## 4 Transponder Calibration Results  
Sea-state bias is one of the largest sources of uncertainty linked with altimetric signals (2015). This is primarily because altimeter measurements are subject to interference originating by multiple and heterogeneous scatterers at sea (2015). A transponder on land overcomes this sea-state and scattering problem. Its image appears as a sharp point target to the radar altimeter, therefore it is easier to distinguish than sea (2015). Thus, during an overpass, the process for determining the range from the satellite to the transponder is straightforward (2015; 2015). The PFAC infrastructure has been involved in calibrations in the past with another and older transponders in Gavdos Island for Envisat (2015) and Jason-2 (2015). In this work, calibration results have \begin{table}
\begin{tabular}{c|c c c c} \hline \hline  & **Satellite** & **Jason-1** & **Jason-2** & **Jason-3** \\ \hline  & Product & GDR-E & GDR-D & GDR-D \\  & Cycles & 70-100 & 2-303 & 1-80 \\ \hline \multirow{2}{*}{SSH bias} & Pass No. 18, Descending & +7.0 cm (\(\pm 1.3\) cm) & + 0.63 cm (\(\pm 0.3\) cm) & \(-0.74\) cm (\(\pm 0.4\) cm) \\  & Pass No. 109 Ascending & +3.50 cm (\(\pm 1.3\) cm) & +0.33 cm (\(\pm 0.2\) cm) & \(-0.50\) cm (\(\pm 0.4\) cm) \\ \hline  & Average & +4.10 cm & +0.48 cm & \(-0.62\) cm \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Absolute calibration results for Jason satellites based on sea-surface observations along the ascending pass No. 109 and the descending pass No. 18 at the Gavdos Cal/Val facility. Figure 25: Altimeter bias for the Chinese HY-2A mission based on the CRS1 Cal/Val site, with its descending pass No. 280, west of Crete, Cycles: 23–98, I-GDR products. It seems that the HY-2A exhibits a linear trend on its bias values starting from +60 cm and going down about the same value with an opposite sign (i.e., \(-60\) cm) at the last cycle No. 98.  
been derived based on the new CDN1 Cal/Val transponder site for Jason-2 and Jason-3, as well as for their tandem mission. Finally, first transponder results for Sentinel-3A are also given. Specifically, a series of Jason-2 calibrations at CDN1 Cal/Val site has been performed for its Pass No. 18 and Cycles 267-277, from October 2015 to January 2016. Afterwards, twenty more calibrations for the tandem mission of Jason-2 and Jason-3 have been carried out during March-September 2016, i.e., for Jason-2 Pass No. 18, Cycles 285-303, and for Jason-3 Pass No. 18, Cycles 5-23. Absolute transponder coordinates for its reference point have been determined using various geodetic techniques and during several years of observations. Two permanent GNSS receivers (i.e., CDN0 and CDN2) continuously operating on site since June 2014 have helped in that direction. Station coordinates are determined in ITRF using scientific software. In addition, local geodetic surveys take place every six months to transfer the GNSS station coordinates to the transponder's reference point. Delays in satellite signals as they travel through the transponder electronics need to be known precisely (uncertainties less than \(\pm\)10-30 picosec) as they introduce raised biases for the satellite range. This internal delay in the transponder electronics has been measured thoroughly in 2013 (see also Section 2.1.4). Supplementary measurements on the transponder's internal delay have been conducted in specialized labs in Greece as well to verify previous results conducted at ESA, the Netherlands. Atmospheric propagation delays are precisely estimated via dedicated GNSS processing. Algorithms and subsequent software for processing transponder signals have already been designed and developed for deriving calibration results at the CDN1 Cal/Val transponder site. Steps and explanations of this transponder processing are given in Reference [60]. Some representative transponder responses for Jason-2, Jason-3, CryoSat-2, and Sentinel-3A are portrayed on Figure 26 below.