracies on GNSS-R delay-Doppler maps

Wind speed retrieval from satellite Global Navigation Satellite System Reflectometry (GNSS-R) is expected to notably impact weather forecasts in the upcoming future. For such purpose, GNSS-R wind speed should be retrieved with an accuracy equal to or better than 2 m/s in the range 0-20 m/s. To the best of our knowledge this performance is achieved only in the range 0-7 m/s, while in the remaining range, accuracy can be even worse than 2.5 m/s, with a consequent poor impact on data assimilation in Numerical Weather Prediction (NWP) models. The reasons for such poor accuracy are various, among which the inability to properly exploit the entire information content of the delay Doppler map (DDM) plays an important role. It has been recently demonstrated that satellite DDMs acquired by the UK TechDemo-Sat 1 (TDS-1) mission are affected by some distortions that are strictly correlated to specular point estimation inaccuracies. In this study, it is shown that also the DDMs acquired by the US satellite mission Cyclone GNSS (CYGNSS) are affected by similar distortions. The main aims of this study are a) demonstrating the basic mechanism that causes such distortions, b) assessing their impact on the DDM observables that are operationally used in the wind speed retrieval algorithms, notably the DDM peak value for TDS-1 and the DDM average (DDMA) and leading edge slope (LES) for CYGNSS and c) analyzing their effects on the stare processing approach for wind speed retrieval. For such purpose, a set of CYGNSS tracks operated in the so-called “science mode” have been re-compressed in the DDM format by using the Doppler frequency at the specular point estimated position output with a more accurate geoid model (provided in the metadata of the regular CYGNSS files). The science mode consists of recording the entire raw reflected GNSS-R echo, which is routinely discarded in the operational acquisition mode, and therefore not available for reprocessing. This mode is typically switched