The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards
, Ilias N. Tziavos 6 and Robert Cullen 3Round 1
Reviewer 1 Report
This manuscript provides a complete and detailed cal/val of the sea surface height measurements from Sentinel-3A, Sentinel-3B and Jason-3 satellite altimeters using the Permanent Facility for Altimetry Calibration of ESA. Altimeter performance is assessed using independent in-situ measurements by land transponder and tide gauges, while the coordinates of cal/val sites are referenced to ITRF2014. Three techniques are used to assess the performance of each altimeter, which are range calibration using a transponder, sea-surface-height calibration relying upon sea-surface cal/val sites, and crossover analysis. The manuscript is well organised and written. It is my view that it should be accepted and published after a minor revision that address the following issues.
The FRM appears in the title and throughout the manuscript, but relative information about it is not introduced. For example, what kinds of measurements belong to FRM or FRM-class data? What are FRM standards? Although references are shown, it is better to have a brief introduction to the FRM.
The calibration results with two decimal place in mm are listed in Tables. My concern is whether or not the computed results can be accurate to 0.01 mm, given that the measurements are just in cm accuracy level.
Line 106: “(…, HY-2C and Quanlan)”
Line 107: “(…, wide swath and different frequencies)”
Line 116: calibration/validation (Cal/Val)
Line 118: (e.g., …)
Line 157: Figure 1 or Fig. 1? The figure caption is Figure XX, but is mentioned in text as Fig. XX. Should they be consistent? Figure 1 also appears before it is mentioned in text.
Lines 212-213: Change “sea surface height” to “SSH”.
Line 260: “sigma0” or “sigma-naught”? Be consistent.
Line 462: Here it said that “The uncertainty for each constituent is classified in two categories: Type A and Type B”. Are there uncertainty ranges for A and B? For example, A is for uncertainty < 0.50 mm and B is uncertainty >= 0.50 mm?
Table 4: Why is the confidence level 68% used? It is a quite small confidence level compared to those of 90% or 95% usually used.
Equation (1): What does [F] stand for?
Author Response
Responses to ReviewersJournal: Remote SensingManuscript Number: remotesensing-875092Section/Special Issue: Ocean Remote Sensing/Calibration and Validation of Satellite AltimetryArticle Title: The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 using transponder and sea-surface calibrations with FRM standards. Date: 10 August 2020-------------------------------------------------------------------- Response: We thank the Editor and the reviewers for their excellent work and effort. It is much appreciated. -------------------------------------------------------------------- Here are the responses to all reviewers’ comments and remarks. All line numbers below refer to the revised document with track changes. Responses to Reviewer #1: Reviewer: This manuscript provides a complete and detailed cal/val of the sea surface height measurements from Sentinel-3A, Sentinel-3B and Jason-3 satellite altimeters using the Permanent Facility for Altimetry Calibration of ESA. Altimeter performance is assessed using independent in-situ measurements by land transponder and tide gauges, while the coordinates of cal/val sites are reference to ITRF2014. Three techniques are used to assess the performance of each altimeter, which are range calibration using a transponder, sea-surface-height calibration relying upon sea-surface cal/val sites, and crossover analysis. The manuscript is well organized and written. It is my view that it should be accepted and published after a minor revision that addresses the following issues. -------------------------------------------------------------------- Response:
Thank you very much for been so kind. We have tried to respond to all your comments.
-------------------------------------------------------------------- Reviewer: The FRM appears in the title and throughout the manuscript, but relative information about it is not introduced. For example, what kinds of measurements belong to FRM or FRM-class data? What are FRM standards? Although references are shown, it is better to have a brief introduction to the FRM. -------------------------------------------------------------------- Response:
A small paragraph has been inserted in the “Introduction” section (page 3, lines 123-130) to introduce the FRM strategy.
As regards the type of measurements that belong to FRM, these are detailed in Section 3 and especially in Sections 3.2 and 3.3. for sea-surface and transponder calibration methodologies respectively.
-------------------------------------------------------------------- Reviewer: The calibration results with two decimal place in mm are listed in Tables. My concern is whether or not the computed results can be accurate to 0.01 mm, given that the measurements are just in cm accuracy level. -------------------------------------------------------------------- Response:
You are absolutely right, and we support your stance. That is why the FRM uncertainty at the end is at ±3-4 cm.
-------------------------------------------------------------------- Reviewer: Line 106: “(…., HY-2C and Quanlan). -------------------------------------------------------------------- Response:
Done. Please see revised manuscript (page 3, line 106)
-------------------------------------------------------------------- Reviewer: Line 107: “(…., wide swath and different frequencies). -------------------------------------------------------------------- Response:
Done. Please see revised manuscript (page 3, line 107)
-------------------------------------------------------------------- Reviewer: Line 116: calibration/validation (Cal/Val)-------------------------------------------------------------------- Response:
Done. Please see revised manuscript (page 3, line 117)
-------------------------------------------------------------------- Reviewer: Line 118: “(e.g., ….). -------------------------------------------------------------------- Response:
Done. Please see revised manuscript (page 3, line 118)
-------------------------------------------------------------------- Reviewer: Line 157: Figure 1 or Fig. 1? The figure caption is Figure XX, but it is mentioned in text as Fig. XX. Should they be consistent? Figure 1 also appears before it is mentioned in the text. -------------------------------------------------------------------- Response:
Thank you. You are right. According to journal’s template “All figures and tables should be cited in the main text as Figure 1, Table 1, etc.”. We have replaced all “Fig. XX” in the main text with “Figure XX”.
-------------------------------------------------------------------- Reviewer: Lines 212-213: Change “sea surface height” to “SSH”-------------------------------------------------------------------- Response:
We would prefer to use in this sentence “sea surface height”, thus to allow, please, authors cadences.
-------------------------------------------------------------------- Reviewer: Line 260: “sigma0” or “sigma-naught”-------------------------------------------------------------------- Response:
Thank you. Both expressions are correct and are found in the literature. They have been used interchangeably.
-------------------------------------------------------------------- Reviewer: Line 462: Here it is said that “The uncertainty for each constituent is classified in two categories: Type A and Type B”. Are these uncertainties range for A and B? For example, A is for uncertainty < 0.50 mm and B is uncertainty ≥ 0.50 mm?-------------------------------------------------------------------- Response:
The manuscript describes that “…Type A uncertainties are evaluated by statistical analysis of observations, whereas Type B ones are estimated by scientific judgment based on manufacturer’s specs, prior experience, handbooks, etc.”. Thus, Type A and Type B uncertainties do not indicate an uncertainty range but they characterize the method for uncertainty estimation. Details may be found in the “Bureau Int. des Poids et Measures”.
-------------------------------------------------------------------- Reviewer: Table 4: Why is the confidence level 68% used? It is a quite small confidence level compared to those of 90% or 95% usually used-------------------------------------------------------------------- Response:
Thank you very much for your observation. The confidence level of 68% is indicative of the uncertainty to allow readers appreciate the internal uncertainty of the data set. The next step of 95% confidence level does not increase confidence on the results. All confidence levels rely upon a distribution (mostly Normal distribution), data quality, infinite data, and statistical independence. None of these conditions are really true in our case. Confidence levels are often invalid because of the Gaussian assumption failed. It is rather precarious, and we do not learn much at this stage, to increase the confidence level.
-------------------------------------------------------------------- Reviewer: Equation (1): What does [F] stands for? -------------------------------------------------------------------- Response:
[F] is a temperature dependent factor that is further analyzed in the final part of the equation. Please see revised text, line 556.
Reviewer 2 Report
Review of “The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-surface Calibrations with FRM Standards” by Mertikas, et al.
This paper presents results of absolute radar altimeter range calibrations from transponders and crossover differences similar to previous papers in Remote Sensing (both Mertikas et al. 2018). This manuscript updates results for Jason-3 and Sentinel-3A and provides the first results for Sentinel-3B. While these analyses are critical to the cal/val of the altimetry record, the unique significance of this paper largely relies on the novelty of the results from the Sentinel-3A/3B tandem phase and the first results from the Sentinel-3B operational orbit. I have some concerns that the paper lacks a thorough statistical analysis of the results. In addition, because the Jason-3 results represent an incremental update of previous results, in their current form, the results are somewhat closer to a technical report rather than a scientific publication. However, for the scope of Remote Sensing, I would recommend publication of this manuscript after some minor revisions.
Major comments:
- The introduction is far too lengthy and includes several paragraphs of material not pertinent for Remote Sensing scientific publication. Below I recommend a few deletions and changes, but I would recommend that the authors focus the introduction closer to the scope of the paper.
- The results in the paper are the product of considerable work to create multiple independent assessments of the altimeters using fiducial measurements. Consequently, it is a bit disappointing that this paper does not use these multiple bias estimates in a more thorough statistical analysis. First, I would expect tables 6 and 7 to have included uncertainty estimates for each of the results. More critically, for the results in Table 7, rather than providing the average of the independent measurements of the biases, it would be more useful to subject the results to statistical tests (e.g. Student’s t-test) to demonstrate if the absolute and relative biases are significantly different from each other. Finally, I can not find the results of Figure 32 cited or discussed in the text. Given the crucial nature of the results in the figure, they should be at least summarized in the text.
Minor comments
Line 26: Change “Radar altimeters are to provide” to “Radar altimeters are used to provide”
Line 50: Please rephrase “have been establishing”. Perhaps “have established” would be better.
Line 56: Drop “110” from this sentence and reword to better match the citation. The reference shows that 110 million people are vulnerable at present to coastal flooding. The optimistic sea level rise scenario will make an additional 80 million people vulnerable.
Lines 67-86: Please remove these paragraphs. Very little of this material is germane to this scientific paper and can easily be covered with a citation.
Line 89: Change “is Sentinel-3 and” to “are Sentinel-3 and”
Line 90: Consider adding “ocean heat content” to the list of derived information from altimetry.
Line 94-102: Please reorganize this paragraph. It does not have a clear topic sentence.
Line 95: Consider adding “TOPEX/POSEIDON and” to “the Jason series”.
Line 141: Change “Sentinel-3A, Sentinel-3A” to “Sentinel-3A, Sentinel-3B”
Line 203: Change “which” to “that”
Line 253: The figures aren’t legible, particularly the CryoSat-2 figure. It is not possible to determine what quantities are plotted. I would recommend that the plots be improved or removed. At present, they do not provide any useful illustration for their citation in line 250.
Line 260: Change the period after “wind speed” to a comma.
Line 304: Change “Sentinel-B” to Sentinel-3B”
Line 361: Figure 5 is also difficult to read. The size of the upper plot and lower plot should be similar.
Line 415: The labels on Figure 8 also can’t be easily read.
Line 523: Change “built” to “build”
Lines 545-6: Drop the first sentence. It is not clear why the swath and global coverage are relevant.
Lines 620-621: Please make the references to altimeter/mission consistent (e.g. add Poseidon-3B to Jason-3 and reverse the order of Poseidon-4 and Sentinel-6).
Line 654: Rather than “justified as,” “explained by” may be a better word choice.
Line 657: Does “uncontaminated” mean “uncontaminated by land”. It may be better to clarify.
Lines 672-679: Throughout this paragraph, the standard error or uncertainties in these biases should be included.
Lines 715-718: Again, some errors should be included in this paragraph.
Line 720: Drop “The” before “Jason-3”.
Line 763: Uncertainties should be included in Table 6.
Line 790: I think that a word may be missing. Perhaps “only indicative”?
Line 809: What is meant by “exact SSH value”?
Line 826: Table 7 is somewhat confusing. I would recommend removing the color. It is misleading since the columns change for each sub table. Since these tables are largely independent, it would be clearer if these were clearly separated into 3 tables.
Line 826: I am not sure that I understand the -6 mm in a separate row for S3A pass 335.
Line 832: Can you provide a citation for the specification for the bias?
Line 836-7: Do you need to clarify that #6 applies only to the tandem S3B mission?
Line 900: I suggest that you drop the sentence on scatterometers, which does not seem to be relevant here.
Line 902: Remove “All” from the sentence, which isn’t true. Replace it with “Some”. For example, WaveWatch III does not include altimeter data.
Author Response
Journal: Remote Sensing
Manuscript Number: remotesensing-875092
Section/Special Issue: Ocean Remote Sensing/Calibration and Validation of Satellite Altimetry
Article Title: The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 using transponder and sea-surface calibrations with FRM standards. Date: 10 August 2020--------------------------------------------------------------------
Response: We thank the Editor and the reviewers for their excellent work and effort. It is much appreciated. --------------------------------------------------------------------
Here are the responses to reviewers’ comments and remarks. All line numbers below refer to the revised document with track changes.
Responses to Reviewer #3:
Reviewer: This paper presents results of absolute radar altimeter range calibrations from transponders and crossover differences similar to previous papers in Remote Sensing (both Mertikas et al. 2018). This manuscript updates results for Jason-3 and Sentinel-3A and provides the first results for Sentinel-3B. While these analyses are critical to the cal/val of the altimetry record, the unique significance of this paper largely relies on the novelty of the results from the Sentinel-3A/3B tandem phase and the first results from the Sentinel-3B operational orbit. I have some concerns that the paper lacks a thorough statistical analysis of the results. In addition, because the Jason-3 results represent an incremental update of previous results, in their current form, the results are somewhat closer to a technical report rather than a scientific publication. However, for the scope of Remote Sensing, I would recommend publication of this manuscript after some minor revisions.
-------------------------------------------------------------------- Response:
Thank you very much. We have tried to respond to all your comments (major and minor). Please note that line numbers refer to the manuscript version with track changes enabled.
Reviewer: “I have some concerns that the paper lacks a thorough statistical analysis of the results”:
-------------------------------------------------------------------------------------------------------------------------------------------- Response:
- On the contrary, this works contains an extensive and thorough statistical analysis, presented succinctly in Tables 4 and 5. Every component in each line in those tables comes from an extensive statistical analysis. All this FRM statistical analysis is about 80-page length. This analysis cannot be presented in this paper.
- New procedures with fiducial reference measurements are being implemented to promote extensive evaluations and a realistic appreciation for the uncertainty of altimetry measurements but also to trace uncertainties for sea and water level to metrology standards (i.e., atomic time, speed of light).
- For each category, standard uncertainty budget has been derived including all aspects (Instrument measurement, Retrieval/algorithm, Application, Unknown) leading to a quantification of a root-sum-square (RSS) estimate of uncertainty. This is a challenging exercise but nevertheless, for climate and satellite validation activities, it is a requirement. Establishing an uncertainty budget for FRM is a fundamental step that drives a better understanding of the various components of FRM uncertainty. If reliable and well-defined uncertainties can be provided with each FRM uncertainty constituent (as those lines in both Tables 4 and 5), then these measurements can uniquely provide an SI traceable measurement on a per-measurement basis when matched to satellite measurements – without the need for many observations to reduce the random error (conventional Parametric Statistics).
--------------------------------------------------------------------
Reviewer: The introduction is far too lengthy and includes several paragraphs of material not pertinent for Remote Sensing scientific publication. Below I recommend a few deletions and changes, but I would recommend that the authors focus the introduction closer to the scope of the paper
-------------------------------------------------------------------- ---------------------------------------------------------------------------------Response:
The authors do not agree that the introduction is far too lengthy. There are already several examples of manuscripts published in Remote Sensing journal, some of them even published in the same Special Issue (i.e., https://0-doi-org.brum.beds.ac.uk/10.3390/rs12111763, https://0-doi-org.brum.beds.ac.uk/10.3390/rs12122003) that contain “Introduction” of larger length than ours.
The “Introduction” Section aims at demonstrating the benefit of the huge investments made by Europe (European Space Agency, European Union, EUMETSAT, etc.) in Earth Observation programs such as “Copernicus”. It is thus essential to present the broad range of environmental and societal applications that exploit satellite products from Copernicus satellite missions. In order to maximize these investments, it is also vital to assess the performance of the satellite products by independent means, like the present Cal/Val facilities.
Thus, the “Introduction” starts with a brief description of the Earth Observation via remote sensing technologies, the Copernicus program and its wide range of applications in general, and the satellite altimetry missions in particular. This description does not cover more than 1.5 pages. Then, the main objective and intend of the paper is given.
--------------------------------------------------------------------
Reviewer: The results in the paper are the product of considerable work to create multiple independent assessments of the altimeters using fiducial measurements. Consequently, it is a bit disappointing that this paper does not use these multiple bias estimates in a more thorough statistical analysis. First, I would expect tables 6 and 7 to have included uncertainty estimates for each of the results.
----------------------------------------------------------------------------------------------------------------------------------------------------Response:
Figures No. 16, 17, 18, 20, 21, 23, 24, 26, 27, 28 as well as the text in Section 4 present the bias results of the satellite altimeters (Sentinel-3A, Sentinel-3B and Jason-3) along with the associated uncertainty. Furthermore, the FRM uncertainty per calibration method (i.e., sea-surface, transponder) has been estimated and given in Section 3.
For illustration reasons these uncertainty estimates have not been included in Tables 6 and 7 to keep the Tables clear and easy for the readers to get the gist of the results.
--------------------------------------------------------------------
Reviewer: More critically, for the results in Table 7, rather than providing the average of the independent measurements of the biases, it would be more useful to subject the results to statistical tests (e.g. Student’s t-test) to demonstrate if the absolute and relative biases are significantly different from each other.
-----------------------------------------------------------------Response:
The standard parametric statistical analysis (including the Student’s t Test, statistical significance, etc.) mainly describes the internal uncertainty of the results, but its validity relies upon Gaussian distributions, statistical independence (no serial correlation), infinite number of data, large degrees of freedom. None of these assumptions truly apply to altimetry data and results.
-----------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer: Finally, I cannot find the results of Figure 32 cited or discussed in the text. Given the crucial nature of the results in the figure, they should be at least summarized in the text.
--------------------------------------------------------------------
Response:
Thank you for pointing this out. A short interpretation of the results presented in Figure 32 has been inserted in the manuscript. Please see Section 5, lines 949-952.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer: Line 26: Change “radar altimeters are to provide” to “Radar altimeters are used to provide”.
--------------------------------------------------------------------
Response:
Done. Please see Line 26 in the revised manuscript.
--------------------------------------------------------------------
Reviewer: Line 26: Change “radar altimeters are to provide” to “Radar altimeters are used to provide”.
--------------------------------------------------------------------
Response:
Done. Please see Line 26 in the revised manuscript.
--------------------------------------------------------------------
Reviewer: Line 50: Please rephrase “have been establishing”. Perhaps “have established” would be better.
--------------------------------------------------------------------
Response:
Done. Please see Line 50 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 56: Drop “110” from this sentence and reword to better match the citation. The reference shows that 110 million people are vulnerable at present to coastal flooding. The optimistic sea level rise scenario will make an additional 80 million people vulnerable.
---------------------------------
Response:
Done. Please see Lines 56-57 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Lines 67-86: Please remove these paragraphs. Very little of this material is germane to this scientific paper and can easily be covered with a citation.
--------------------------------------------------------------------
Response:
Please see our previous response to your comment about the length of the “Introduction” section.
--------------------------------------------------------------------
Reviewer Line 89: Change “is Sentinel-3 and” to “are Sentinel-3 and”
--------------------------------------------------------------------
Response:
Done. Please see Line 90 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 90: Consider adding “ocean heat content” to the list of derived information from altimetry
.--------------------------------------------------------------------
Response:
The sentence has been rephrased. Please see Line 93 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 94-102: Please reorganize this paragraph. It does not have a clear topic sentence.
--------------------------------------------------------------------
Response:
The paragraph has been restructured. Please see Lines 95-106 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 95: Consider adding “TOPEX/POSEIDON and” to “the Jason series”.
--------------------------------------------------------------------
Response:
Done. Please see Line 97 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 141: Change “Sentinel-3A, Sentinel-3A” to “Sentinel-3A, Sentinel-3B”
--------------------------------------------------------------------
Response:
Done. Please see Line 150 in the revised manuscript (caption of Figure 1).
--------------------------------------------------------------------
Reviewer Line 203: Change “which” to “that”
--------------------------------------------------------------------
Response:
Done. Please see Line 212 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 253: The figures aren’t legible, particularly the CryoSat-2 figure. It is not possible to determine what quantities are plotted. I would recommend that the plots be improved or removed. At present, they do not provide any useful illustration for their citation in line 250.
--------------------------------------------------------------------
Response:
Thank you. The CryoSat-2 figure has been replaced with a clear one.
--------------------------------------------------------------------
Reviewer Line 260: Change the period after “wind speed” to a comma.
--------------------------------------------------------------------
Response:
Done. Please see Line 271 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 304: Change “Sentinel-B” to Sentinel-3B”
--------------------------------------------------------------------
Response:
Done. Please see Line 314 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 361: Figure 5 is also difficult to read. The size of the upper plot and lower plot should be similar.
--------------------------------------------------------------------
Response:
Done. Please see revised Figure 5 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Line 415: The labels on Figure 8 also can’t be easily read.
--------------------------------------------------------------------
Response:
Done. Please see new Figure 8 in the revised manuscript.
--------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 523: Change “built” to “build”
--------------------------------------------------------------------
Response:
Please see Line 537 in the revised manuscript.
--------------------------------------------------------------------
Reviewer Lines 545-6: Drop the first sentence. It is not clear why the swath and global coverage are relevant.
--------------------------------------------------------------------
Response:
The sentence has been rephrased. Please see Line 569 in the revised manuscript.
----------------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Lines 620-621: Please make the references to altimeter/mission consistent (e.g. add Poseidon-3B to Jason-3 and reverse the order of Poseidon-4 and Sentinel-6).
------------------------------------------------
Response:
Done. Please see Lines 636-637 in the revised manuscript.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 654: Rather than “justified as,” “explained by” may be a better word choice.
----------------------------------------------
Response:
Text revised. Please see Lines 710-715 in the revised manuscript.
--------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 657: Does “uncontaminated” mean “uncontaminated by land”. It may be better to clarify.
------------------------------
Response:
Done. Please see Line 710-715 in the revised manuscript.
-----------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Lines 672-679: Throughout this paragraph, the standard error or uncertainties in these biases should be included.
-------------------
Response:
Done. Please see Lines 691-692 in the revised manuscript.
-------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Lines 715-718: Again, some errors should be included in this paragraph.
------------------
Response:
Done. Please see Lines 737-738 in the revised manuscript.
--------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 720: Drop “The” before “Jason-3”.
-----------------------------
Response:
Done. Please see Line 742 in the revised manuscript.
--------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 763: Uncertainties should be included in Table 6.
-----------------------------
Response:
Please see our response to one of your previous comments.
----------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 790: I think that a word may be missing. Perhaps “only indicative”?
--------------------------------
Response:
Done. Please see Line 811 in the revised manuscript.
-------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 809: What is meant by “exact SSH value”?
-------------------------------
Response:
‘Exact” was replaced by “the exact SSH value at the crossover location for each…”.
--------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 826: Table 7 is somewhat confusing. I would recommend removing the color. It is misleading since the columns change for each sub table. Since these tables are largely independent, it would be clearer if these were clearly separated into 3 tables.
------------------------------------
Response:
Each color corresponds to a different satellite mission. The purpose of Table 7 is to summarize the results already presented in Section 4 (text and figures). This is a typical form for summarizing results and from our perspective is not confusing but it helps the reader to clearly understand the given values.
Splitting Table 7 to three independent tables (per satellite mission? Per calibration method?) as per your recommendation would lead to repetition of the same information.
---------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 826: I am not sure that I understand the -6 mm in a separate row for S3A pass 335.
-------------------------
Response:
Table has been updated. Sentinel-3A Pass No. 335 is calibrated using two Cal/Val sites: Gavdos and RDK1. The -6.00 mm bias refers to RDK1 Cal/Val site, whereas -10 mm corresponds to the bias as determined by the Gavdos Cal/Val facility.
-------------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 832: Can you provide a citation for the specification for the bias?
-------------------------------
Response:
Done. Please see Line 843 in the revised manuscript.
-------------------------------------------------------------------- -------------------------------------------------------------------------------
Reviewer Line 836-7: Do you need to clarify that #6 applies only to the tandem S3B mission?
--------------------------------
Response:
The positive sign of the Sentinel-3B SSH bias as determined during its operational phase (nominal orbit) entails that Sentinel-3B measures the SSH higher than the “true/reference” SSH. The opposite stands for Sentinel-3A. Thus, the statement that Sentinel-3B seems to measure SSH higher than Sentinel-3A is valid for both tandem and operational phases.
-----------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 900: I suggest that you drop the sentence on scatterometers, which does not seem to be relevant here.
----------------------------
Response:
The aim of the sentence is to demonstrate the usefulness of the backscatter coefficient in multi-disciplinary Earth Observation satellite instruments, and subsequently the need for the development of a sigma-naught transponder. We would prefer of keeping this sentence as it is, please.
-----------------------------------------------------------------------------------------------------------------------------------------------------
Reviewer Line 902: Remove “All” from the sentence, which isn’t true. Replace it with “Some”. For example, WaveWatch III does not include altimeter data.
------------------------------
Response:
Done. Please see Line 924 in the revised manuscript.
-------------------------------------------------------------------- Again, thank you very much for your meticulous and detailed review of this manuscript.
Reviewer 3 Report
The manuscript is well written but it is not easy to find how the calibration is actually performed. How the altimeter data were calibrated against ground truth data.
HY-2B has been mentioned in the caption of Figure 1 but I could find on the figure the track of HY-2B.
Moreover, if the track of HY-2B has been included, where the HY-2B data have been obtained?
Author Response
Journal: Remote SensingManuscript Number: remotesensing-875092Section/Special Issue: Ocean Remote Sensing/Calibration and Validation of Satellite AltimetryArticle Title: The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 using transponder and sea-surface calibrations with FRM standards. Date: 10 August 2020--------------------------------------------------------------------
Response: We thank the Editor and the reviewers for their excellent work and effort. It is much appreciated. --------------------------------------------------------------------
Here are the responses to all reviewers’ comments and remarks. All line numbers below refer to the revised document with track changes.
Responses to Reviewer #2:
Reviewer: The manuscript is well written but it is not easy to find how the calibration is actually performed. How the altimeter data were calibrated against ground truth data.
--------------------------------------------------------------------
Response:
Thank you very much. The calibration procedures have been described in several publications. Also, in Section 2.2, line 218 it is stated that: “Details on PFAC-employed Cal/Val techniques can be found in [14]”. Furthermore, Sections 2.2.1 and 2.2.2 provide a brief description of the sea-surface and transponder calibrations.
--------------------------------------------------------------------
Reviewer: HY-2B has been mentioned in the caption of Figure 1 but I could not find on the figure the track of HY-2B.
--------------------------------------------------------------------
Response:
The ground track of HY-2 satellites (both HY-2A and HY-2B) are given in Figure 1 (top right). Please see the right image in the top row. The HY-2 ground tracks are marked with pink color.
--------------------------------------------------------------------
Reviewer: Moreover, if the track of HY-2B has been included, where the HY-2B data have been obtained?
--------------------------------------------------------------------
Response:The Gavdos/Crete Cal/Val facilities supports the calibration and validation activities for several international altimetry missions such as S3A, S3B, Jason-3, CryoSat-2, HY-2B, etc. In the past it has been involved in the calibration of Jason-1, Jason-2, Envisat, SARAL/AltiKa, and HY-2A altimetry missions. This work presents the latest calibration results for some of these missions and specifically Sentinel-3A, Sentinel-3B and Jason-3. The HY-2B calibration results as determined by the Gavdos/Crete facilities will be presented in an upcoming publication.
