## 5 Crossover Calibration Results  
On 11 November 2016, Sentinel-3A (Cycle No. 11) and Jason-3 (Cycle No. 28) flew almost simultaneously over the transponder site, in ascending and descending passes, respectively. They were separated in orbit by 24 s (Figure 34). Therefore, atmospheric conditions, tidal effects, transponder characteristics, etc. are exactly the same for both S3A and Jason-3 calibrations. The range bias of Sentinel-3A was estimated for this pass as S3A = +20 mm, while the range bias of Jason-3 was JA3 = +45.0 mm. Thus, the range as measured by Jason-3 was longer than that of S3A by +45.0 mm \(-\) 20 mm = +25.00 mm. This shows that Jason-3 at the sea surface will observe shorter sea-surface heights than Sentinel-3A by 25 mm. Although this is a very rare event to happen over the transponder, it is a reliable indication of the offset of Jason-3 with respect to Sentinel-3A. Comparisons have been made between different altimeters at sea within a time window of 3 days or shorter. Sea surface heights observed using SARAL/AltiKa with respect to the reference mission of Jason-2 at a crossover point south of Gavdos (Figure 34) have been presented in Reference [41]. The sea surface height measured by altimeters has to be estimated exactly at the point of their crossover. This process involves errors as satellites do not pass at the same time over the crossover point and different conditions and heights may prevail at sea during a satellite pass. Nonetheless, crossover analysis could provide relative performance of one altimeter against another. Figure 34: The almost simultaneous pass of Sentinel-3A (ascending) and Jason-3 (descending) over the transponder CDN1 Cal/Val site on 11 November 2016. Sentinel-3A passed at 20:00:12 UTC, while Jason-3 followed at 20:00:36 UTC over the CDN1 Cal/Val site, separated by only 24 s. The same conditions and observation site settings existed for both satellites. Figure 33: Bias profiles of Jason-3 and Jason-2 with values determined from a 300 m running median window with respect to distance from the point of closest approach. Altimeters’ profiles are along the descending Pass No. 18 and south of Gavdos Cal/Val. Figure 35 shows the ground tracks of SARAL/AltiKa Pass No. 571 (yellow) and Jason-2 Pass No. 109 (red) (both ascending) where they converge at a point about 4 km south of the Gavdos Island. Sea-surface heights as measured by these two altimetric missions have been compared within a time window of less than 3 days. It can be seen from Figure 32 that SARAL/AltiKa measures sea-surface heights as a general rule less than the Jason-2 at this specific point at sea. Also, in the same Figure, the linear degradation of HY-2A bias with respect to Jason-2 is quite clear at a point north of Crete where both satellites converge. Finally, Sentinel-3A exhibits an average sea surface higher than Jason-3 by about +4 cm when compared at a crossover location south of Gavdos. Figure 35: **(Upper)** The ground tracks of SARAL/AltiKa Pass No. 571 (yellow) and Jason-2 Pass No. 109 (red) (both ascending) converge at a point of about 4 km south of the Gavdos Island, Greece coast **(left)**. The sea-surface heights as measured by these two altimetric missions within a time window of less than 3 days **(right)**. Results indicate that SARAL/AltiKa always measures sea surface heights smaller than Jason-2. **(Middle)** Analysis of the sea-surface heights as estimated by HY-2A and Jason-2 at a crossover point north of Crete. Left image shows the cross over location at Northwest of Crete and the right image portrays the results for each Jason-2 and HY-2A satellite for the sea surface heights. **(Lower)** Analysis of the sea-surface heights as estimated by the Sentinel-3A and Jason-3 altimeters at the crossover location south of Gavdos. Left lower image shows the cross over location south of Gavdos and the right image portrays differences of Sentinel-3A with respect to Jason03.The results are given with respect to the Jason-3 reference mission.