Evaluate the impact of a 4-hour tandem phase on the continuity of nadir altimetry measurements between S3 and S3NG-T

Abstract. The upcoming Sentinel-3 Next Generation Topography (S3NG-T) mission, designed to succeed the current Sentinel-3 (S3) mission, will operate on the same ground tracks as the current S3 constellation to maximise continuity of measurements, but with a fixed 4-hour temporal lag due to satellite design constraints. This configuration prevents the implementation of a classical near-simultaneous tandem phase, traditionally used for inter-mission cross-calibration, and raises concerns regarding the impact of short-term oceanic variability on continuity assessment. In this study, we evaluate the feasibility and expected performance of a 4-hour delayed tandem phase for cross-calibrating S3 and S3NG-T. Using tandem datasets from Sentinel-3A/B and Jason-3/Sentinel-6 missions, combined with SWOT KaRIn observations, we develop a methodology to quantify the oceanic variability introduced by a 4-hour delay and to evaluate its effect on the accuracy of inter-mission offset estimates. Results indicate that the classical tandem configuration achieves regional offset uncertainties of approximately 2 mm over a three-month period. In contrast, a 4-hour delayed tandem phase increases this uncertainty to about 7 mm in the same period, but still performs significantly better than non-tandem scenarios. Extending the 4-hour tandem phase to one year enables the detection of systematic errors of ± 3.5 mm amplitude, sufficient to ensure continuity between S3 and S3NG-T. These findings demonstrate that, despite additional oceanic variability, a 4-hour tandem configuration remains a viable and effective strategy for cross-calibration, especially when supported by improved environmental corrections and extended observation duration.