# Validating CYGNSS Wind Speeds with Surface-Based Observations and Triple Collocation Analysis
## 3 Results
### Surface-Based Analysis
#### 3.1.1 All Wind Events  
Summary statistics from each of the gauges to CYGNSS measurements are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Height Above** & \multirow{2}{*}{**Lat**} & \multirow{2}{*}{**Lon**} & \multicolumn{2}{c}{**No. of**} & \multirow{2}{*}{**R \({}^{*}\)**} & \multirow{2}{*}{**RMSD (m/s)**} & \multirow{2}{*}{**Bias (m/s)**} \\  & & & & & & & \\ \hline \multicolumn{8}{c}{Australia} \\ Troughton Island & 8 & \(-\)13.75 & 126.15 & 751 & 0.81 & 1.96 & 1.11 \\ Legendre Island & 29.5 & \(-\)20.36 & 116.84 & 6362 & 0.73 & 2.27 & 1.01 \\ Bedout Island & 9.4 & \(-\)19.59 & 119.10 & 7057 & 0.77 & 2.67 & 1.70 \\ North Island & 2.7 & \(-\)28.30 & 113.59 & 11,755 & 0.67 & 2.17 & 0.24 \\ Morning Island Airport & 11.5 & \(-\)16.66 & 139.17 & 2245 & 0.67 & 1.50 & \(-\)0.22 \\ Willis Island & 9.8 & \(-\)16.29 & 149.97 & 10,608 & 0.85 & 1.90 & 1.04 \\ Rowley Shoals & 7.6 & \(-\)17.52 & 118.95 & 13,274 & 0.82 & 1.69 & 0.63 \\ Point Fawcett & 6.5 & \(-\)11.76 & 130.03 & 3849 & 0.60 & 2.22 & \(-\)1.13 \\ Holmes Reef & 6.3 & \(-\)16.47 & 147.87 & 12,604 & 0.85 & 2.09 & 1.24 \\ Adele Island & 5.5 & \(-\)15.51 & 123.16 & 14,176 & 0.73 & 1.62 & 0.33 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of each gauge against FDS.  
The results show slightly higher errors than those used in the previous buoy coastal analysis, with an average RMSE of 2.02 m/s compared to 1.86 m/s in [21]. The correlations are lower (0.75 compared to 0.83), which may be due to the use of FDS rather than the machine learning model developed in their paper. The positive bias is higher than observed in other studies (0.65 compared to \(-\)0.44 [21] and \(-\)0.55 [32]), presumably due to coastal effects.  
Density plots comparing the two CYGNSS algorithms and coastal anemometer data for the Australian and Pacific regions are shown in Figure 2.  
NOAA v1.2 presented the best results with higher correlations and lower RMSD, which is consistent with previous buoy studies [32] due to its stricter quality control and de-biassing techniques [20]. There is a considerable amount of noise across all wind speeds, with the percentage error of winds 0-4 m/s being particularly large.  
RMSD values over Australia were higher than compared to tropical ocean buoys. RMSD values FDS over Australia (Figure 2a) and Pacific (Figure 2c) was 1.95 and 2.18 m/s, respectively, compared to a previously recorded RMSD value of 1.75 m/s using FDS v3.0 and equatorial buoys [32]. NOAA's RMSD for Australia (Figure 2b, 1.58 m/s) and the Pacific (Figure 2d, 2.03 m/s) were similarly higher than the 1.21 m/s recorded using NOAA v1.1 [32]. The results of FDS over Australia (R = 0.80, RMSD = 1.95) are similar to values determined from NOAA coastal buoys (R = 0.82, RMSD = 1.65), which were produced using an older version (2.1) of CYGNSS [21]. Overall, the results for all wind speeds compared to Australian anemometers show weaker performance than previously observed with buoys, but still within the CYGNSS mission guidelines of RMSD being below 2 m/s for winds below 20 m/s [52].  
The Pacific dataset (Figure 2c,d) shows a reduced correlation, higher RMSD, and negative bias trend compared to the Australian dataset. This is consistent with using wind-gust measurements, which would increase the station's recorded speed, rather than the average for the hour. Figure 2c shows the degradation in performance at gusts above 12 m/s, as FDS consistently underestimates the anemometer value.