The main advantage of the PFAC’s geographical location of ESA is that it allows calibration of Sentinel-3A and Sentinel-3B with diverse and independent calibration techniques. Let us examine, for example, Sentinel-3A Pass No.14. This is an ascending orbit. As the satellite approaches the calibration facility from the south, the sea-surface calibration is employed with the Gavdos Cal/Val site. A few seconds later, the transponder calibration takes place at the CDN1 Cal/Val site on the mountains of west Crete at an elevation of 1100m.
For the descending Sentinel-3B Pass No. 335, a similar procedure takes place but with a reverse order, that is transponder first and a few seconds later sea-surface calibrations. To strengthen results of sea-surface calibrations, a new sea-surface Cal/Val site (“SUG1”) in the south coast of Crete is getting ready for operations soon (see also Cal/Val Techniques: Sea-Surface).
Sea-surface calibration is also carried out for Sentinel-3A Pass No. 335 (with the Gavdos Cal/Val site) and Pass No. 278 (with the CRS1 Cal/Val site). Also, Sentinel-3B Pass No. 71 and Pass No. 14 are also calibrated using ground-truth measurements obtained by the RDK1 and CRS1 Cal/Val sites, respectively.
The following transponder calibrations have been carried out so far at the CDN1 Cal/Val site for Sentinel-3A and Sentinel-3B:
The performance of Sentinel-3A and Sentinel-3B is also monitored and validated against other international missions such as Jason-3, HY-2, CryoSat-2. Building on the heritage of TOPEX/Poseidon, Jason-1 and Jason-2, Jason-3 also flies along the same orbit thus creating long-term and reliable time-series of sea-surface heights. When the same Cal/Val site is employed for the calibration of Jason-3, as well as Sentinel-3, it is possible to recover and then eliminate (or significantly reduce) errors associated with the specific Cal/Val site. In the PFAC this occurs for Jason series with the Gavdos and RDK1 Cal/Val facilities. It is thus feasible to estimate any relative bias between Jason and Sentinel-3 and integrate them for a single time series of sea surface heights.
