In this paper the authors present a novel validation method to assess the instrumental drifts in the global mean sea level trends based on the implementation of a second phase in which the two successive reference altimetry satellites fly in tandem a few years after the first. They quantify the uncertainties of the trend such method would achieve depending on the duration of the second tandem flight phase and the length of time between the two phases and compare the results with other validation methods, proving the usefulness of implementing the second tandem phase.

The manuscript represents a significant scientific contribution to achieving stable and consistent sea level measurements. However, while the methods applied in this study are most likely valid, it is hard to fully assess that because they are not presented in a clear way or sometimes at all, leaving some questions about them unanswered.

Major comments

1. It is hard to understand the methodology and follow the paper the way it is written because many parts are not explained, but only reference other papers where the method is described. It is perhaps not necessary to go into every detail as in the papers where the method is used first, but the reader should not need to read several other papers to understand what was done in this one. The paper should include a better explanation of how the covariance matrix for one tandem phase is created and how the number of independent observations is calculated, as well as what that number means for the calculation of the uncertainties. An additional problem is the way the manuscript is structured: Sect. 2 describes the method to quantify the uncertainty of the 2-tandem phase, which requires the error covariance matrix for each of the phases. It is not explained well enough how these error covariance matrices are calculated, but also the description of what they contain only comes after, in Sect. 3, which completely disrupts the flow of text.

2. It could be because of my misunderstanding due to lacking methodology explanations, but the sensitivity study and its conclusions do not seem convincing at all. You claim that the sensitivity tests showed that there is fairly low sensitivity of the uncertainty to the temporal correlation of errors and/or the variance because the uncertainty varies between 0.06-0.18 and 0.11-0.17, which correspond to 0.12 and 0.06 range, respectively. However, based on Fig. 2, the uncertainty varies between 0.12 and 0.18 depending on the duration of the second phase (with 2 years and 9 months between phases), so also only 0.06 difference. Same if you change the length between the two tandem phases from 3 to 6 years (double!), the uncertainty is only decreased by 0.06, from 0.13 to 0.07. To me that seems like the uncertainty is a lot more sensitive to the choice of decorrelation time than to the length of the second tandem phase or even the time between the two phases. Or am I misunderstanding something here? It would help to explain better how you chose the 1 month for the temporal correlation to make sure it is a good choice, since it seems to affect the results quite a lot.

Other comments

L61-65: This is a very long and quite hard to read sentence.

L86: This seems like a wrong unit or a very wrong number.

L106: dot missing

L? (p5 bottom): “contain the along-track sea level anomaly at 1Hz (SLA, see Eq. (1))”

What does Eq. (1), which is for the estimator of beta, have to do with SLA? Seems like it is referring to the wrong equation.

L? (p6 top): “The along-track SLA provided in the L2P products is derived from the following equation:

SLA = Orbit − Range − Σ_i Correction_i − MeanSeaSurface”

What is the meaning of each of the variables in this equation?

L137-145 It is not clear to me whether you here describe how the dataset you downloaded and used was created or the processing steps you applied to the dataset before using it. It is especially unclear what was done here considering the “regional scales” you refer to throughout the rest of the manuscript are using different longitude-latitude box sizes that the 1 degree latitude  per 3 degrees longitude mentioned here. Additional minor comment: You here use degree (word), and in other places in the manuscript degree symbol when describing the size of the box. Please be consistent.

L147-152 This is a general statement, not specifically for the global scale, so it should be in the previous subsection.

L151-152 Why exactly are you referring to the previous section here? Seems completely unnecessary and it disrupts the reading.

L156-157 1. Is the dashed line the periodic signal or the differences before removing it? The way this sentence is phrased, it could also be understood as the dashed line being the difference with the periodic signal removed, which does not seem the case when looking at the figure. The explanation of what the grey dashed line is should also be in the figure caption; 2. How much did the removal of the signal reduce the standard deviation (or what was the standard deviation before removing this signal)?

L164-167 This could be because there was no proper explanation of what is the meaning of n and its calculation, but I cannot follow this conclusion. Could you please explain the reasoning behind it?

L180 In this section you seem to focus on the Jason-3 and S6-MF tandem phases, but you do not clearly state that, you only mention that you use the duration of the first tandem phase from those missions and refer to Tab. 1 for the uncertainty budget, which contains the uncertainties for all 3 pairs of satellites. Could you please make this clearer.

L193 10.16 seems to be the wrong number, probably a typo.

L194-195 This conclusion comes out of nowhere because the specific Jason-3/S6-MF 2-tandem phase scenario has only been mentioned once before in the manuscript, at the end of the introduction, where the duration of the second phase is not mentioned at all, just when the second phase would be. You need to elaborate this scenario better before discussing results and conclusions about it, not after. It would also be good to know how and why was this particular scenario chosen.

L199-206 You might want to put the sensitivity tests into a separate paragraph to improve readability of the manuscript.

L212 I do not understand what does “corresponding to regional scales” mean in this context.

L221 Nothing is actually marked as a results section and there are two sections that describe the results, the previous one (Sect. 4) and this one (Sect. 5).

L228 These are not the same satellites as in Sect. 4. Are you comparing the results for the without-tandem method with the results from Sect. 4. for the Jason-3 and S6-MF or are you re-calculating the 2-tandem uncertainties for other satellites with different short-term time-correlated errors?

L232 & L240 You refer to Tab. B1 and Tab. C1 before Tab. A1.

L285 methods, there are two of them

L301-302 "since" is written twice

Fig. 2. What is the unit of lag? When discussing the time with no autocorrelation, you use months, so could you explicitly relate that to whatever is shown here. Also, please note clearly what is the grey line in (f) here, not just in the main text.

Fig. 3. Should be space agency, not spatial.

Fig. 4. Since in the text you sometimes refer to box sizes in kilometers, could you please add that in the figure, it would make it easier to follow.

Tab. 2. Could you please also provide the values for other box sizes used in the study?

There is an error with the format of citations throughout the paper, most of them have too many brackets.

Review of "Benefits of a second tandem flight phase between two successive satellite altimetry missions for assessing the instrumental stability" by Ablain et al.

This papers describes a potential second tandem flight phase and the benefits this would bring to the calibration of two reference altimeters. 

While the topic is of interest, he paper is often too brief in describing what has been done, and the reader needs to look for the cited references in order to understand the methodology (and sometimes even those references don't give the information we're after). For example, in section "5.1 Uncertainty budgets of other validation methods" the authors say "the altimeter measurements are not performed exactly at the same time and location". There is no information about the maximum allowed differences in space and time between altimeter and tide gauge data, or between 2 altimeters, so the reader is unable to assess how this comparison is done. The cited references do not give this information either, or I have been unable to find them (one of the references, Ablain et al 2018, is not complete in the list of references and I have been unable to find it).

The manuscript contains also various vague statements or definitions, in particular when using the word "uncertainty" throughout the manuscript. It should be clear at each moment of the paper which uncertainty are they talking about (example, the title of section 2 "Method to quantify the uncertainty of the 2-tandem phase method", which uncertainty? As it is phrased, it sounds like the uncertainty that the 2-tandem phase takes place or not...). Also, there are  2 "method" in the same sentence. The term "estimated" should also go with uncertainty in many parts of the manuscript.

I'll specify my comments in order of appearance:

Line 39, but general comment: please check the parentheses of your citations, they are often wrong ("identified and characterised by (Ablain et al., 2019b; Guérou et al., 2023; Prandi et al., 2021)." should be "identified and characterised by Ablain et al. (2019b), Guérou et al. (2023) and Prandi et al. (2021)."

line 81: "drifts in the seal level data record". Strange wording, it is not the sea level that drifts (it changes, and it is not that change the authors are after) but the satellite sensors' accuracies. 

Line 116: "This uncertainty budget enumerates the various sources of uncertainty", please enumerate which ones.

Line 117: "standard deviation associated with errors": which errors are you talking about here?

Line 120 \Sigma_TP should be \Sigma_tp

Equations (and text), please put matrices in bold characters, especially \Sigma since it is used later as the summation symbol.

Line 136 The equation there is given without describing each of its components, and what the components i of the correction are.

Line 128: "relative errors observed during the two tandem phases". No information is given about these "relative errors", which errors and how are they estimated/calculated. Also, in line 134 it says "we can study the uncertainty of the 2-tandem phase method without having yet executed the second tandem phase" so the use of "observed" in the first sentence is a bit misleading.

Line 142: the choice of 1 degree in latitude and 3 degree in longitude has been used elsewhere, but some explanation about this choice should be provided.

Line 145. Explain the GMSL AVISO method briefly

Line 154 (and figure 2). A 2-month periodic signal is mentioned in panel f. The attribution to POD is given without reasoning, and in fact POD uncertainties are part of the uncertainty budget you are trying to assess, so why removing it? Also, you say that the data without this 2-month signal is the dashed line in figure 2f, but for me this line shows a much more evident periodic signal. Further down the text it is mentioned that "the autocorrelation of each GMSL difference does not show an obvious time dependency", and that the "the GMSL differences are fully decorrelated beyond one month" which is expected since I guess this has been done after removing this 2-month signal?

Line 176 "The standard deviation is assigned to the uncertainty budget for the 1-month correlated error (see Tab. 2), homogeneously to the global scale." Sentence not complete?

line 193: 10.16 mm yr-1 sounds huge

Line 229: provide details about the uncertainty budgets and what are the maximum delta_t and delta_{x,y} used to compare 2 altimeters and an altimeter to tide gauge data

Line 246 You repeat that you are doing two additional methods (line 246-248)

line 324 Incomplete reference (pages, journal?) 

Figure 1. Line blue is the "relative sea level drift" Again, I think this wording is confusing. 

Figure 2. central figures should have a reduced y-axis to see better the variability. Same for bottom panels. Indicate what the dashed line is in 2f.

Figures 3, 4 and 5: the symbols are quite small and difficult to tell apart (especially in figures 3 and 4). The legend contains "uncertainty at 68%" but it is not shown in the figures. The vertical and horizontal lines within the figures seem to be added rather randomly, and in figure 4 there is an extra vertical dashed line that is not explained