# First In-Orbit Validation of Interferometric GNSS-R Altimetry: Mission Overview and Initial Results
## 3 Results
### Intercomparisons with Sentinel-6 SSH Products  
During the study period (11 July-29 December 2023), we identified 136,738 crossover points for SSH product accuracy assessment. Figure 14 shows their global distribution, demonstrating similar spatial patterns between Sentinel-6/iGNSS-R altimeter and Jason-3/iGNSS-R altimeter pairs. This similarity stems from nearly identical orbital parameters, with Sentinel-6's orbit offset by \(\sim\)157 km (half the intertrack distance) from Jason-3. This deliberate offset improves spatial sampling and enhances both the coverage and representativeness of our SSH evaluation. Figure 15 presents the evaluation of iGNSS-R SSH against Sentinel-6 SSH at crossover points. Figure 15a shows the scatter plot comparison between the two datasets. To better illustrate the retrieval errors of iGNSS-R SSH, Figure 15b displays the statistical distribution of errors across 136738 crossover points. The results indicate that the iGNSS-R SSH has an RMSE of 17.19 cm and a bias of \(-\)0.35 cm, with the overall error distribution closely approximating a normal distribution. Compared to the evaluation based on Jason-3 data, the SSH deviations derived from Sentinel-6 data exhibit slightly larger magnitudes. This difference primarily originates from our calibration methodology, in which Jason-3 data were used as the reference standard for SSH calibration. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Validation Parameters** & **STD (cm)** & **Bias (cm)** & **RMSE (cm)** & **R** \\ \hline SSH & 17.140 & \(-\)0.010 & 17.140 & 1 \\ Wet tropospheric delay * & 1.063 & 0.755 & 1.303 & 0.993 \\ Dry tropospheric delay * & 0.405 & \(-\)0.637 & 0.755 & 0.992 \\ Ocean tide & 3.225 & \(-\)0.219 & 3.230 & 0.994 \\ Polar tide & 0.492 & \(-\)0.003 & 0.492 & 0.902 \\ Solid earth tide & 0.435 & \(-\)0.002 & 0.434 & 0.998 \\ DAC & 0.390 & \(-\)0.013 & 0.390 & 1 \\ \hline \hline \end{tabular}
* Statistical results at 0\({}^{\circ}\) incidence angle.  
\end{table} 
Table 2: Comparative assessment with Jason-3 SSH products. Figure 13: Comparison scatter plots of Jason-3 vs. iGNSS-R SSH corrections: (**a**) wet tropospheric delay, (**b**) dry tropospheric delay, (**c**) ocean tide, (**d**) polar tide, (**e**) solid Earth tide, (**f**) DAC. Figure 16 presents the evaluation results of iGNSS-R altimeter SSH error corrections using Sentinel-6-derived parameters at crossover points, including dry/wet tropospheric delays, ocean tide, pole tide, solid Earth tide, and DAC. In the subfigures, the blue dashed lines represent the 1:1 lines, while the red solid lines show the results of the least-squares linear fit. With a 0.2 h time window for ocean tide, pole tide, solid Earth tide, and DAC matching, 692 crossover points were available for comparison. With a 0.5 h time window for dry/wet tropospheric delay correction matching, more than 1660 crossover points were available for comparison. Due to orbital geometry, the number of crossover points available for dry/wet tropospheric delay evaluation was approximately 2.5 times greater for Sentinel-6 than for Jason-3. Figure 16a,b show RMSEs of 1.282 cm for wet tropospheric delay correction and 0.753 cm for dry tropospheric delay correction at a 0\({}^{\circ}\) incidence angle. When scaled by the cosine of the incidence angle to account for the maximum 35\({}^{\circ}\) viewing geometry of iGNSS-R, these errors increase to 1.57 cm (wet) and 0.92 cm (dry), respectively. Figure 16c-f reveal the RMSEs for the other correction terms: 3.49 cm (ocean tide), 0.49 cm (pole tide), 0.46 cm (solid Earth tide), and 0.59 cm (DAC). Table 3 summarizes the overall accuracy assessment of the iGNSS-R SSH products, using Sentinel-6 as a reference. Figure 14: Crossover points between Sentinel-6 and iGNSS-R altimeter. Figure 15: Comparative analysis of sea surface height measurements between Sentinel-6 radar altimeter and iGNSS-R altimeter. (**a**) Scatter plot of coincident measurements. (**b**) Distribution histogram of measurement differences.