| 09 Jan 2026
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.
Initial Comments
I recommend publication after revision, as detailed below
Scientific Significance – Good (2)
This paper addresses an important issue, the need to provide a sound scientific basis to evaluate the performance of the Sentinel-3 Next Generation Topography Mission, in terms of the cross-calibration with the fore-running Sentinel-3 mission during a tandem phase mission. The approach described is new, developed from previous approaches applied to previous satellite altimeter tandem phase validation studies.
Thus the prime objective of the study is to support understanding of the impact of a proposed orbit selection on measurements from the S3 and S3NG-T missions, rather than to present new scientific results on the state and behaviour of the ocean. I therefore agree the paper is most appropriately included as a technical note.
Scientific Quality – Fair (3)
The scientific approach and applied methods are valid and developed from relevant previous work. Appropriate references are provided throughout.
The discussions in Sections 4 (Results) and 5 (Assessment) do not provide sufficient detail to enable a good understanding on how figures have been generated, what can be seen in these figures, and to justify the conclusions that have been drawn.
Presentation Quality – Good (2)
The text is well written and structured, although as mentioned above, more detail is needed in the description and discussion of the figures.
Thus I would recommend some revisions primarily to sections 4 and 5 to provide more detailed presentation and discussions of the results, to provide a more solid justification of the main findings.
Two of the figures are listed as A1 and A2, and the contents of these figures provide useful and important information on the geographical variability in the processed and gridded data. I cannot find any guidance regarding constraints on the number of figures within a paper. As there are only 4 other figures, I would recommend including these figures in the main body of the paper.
The labelling of axes on the figures and the content of the captions should be improved to allow readers to understand what is presented. For all Figures 1-4 the y axes are labelled “Uncertainty”, and the captions do not explain what the uncertainty is in.
Detailed Comments
Abstract
The abstract is well written and accurately summarises the contents of the paper.
10 – “…uncertainties of approximately 2mm” – in what?
13 -Extending tandem phase to one year enables the detection of errors of ±3.5mm amplitude – see later comments around Figure 4 (Line 274 onwards).
The background and context for the study are in general well described.
41 – I understand that during the tandem phases the interval between successive satellites for the Jason series reference missions was around 60 seconds and for Sentinel 3A/3B it was 30 seconds. However that for ERS-2 and ENVISAT it was longer - 30 minutes. The text gives the impression the delay was under one minute for all tandem phases.
Please rewrite this section to more accurately describe the tandem missions referred to.
44- LEULIETTE et al - correct link and reference to correct capitalisation.
The approach only considers using recorded satellite altimeter data sets. Was using ocean model hindcast data, sampled along the exact ground tracks, considered? It would be useful to include a short discussion on the potential to use model data for these types of study, and why it was not considered appropriate for this study.
83 Suggest to note here that S3A and S3B were only 30 s apart on the same ground track.
100 – Link to Jason CS Sentinel 6 product handbook
3.1 Comparison between two missions
I find the description in this subsection confusing. Has the same gridding process been applied to S3A and S3B data and also to J3 and S6A data, covering periods in and out of the tandem phase, so that there are effectively four data sets? A table listing the gridded data sets as generated might help to clarify the situation.
127 “For each cycle” - Does this refer to the S3A, S3B 27 day cycle, or the earlier defined 12 day “sub-cycle” (itself made up of 3 4-day subcycles)
If these data are used to generate Figure A1 provide an explanation and a discussion. (Note the recommendation to include this figure in the main body of the paper.)
3.2 Uncertainty Computation
Two approaches are given for the calculation of the uncertainty in the estimates of regional offsets of sea surface height anomaly. Results from the two approaches are compared in Figure 3, but it is not clear which approach was used to generate the values presented in Figures 1 and 2. Please clarify.
3.3 Accounting for 4-hour variability
185 Figure A2 – is the first panel for zero time offset? – Please provide a more detailed discussion on what is in the figure, and of the important features in the figure..
193 -remind readers that “Classic tandem phase” implies near zero delay (less than one minute).
4 Results
This is a key section, as it presents the results of the estimated uncertainty in regional SSHA for the 4 hour delay tandem phase. As identified in the initial remarks, more detailed discussion is required in this section to provide the reader with a better understanding of these key findings
194 How exactly are the values for the “non-tandem” scenario calculated – from data outside tandem phase.. (duh).
195 Provide a more detailed discussion of Figure 1.
Which version of Regional Inter Mission Bias Uncertainty is being plotted?
Figure 1 (and 2, 3 4)– Include parameter for uncertainty ( on y-axis) in caption – i.e. “Regional intermission bias uncertainty in SSHA”
5 Assessment
Comparison with J3/S6A missions
Again – provide a more detailed description and discussion of Figure 2. What measure of uncertainty has been calculated (3.2.1 or 3.2.2)?
Comparison with Cross-Over Based Approach.
231 - The text does not describe how uncertainty values are derived from the SLA difference values and the regional offset.
Comparison of two uncertainty estimation methods
258 Again provide a more detailed discussion of Figure 3.
What conclusions can be drawn from Figure 3? Which calculation approach is better?
Conclusions
261 – “Traditional tandem phase(s), where missions fly in close formation, are crucial for mission continuity.” - add the (s)
273 …through (an) extended calibration period. Add the (an)
276 – Insufficient description in the text on how the curves in figure 4 were calculated, and also insufficient discussion of the details in Figure 4. How were the results from the J3/S6a comparison used to fit a curve to the S3A S3B results?
What are the 1, 2, 3 year thresholds?
Without this information it is not possible to assess whether the key findings of the paper are justified :
“A 4 hour tandem phase would require approximately two years of continuous observations to reach a similar level of calibration precision (Fig. 4).
“However, the demonstrated ability to detect systematic differences of ±3.5 mm within one year highlights the feasibility of this approach….)”
327 – References –
Caps on Surnames of authors and co-authors. Leulliette et al (2004)
Zhao Z (2024) – non standard characters (${\mathrm{M}}_{2}$ )