## 3 Sea-Surface Calibration Results  
The first calibration results for Jason-1 with Gavdos Cal/Val were reported in 2004 [45]. Since then, a standardized sea-surface methodology has come out for calibrating satellite altimeters using the ascending orbit No. 109 and the descending orbit No. 18 for Jason satellites [24; 40; 46; 47; 48; 49]. It is not the purpose of this paper to provide details on calibration procedures over the sea surface. The reader is referred to the previous literature for details on the methodology.  
The latest results for the Jason satellites, when the sea-surface calibration methodology is applied, are presented in Figure 24 with the following product versions: Jason-1 (GDR-E), Jason-2 (GDR-D), and Jason-3 (GDR-D) (Table 4).  
Calibration is carried out at sea zones between 14.5 km to 24 km for Pass No. 109 (ascending), and 9 km to 20 km for the Pass No. 18 (descending). Sea calibrating zones are with respect to the point of closest approach and south of Gavdos. The segment of the ascending orbit No. 109 in Jason, north of Gavdos (extending 12-20 km North), provides bias results as well, but based on two Cal/Val sites, i.e., Gavdos and RDK1 Cal/Val. These calibrating zones have been selected after careful and detailed examination of geoid and MDT models at these regions against bias performance, bathymetry changes, and are also measured and verified by boat campaigns [40].  
For Jason-1, sea-surface calibrations started at cycle No. 70 (December 2003), as site constructions had not been concluded earlier, along with instrument set up at the Cal/Val site in Gavdos. The Jason-1 calibration ended at cycle No. 101 (October 2004), as at this time works for building a new harbor in Gavdos had been initiated and instruments were temporarily removed from the Cal/Val area.  
Calibration results for Jason-2 are complete for all of its cycles (2-303) and presented in Figure 24 and Table 4. The Jason-3 biases have been determined as well for cycles 1-80 (18 February 2016, till 11 April 2018) and also shown in Figure 24e,f and Table 4.  
Figure 24: _Cont._  
In the above results, altimeter biases represent mean values. Uncertainties, as shown in Table 4 and Figure 23, declare a statistical measure (standard deviation of the mean) for the dispersion of the values in the derived bias for each satellite. They, however, reflect a rather misleading and incomplete indicator for the bias uncertainty of each satellite. The \"true\" uncertainty for the altimeter bias is approached and should be evaluated using the concept of fiducial reference measurements [50,51].  
Figure 24: Altimeter biases for Jason-1 (**a**,**b**), Jason-2 (**c**,**d**), and Jason-3 (**e**,**f**) along descending and ascending orbits over the sea surface around Gavdos Island.  
It seems that some minor directional errors are revealed on the derived bias for Jason altimeters along its ascending and descending orbits. This could be justified as a similar pattern for orbital errors for those satellites that seem to exist over the globe as well (OSTST Report 2013). Nonetheless, peculiarities in this calibration locale, instrumentation, models, and processing might also influence and attribute to this discrepancy in bias along different orbit directions.  
Sea-surface calibration of Sentinel-3A has been also carried out using the same infrastructure for both its ascending (Pass No. 14) and descending (No. 335) orbits around and in proximity to Gavdos (2015). The same Cal/Val infrastructure has also been used for the calibration of the SARAL/AltiKa altimetric mission along its ascending Pass No. 571 (2015) as well as for the Chinese mission HY-2A (descending Pass No. 280) (2015). Some recent results for the Chinese HY-2A mission are given in Figure 25, based on the CRS1 Cal/Val site on southwestern Crete for cycles 23 to 98 and for I-GDR products. It can be observed using Figure 23 that the altimeter of HY-2A is not stable but presents a linear and downgrading trend.