# Evaluation of Spaceborne GNSS-R Retrieved Ocean Surface Wind Speed with Multiple Datasets
## 3 Results and Validation
### GNSS-R Wind Speed Validation with Buoy Observation  
Continuous high temporal-resolution wind speed data were obtained from moored buoy measurements provided by the National Data Buoy Center (NDBC). The time resolution of the wind speed product is 10 minutes, the dynamic range of wind speed is 0.0-35 m/s, and the measurement uncertainty is 0.3 m/s or 3%. Figure 4 shows the distribution of the 93 moored buoys used in this study, buoy wind is employed as an additional data source to validate NWP-derived and CCMP-derived wind speed. It should be noted that the wind of the Tropical Atmosphere Ocean (TAO) program buoy array provided by the NDBC is not calibrated to the standard wind speed reference height U10, and is directly corrected by the following formula [33]:  
\[\mathrm{U}_{10}=8.87403\times U_{Z}/ln(z/0.0016) \tag{6}\]  
where \(U_{Z}\) is the measured wind speed at the anemometer height of \(z\) above the sea surface in meters. When matching the buoy wind speed with GNSS-R retrieved wind speed, the distance between the specular point of observables and the buoy is limited to less than 50 km, and the reference wind speed was obtained by linear interpolation in the time domain.  
Buoy wind speed is obtained by in situ observations, so it has the highest accuracy. Since most of the buoys are located on the offshore coast, the number of training samples collected under the matching condition is very small, so it is difficult to model a 2D GMF. Through analyzing the training dataset, it is found that the buoy reference wind speed is almost below 15 m/s in the matched dataset, even if we tried to neglect the influence of incident angle, the 1D GMF still lacks the reference value for application, so we only used the buoy wind speed as external data source to further evaluate the wind speed inversion results of the other two datasets.  
Figure 12 compares the NWP-derived and CCMP-derived winds using the buoy wind as a reference value, where RLM represents a robust regression line, and the histogram shows the distribution of winds. It can be seen from the wind distribution histogram that the wind speed range of the matched samples was basically in the range of 0-15 m/s. The test wind speed pairs from NWP-derived winds and buoy winds only had 23,812 pairs in December 2017, only the sample less than four times of the standard deviation was selected for accuracy statistics, which are contained in the patch of the figure. The bias and RMS are \(-0.18\) m/s and \(1.91\) m/s, respectively, and the Pearson correlation coefficient between the two sets of wind is 0.78. The CCMP-derived inversed winds demonstrated a slightly better performance compared to NWP, where there are 20,604 groups of opportunity matched wind speed pairs obtained, with the bias and RMS of 0.11 m/s and 1.87 m/s, respectively, the Pearson correlation coefficient between the estimated winds and buoy winds is 0.78, at the same time, the linear regression line had a larger slope. Generally, the accuracy of the retrieved wind speed from two sets of reference wind data sources is considerable. Both the NWP and CCMP wind speed products can be employed as the truth reference wind speed for spaceborne GNSS-R ocean wind speed remote sensing.