The performance of Sentinel-3 will be validated against other altimeter missions (i.e., Jason-3/Jason-CS reference missions, CryoSat-2, HY-2B and its follow on, etc.). This is carried out on land and at open seas employing the transponder and sea-surface calibration techniques.

Multi-mission crossover analysis constitutes an alternative approach to estimate the relative bias between two (or more) satellite altimeters. This analysis involves the examination of the sea-surface heights, as estimated by each satellite altimeter when they happen to fly over the same location at sea within a short period of time (i.e., 1-3 days) (Fig. 1). Note that CryoSat-2 ground-tracks are not presented in Fig. 1, because of its drifting orbits.

Fig. 1. Crossover locations at sea and on land, over the Permanent Facility for Altimetry Calibration, for the Sentinel-3 missions and other altimeters.

Implementation of crossover analysis at sea does not require any in-situ measurements. This is not the case for transponder calibrations. Crossover analysis over land may be employed only when two satellites overpass the transponder within a short period of time (i.e., a few seconds up to a few hours).

The following crossover locations of satellite altimetry missions have been identified in west Crete, Greece.

The main advantage of the sea-surface crossover analysis is that it can be employed globally as it only requires the sea-surface heights as measured by each altimeter. No permanent Cal/Val site is necessary. Without doubt, the presence and continuous operation of the sea-surface and transponder infrastructure in this PFAC enhance accuracy, robustness, redundancy and reliability of the Cal/Val results.