## 4 PFAC Calibration Results
### Sentinel-3A  
Three Sentinel-3A passes are calibrated at the PFAC (Figure 14): The ascending Pass No. 14 with sea-surface and transponder calibration at the Gavdos and the CDN1 Cal/Val sites, respectively, the descending Pass No. 335 also using the Gavdos Cal/Val site, and the descending Pass No. 278 with the CRS1 Cal/Val site. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Error Constituent** & **Type** & **Uncertainty Estimate** &
\begin{tabular}{c} **Standard Uncertainty** \\ **(Confidence 68\%)** \\ \end{tabular} \\ \hline GNSS Height 1  & A & 0.13 mm & \(\pm\)0.13 mm \\ GNSS Receiver 2  & B & 6.00 mm & \(\pm\)3.50 mm \\ GNSS Antenna Reference Point 1  & B & 2.00 mm & \(\pm\)2.00 mm \\ Measured Range 2  & B & 3.00 mm & \(\pm\)1.73 mm \\ Transponder Internal Delay 1  & B & 30.00 mm & \(\pm\)15.00 mm \\ Dry Tropospheric Delay 2  & B & 2.00 mm & \(\pm\)1.15 mm \\ Wet Tropospheric Delay 2  & B & 14.00 mm & \(\pm\)8.08 mm \\ Ionospheric Delay 2  & B & 4.00 mm & \(\pm\)2.31 mm \\ Geophysical Corrections 2  & B & 20.00 mm & \(\pm\)11.55 mm \\ Satellite Orbit Height 2  & B & 20.00 mm & \(\pm\)17.32 mm \\ Pseudo-Doppler Correction 1  & B & 2.00 mm & \(\pm\)2.00 mm \\ Leveling Instrument/Method 1  & B & 1.00 mm & \(\pm\)1.00 mm \\ Transponder Leveling 1  & A & 0.50 mm & \(\pm\)0.16 mm \\ Processing and Approximations 2  & B & 30.00 mm & \(\pm\)17.32 mm \\ Orbit Interpolations 2  & B & 0.30 mm & \(\pm\)0.17 mm \\ Unaccounted effects 2  & B & 20.00 mm & \(\pm\)11.55 mm \\ Root-Sum-Squared Uncertainty & & & \(\pm\)34.46 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Uncertainty budget for the transponder calibration. The final standardized uncertainty (at 68% confidence level), described as Root-Sum-Square value, has been computed as: \(FRM_{TRP}(Uncertainty)=\pm 34.46\,mm=\sqrt{\left(0.13\right)^{2}+\left(3.50\right)^{2}+ \left(2.00\right)^{2}+\cdots+\left(17.32\right)^{2}+\left(0.17\right)^{2}+ \left(11.55\right)^{2}}\). In Sections 3.1 and 3.2, the calibration processing of the in-situ ground measurements and models has been described. Satellite measurements also require some kind of rectification, as the altimeter and radiometer's observations are contaminated by the land mass of Gavdos and Crete. Regions with valid satellite measurements have to be identified and selected as calibrating regions for comparison with in-situ Cal/Val measurements. This process is carried out for each satellite's pass. Using the latest processing baseline for Sentinel-3A (version 2.61) that is common for all of its cycles, three sea-calibrating regions have been defined at the PFAC, as illustrated in Figure 14.  
For a successful transponder calibration, the satellite altimeter generally needs to operate in \"open-loop\" mode. This is necessary to adjust its range gate to operate over the transponder's elevation and collect range measurements. The \"open-loop\" mode relies upon high-resolution Digital Elevation Models (DEM) stored in the altimeter or additionally with a direct commanding as in the Sentinel-6 case that additionally allows the fixing of the instrument gain given the transponder characteristics. Using this a-priori knowledge, the altimeter adjusts its internal receive window appropriately, and thus the signal amplified by the transponder can be received by the satellite altimeter. It is not the purpose of this work to explain the steps followed to process the waveforms for pulse-limited (e.g., Poseidon-3B in Jason-3) and delay-Doppler altimeters (i.e., SRAL/Sentinel-3A/B and Poseidon-4 Sentinel-6) to arrive at the value of range bias, as details can be found in [14; 31]. Processing of Sentinel-3 waveforms is a complex procedure applied to about 200 radar bursts (of which about 30-40 bursts contain a useful transponder response over 1.5 to 2 s integration time) each of which contains 64 Ku-band pulses.