Status of CYGNSS Level 2 Winds

The Cyclone Global Navigation Satellite System (CYGNSS) Mission, comprised of eight small satellites using reflected GPS signals (GNSS-R) to make estimates of ocean winds, has been operating for well over 2 years (launch - December 2016, operational phase started May, 2017). Since the start of the operational phase, a number of updates have been made to the Level 1 and Level 2 data to improve data quality. In addition to including updates to the calibration of the CYGNSS receivers, The Level 1 calibration was recently updated to address a large source of uncertainty in the data - variability in the transmit power of the GPS constellation. Along with Level 1 updates, the Level 2 wind retrieval algorithm, based on empirically tuned geophysical model functions (GMFs), was updated to accommodate Level 1 updates. This submission will briefly describe the Level 1 updates to the CYGNSS observations as background to presenting the details of the updated Level 2 CYGNSS winds, focusing on both updates to the algorithms and on an assessment of the resulting wind speed estimates.

In order to address transmit power variability within the GPS constellation, The CYGNSS mission modified the operation of its receivers to use the GPS navigation antenna as a GPS power monitor. This modification is only available after August 1, 2018. For these data, the latest version of the data (version 3.0), uses real-time estimates of GPS transmit power to calibrate the CYGNSS observables (normalized bistatic radar cross-section NBRCS, and leading edge slope LES), which are input to updated GMFs to produce Level 2 winds. For these, separate GMFs are used for fully developed seas and low winds (<20 m/s) and for young seas with limited fetch and high winds (> 20 m/s) typical of Tropical cyclones. For prior to August 1, 2018, an update to the previous CYGNSS data (version 2.1) was developed. The new version 2.2 algorithm adjusts NBRCS and LES estimates on a track-by-track basis by comparing them to estimates produced from wind speed estimates from NASA's Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) input to the CYGNSS GMF. This can in a mean sense account for fluctuations in the GPS transmitter across that track. A CYGNSS track is a series of observations made by a given CYGNSS receiver, GPS satellite pair, which appear as an along-track path of wind speed estimates. A statistical summary of on-orbit performance against independent estimates (e.g., bias and root mean square difference as a function of wind speed) as well general on-orbit trending of the new version 2.2 and 3.0 winds will be presented.