# Characterization of CYGNSS Ocean Surface Wind Speed Products
## 5 Conclusions
The RMSE of the MRG product was 3.2 m/s (wind speed \(<\) 20 m/s), 3.2-4.6 m/s (wind speed 20-30 m/s), and 4.6-6.8 m/s (wind speed 30-45 m/s). The corresponding RMSE values for the NOAA product were 4.2 m/s (wind speed \(<\) 20 m/s),4.2-7.0 m/s (wind speed 20-30 m/s), and 7.0-8.0 m/s (wind speed 30-45 m/s). The RMSE ranges given are the high and low values over the specified range of wind speeds. The 45 m/s maximum for the wind speed range is limited by the availability of three-way coincident matchups between all the products. The RMSE can be expected to continue to rise above 45 m/s due to fundamental limitations in the sensitivity of the CYGNSS radar [43]. Triple colocation was also used to assess the performance of the 34-knot significant wind radius (R34) that is derived from the MRG wind field. R34 is a measure of storm size, and its variation with quadrant a measure of storm structure, that is frequently used by operational agencies. There are more alternate sources of R34 information because it is based on measurements in the 15-20 m/s range, and an RMSE decomposition is performed using each triplet drawn from them. Averaging over all such RMSE values, the resulting RMSE was 38 km for ASCAT, 44 km for Best Track, 49 km for SMAP, and 54 km CYGNSS MRG. While CYGNSS has the largest error, it also has a lower revisit time than the other satellite products, suggesting that it could serve as a complimentary gap filler product to better track changes on short time scales in storm size and structure. See Sections 3.2.1 and 3.2.2 for details about these triple colocation analyses. In addition to the overall performance assessments, which are based on populations of coincident matchups that include all CYGNSS receiving satellites and all GPS transmitting satellites, and span multiple years, the bias and RMSD with respect to ERA5 were also evaluated individually for each of the two radars on each of the eight CYGNSS satellites, for each of the six versions of GPS satellites, and as a function of time from 2018 to the present. The performance was found to be consistent across all subsets of the hardware and stable over time throughout the mission lifetime. Both the FDS and NOAA wind speed products were also evaluated with respect to their sensitivity to the Significant Wave Height (SWH) of the ocean surface at the time of the measurement. This assessment was motivated by the secondary sensitivity of GNSS-R measurements to the long wave portion of the surface roughness spectrum, in addition to its primary sensitivity to the shorter capillary waves that are more directly forced by the local wind stress. In recognition of this sensitivity, both FDS and NOAA retrieval algorithms incorporate corrections for SWH. The evaluation performed here illustrates the need for such a correction and shows that none is needed for wind speed products produced by passive microwave radiometers or scatterometer radars. It also demonstrates that the corrections implemented by the FDS and NOAA algorithms are largely successful at removing the dependence of retrieval error on SWH. One final characterization of the behavior of the CYGSS wind speed products considered the dependence of their bias on the persistence of the wind at the location of the measurement in the 2 h prior to said measurement. In the FDS case at all wind speeds and in the NOAA case at lower wind speeds, if the winds have not changed much in the 2 h prior to a CYGNSS observation, the bias is quite low. This represents conditions in which the sea state is considered to be fully developed. If, on the other hand, the wind speed has changed significantly in the 2 h prior to a measurement, a significant bias is found to be present and that bias increases with increasing change in wind speed.