An Action Plan Towards Fiducial Reference Measurements for Satellite Altimetry
Round 1
Reviewer 1 Report
The authors share their rich experience gained during active CalVal activities with many altimetric satellite missions. They demonstrate great technical skills and in this paper they sometimes go into small details that can only be useful for equally experienced specialist.
Because the system has proven working for several decades (including other CalVal sites), the article has practical importance and will be useful to experts in Europe and other countries. I think it should be published after addressing the following issues.
1. Diversity of satellite instruments. The article focuses on accuracy of point observations of sea level and their use to calibrate some “standard” satellite altimeter. However, altimeters evolved over time from radars to lidars and to SAR interferometers that raises important questions not mentioned in the article. How to inter-calibrate a point pressure gauge and a 25-km footprint of a radar altimeter? How to inter-calibrate the same gauge with a tiny lidar footprint that does not even always crosses the gauge site? How can a single-point site help to calibrate wide-swath instrument?
2. Pluralism of tasks and products associated with altimetry. Currently, altimetry has many different applications. This includes sea level rise (mentioned in the paper), mapping eddies, geostrophic currents, basin-wide processes (ENSO, PDO, etc), etc. Each application has different requirements (resolution, accuracy of along-track gradient, large-scale biases). I don’t see how one reference site on Crete can provide global calibration. The authors should include discussion of the global network (or regional network if they want to focus on Europe).
3. Line 110-118. This one of “philosophical” paragraphs that feels strange. It starts with the suggestion to call “measure of uncertainty” a “scale” while to me it’s logical to use “scale” to characterize the useful signal, not its noise or errors. And it ends the statement that one can read as “the better the CalVal site the better is the altimeter performance” – do the authors mean that better CalVal will produce better PRODUCTS?
4. Lines 185-187. Sentence 1: “Earth is warming up in recent decades. Much of ice melting from the north and south caps, has gone into the ocean.” Correct.
Sentence 2: “This process of extra water mass is responsible for sea level rise, mostly by thermal expansion of the water as it warms.” Wrong! There are two processes: adding new water (regardless of the temperature) and steric effects due to warming of existing water, no new water is required.
5. Line 176. Typo: “Develop an approach”.
6. Line 698. “In general, the final discrepancy in absolute sense, between the geoid and the ellipsoid is no less than ±35 cm.” Wrong. Maximum deviation (globally) is greater than 100 meters! Geoid is the shape of an equipotential surface, exact value at specific location (e.g. 200px in line 701) is not relevant to anything.
7. In section 7.1.4. and Table 1 the authors discuss the geoid and mean dynamic topography. In practice, altimetry provides such critical product as the mean seas surface (MSS, the absolute shape of the ocean surface), which is then combined with the geoid to produce MDT (ocean surface signal, associated with ocean currents). MSS should be included in the discussion. Other important product that is not mentioned is sea level anomaly (SLA).
8. Abstract. Line 21. I am not sure what “products and services” are “expected to come into being” here. Please revise the abstract to align it with the actual manuscript.
Author Response
Responses to Reviewer #1: General comments: Reviewer: The authors share their rich experience gained during active CalVal activities with many altimetric satellite missions. They demonstrate great technical skills and in this paper they sometimes go into small details that can only be useful for equally experienced specialist. Because the system has proven working for several decades (including other CalVal sites), the article has practical importance and will be useful to experts in Europe and other countries. I think it should be published after addressing the following issues.
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Response: Thank you very much. Much appreciated. --------------------------------------------------------------------
Specific comments Reviewer: Diversity of satellite instruments. The article focuses on accuracy of point observations of sea level and their use to calibrate some “standard” satellite altimeter. However, altimeters evolved over time from radars to lidars and to SAR interferometers that raises important questions not mentioned in the article. How to inter-calibrate a point pressure gauge and a 25-km footprint of a radar altimeter? How to inter-calibrate the same gauge with a tiny lidar footprint that does not even always crosses the gauge site? How can a single-point site help to calibrate wide-swath instrument?.
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Response: The Permanent Facility for Altimetry Calibration in west Crete, Greece provides calibration and validation services attaining FRM standards not only for “standard” nadir-looking (i.e., Jason-1, -2, -3, HY-2A) but also for SAR altimeters (i.e., Sentinel-3A, -3B, etc.). It will be extended to wide-swath altimeters. It is not using point observations as its reference and benchmarking surfaces used for calibration covers a wide area around the Cal/Val site. It is not the purpose of this paper to present details and recommend on specific Cal/Val techniques to be applied for the calibration of nadir-looking, SAR-altimetry, wide-swath, etc. satellite altimetry missions. It intends to present a new strategy irrespectively of the Cal/Val technique used. --------------------------------------------------------------------
Reviewer: Pluralism of tasks and products associated with altimetry. Currently, altimetry has many different applications. This includes sea level rise (mentioned in the paper), mapping eddies, geostrophic currents, basin-wide processes (ENSO, PDO, etc), etc. Each application has different requirements (resolution, accuracy of along-track gradient, large-scale biases). I don’t see how one reference site on Crete can provide global calibration. The authors should include discussion of the global network (or regional network if they want to focus on Europe)
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Response: This paper presents guidelines and recommendations on how satellite altimetry calibration shall be implemented in practice based upon the experience gained by the Permanent Facility for Altimetry Calibration in west Crete, Greece. It is not our intention to claim that this site shall be unique but to provide the framework under which other, existing and future Cal/Val sites, shall operate and disseminate their results. This standardization process constitutes a way for integrating the results obtained by a global network of Cal/Val sites. A description of the global network of Cal/Val sites as been inserted in Section 2. We agree with the reviewer that different satellite altimetry applications have different requirements. However, these requirements have been taken into consideration by the corresponding agency in the design phase of any satellite altimetry mission. A Cal/Val site evaluates the performance of satellite products against mission requirements (basically calibrating its fundamental measurements, i.e., the range) prescribed in the early stages of the mission development.
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Reviewer: Lines 110-118. This one of “philosophical” paragraphs that feels strange. It starts with the suggestion to call “measure of uncertainty” a “scale” while to me it’s logical to use “scale” to characterize the useful signal, not its noise or errors. And it ends the statement that one can read as “the better the CalVal site the better is the altimeter performance” – do the authors mean that better CalVal will produce better PRODUCTS? --------------------------------------------------------------------
Response: The said paragraph aims to introduce the proper definition of location and scale of the critical parameters (e.g., range, sea-surface height, wet troposphere correction, etc.) that impact and characterize the satellite altimeter performance. The paragraph has been rewritten in the revised manuscript.
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Reviewer: Lines 185-187. Sentence 1: “Earth is warming up in recent decades. Much of ice melting from the north and south caps, has gone into the ocean.” Correct. Sentence 2: “This process of extra water mass is responsible for sea level rise, mostly by thermal expansion of the water as it warms.” Wrong! There are two processes: adding new water (regardless of the temperature) and steric effects due to warming of existing water, no new water is required.
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Response: Sentence has been revised.
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Reviewer: Line 698. “In general, the final discrepancy in absolute sense, between the geoid and the ellipsoid is no less than ±35 cm.” Wrong. Maximum deviation (globally) is greater than 100 meters! Geoid is the shape of an equipotential surface, exact value at specific location (e.g. 200px in line 701) is not relevant to anything.
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Response:
It has been revised. Thank you for spotting this mistake.
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Reviewer: In section 7.1.4. and Table 1 the authors discuss the geoid and mean dynamic topography. In practice, altimetry provides such critical product as the mean seas surface (MSS, the absolute shape of the ocean surface), which is then combined with the geoid to produce MDT (ocean surface signal, associated with ocean currents). MSS should be included in the discussion. Other important product that is not mentioned is sea level anomaly (SLA).
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Response:
Section 7.1 presents the constituents in sea-surface calibration technique and provides qualitative and quantitative guidelines for their estimation. In this respect, in Section 7.1.4 the reference surface (geoid, MDT, MSS) to be employed for implementing this Cal/Val technique is discussed. The altimeter’s performance has to be carried out independent and unrelated to its measurements but against reference surfaces (geoid, MDT, MSS) determined by other means. This is especially true when the Cal/Val site is not located exactly under the satellite ground track and transfer of the ground sea-surface measurements to the open sea has to be carried out.
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Reviewer: Abstract. Line 21. I am not sure what “products and services” are “expected to come into being” here. Please revise the abstract to align it with the actual manuscript.
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Response:
The said sentence refers to the products and services provided by satellite altimetry Cal/Val sites that attain the FRM concept. It has been revised.
Reviewer 2 Report
Review of the paper: remotesensing-558058
Title: An Action Plan Towards Fiducial Reference Measurements for Satellite Altimetry
General assessment
This paper presents the work performed under an EU/ESA project aimed at establishing Fiducial Reference Measurements for Altimetry (FRM4ALT). The authors introduce the problem and then, in a relatively exhaustive way, address the various issues related with the topic, namely the expected products and services, the underlying technologies and the required resources, skills, partnerships and facilities.
Regarding the content of the paper, in spite of the fact that it is not a typical research article, there is no doubt that it is useful, particularly section 7. However, some parts are too wordy, partly repetitive, making the paper a bit boring. Indeed the format of the paper is more of a project report than of a scientific paper (even of a review one).
I therefore recommend revision to account for the issues mentioned below.
Major points
1) Paper structure
Try to reorganize the information in fewer major sections, as mentioned in the abstract. Avoid “philosophical” and “common knowledge” statements and try to concentrate on the scientific content.
2) In spite of the exhaustive analysis performed by the authors, in my view there is an important point missing, possibly in section 6. The fiducial measurements are meant to calibrate altimeter observations. The calibration procedure includes not only the good practices required in the establishment, processing and monitoring of the fiducial measurements (well addressed in the paper) but also of the “processing of the altimeter measurements” themselves. This is even more relevant as the altimeter and fiducial measurements are usually not collocated. So, the calibration results will depend on many factors such as:
Adopted retracking. Should the standard GDR measurements be adopted or those from an improved coastal retracker? In this regard, mention should be given to efforts made in the development of coastal retrackers, very relevant in retrieving valid SSH closer to coast by e.g. Passaro, Dinardo, etc. Range and geophysical corrections. Which corrections should be adopted in the coastal strip? These are very relevant issues, particularly regarding the wet path delay (as near the coast the microwave radiometer measurements cannot be used), the ocean tides (global models fail in certain coastal regions) and the SSB (no coastal SSB models exist so far). I suggest that authors include mention to e.g. the review chapter by Cipollini in the recent CRC book, works on the wet path delay mainly by Brown and Fernandes, reference e.g. to the Fes2014 tide model and the most recent developments in the SSB (by e.g. Tran, Pires and Quartly). How altimeter and the various fiducial measurements should be compared? How far from the coast should the altimeter measurements be selected (far enough to avoid land contamination, both in the altimeter and in the corrections or should improved corrections be used, e.g. for tides and wet path delay)? Shouldn’t all calibration sites adopt the same processing methodologies, namely type of reatracking, corrections, etc.? Do LRM, SAR and future SWOT altimeters have the same calibration requirements?
3) Authors only give reference to Crete sites. For completeness, the other major existing sites should also be mentioned.
Detailed comments
Section2 - “We could thereafter state that the smaller the location estimate (e.g., range bias, sea-surface height bias, etc.) and its uncertainty, the better the performance of the satellite altimeter, and vice versa”. I suggest rephrasing as the magnitude of the bias is not related with the instrument performance in the sense that a small bias does not mean a better performance.
Authors should make a careful check of the language as e.g. in the caption of Fig.1.
Author Response
General comments:Reviewer: This paper presents the work performed under an EU/ESA project aimed at establishing Fiducial Reference Measurements for Altimetry (FRM4ALT). The authors introduce the problem and then, in a relatively exhaustive way, address the various issues related with the topic, namely the expected products and services, the underlying technologies and the required resources, skills, partnerships and facilities. Regarding the content of the paper, in spite of the fact that it is not a typical research article, there is no doubt that it is useful, particularly section 7. However, some parts are too wordy, partly repetitive, making the paper a bit boring. Indeed the format of the paper is more of a project report than of a scientific paper (even of a review one).I therefore recommend revision to account for the issues mentioned below.
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Reviewer: Paper structure. Try to reorganize the information in fewer major sections, as mentioned in the abstract. Avoid “philosophical” and “common knowledge” statements and try to concentrate on the scientific content.
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Response: Section 2 which is of more theoretical nature has been revised and a few figures have been inserted to represent its purpose but also reflect your comments. We consider that this Section presents the common practice followed in all system performance evaluation and has to be included for satellite altimetry Cal/Val. Please see revised manuscript.
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Reviewer: In spite of the exhaustive analysis performed by the authors, in my view there is an important point missing, possibly in section 6. The fiducial measurements are meant to calibrate altimeter observations. The calibration procedure includes not only the good practices required in the establishment, processing and monitoring of the fiducial measurements (well addressed in the paper) but also of the “processing of the altimeter measurements” themselves. This is even more relevant as the altimeter and fiducial measurements are usually not collocated. So, the calibration results will depend on many factors such as: Adopted retracking. Should the standard GDR measurements be adopted or those from an improved coastal retracker? In this regard, mention should be given to efforts made in the development of coastal retrackers, very relevant in retrieving valid SSH closer to coast by e.g. Passaro, Dinardo, etc. Range and geophysical corrections. Which corrections should be adopted in the coastal strip? These are very relevant issues, particularly regarding the wet path delay (as near the coast the microwave radiometer measurements cannot be used), the ocean tides (global models fail in certain coastal regions) and the SSB (no coastal SSB models exist so far). I suggest that authors include mention to e.g. the review chapter by Cipollini in the recent CRC book, works on the wet path delay mainly by Brown and Fernandes, reference e.g. to the Fes2014 tide model and the most recent developments in the SSB (by e.g. Tran, Pires and Quartly). How altimeter and the various fiducial measurements should be compared? How far from the coast should the altimeter measurements be selected (far enough to avoid land contamination, both in the altimeter and in the corrections or should improved corrections be used, e.g. for tides and wet path delay)? Shouldn’t all calibration sites adopt the same processing methodologies, namely type of reatracking, corrections, etc.? Do LRM, SAR and future SWOT altimeters have the same calibration requirements?
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Response: The purpose of this FRM for altimetry calibration is not meant to be applied for the generation of products and processing of altimetry measurements. Please see revised manuscript line 52-54.
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Reviewer: Authors only give reference to Crete sites. For completeness, the other major existing sites should also be mentioned.
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Response: All world Cal/Val site network is mentioned in the revised manuscript. See new Figures added and lines 95-100.
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Reviewer: Section2 - “We could thereafter state that the smaller the location estimate (e.g., range bias, sea-surface height bias, etc.) and its uncertainty, the better the performance of the satellite altimeter, and vice versa”. I suggest rephrasing as the magnitude of the bias is not related with the instrument performance in the sense that a small bias does not mean a better performance.
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Response: This has been revised as per your comments.
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Reviewer: Authors should make a careful check of the language as e.g. in the caption of Fig.1.
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Response: It has been revised. Thank you.
Reviewer 3 Report
General comments
According to the authors, objectives of this manuscripts are fourfold: 1) to examine the problem and the need behind Fiducial Reference Measurements for Altimetry principle to achieve an objective Earth observation; 2) to describe the expected products and services which are to come into being out of this new observational strategy; 3) to outline the technology and the services required for reaching this goal; 4) to elaborate upon the necessary resources, skills, partnerships and facilities for establishing FRM standardization for altimetry.
Except for the 1st objective, other objectives do not constitute scientific questions. Objectives given under introduction of the article have not presented rationally and scientifically. Therefore, the manuscript looks like a descriptive report rather than a scientific article.
The article, in particular, section 2 has to be rewritten as its language is not suitable for a scientific article. The given explanations should be focus exactly to the subject of the actual work. It should be coherent, brief and precise. There is no reference cited in section 2. There are many sentences with vague meanings.
It is better to describe previous definitions of performance by including their citations. Then the authors can define their definition for the phenomenon of performance and the method of its quantitative estimation.
There are some analysis, calculations and graphs. But at least those aspects have to be explained in a dedicated section for methodology.
Some of the specific comments
Line 93: There is an issue of line numbers.
Line 108: The sentence may not be suitable for a scientific article. Do you want to suggest that the phenomenon of performance is subjective and it has to be defined in the context of the subject?
Line 109-100: It is better to define the phenomenon of performance quantitatively (mathematically). Authors definitions are qualitative and do not show any idea of the scale.
Line 126: Loosely speaking? This terminology is not appropriate for a scientific article.
Line 152: The sentence is vague and not clear.
Line 159: give examples for previous definitions with their citations.
Line 162: Which criteria. Cite it.
Line 166: By the same token… These phrases are not appropriate.
Line 172: Fortunately, pure logic… remove such phrases.
Line 176-183: Objectives given here are general objectives of the FRM4ALT strategy of ESA. Explain how these objectives are relevant to the objectives of this article. There is no explanation of the methodology of achieving 5 objectives given here. For instance, the methodology to conduct full data analysis is not explained nor there an indication of the exact number of variables that are to be analysed in this article.
Line 315: “have be revamped”?
Line 391: Let us take a real-life example... rewrite.
Line 505: In the main text, explain how the GVD8 altitude daily solutions were obtained.
Line 518-510: Was figure 9 obtained by authors of this article or is it a reproduction from the cited article (29). If the authors created this graph, it should be explained properly. There should be a distinct section for methodology.
Line 544: Explain how these figures were created or give citations if they are reproduced from other sources.
Line 849: This is not table 1. The table should be named as table 2. This table should be included in the before the conclusion. The methodology of calculating uncertainties should be given in a different section that is dedicated to explaining the methodology.
Author Response
General comments:
Reviewer: According to the authors, objectives of this manuscripts are fourfold: 1) to examine the problem and the need behind Fiducial Reference Measurements for Altimetry principle to achieve an objective Earth observation; 2) to describe the expected products and services which are to come into being out of this new observational strategy; 3) to outline the technology and the services required for reaching this goal; 4) to elaborate upon the necessary resources, skills, partnerships and facilities for establishing FRM standardization for altimetry. Except for the 1st objective, other objectives do not constitute scientific questions. Objectives given under introduction of the article have not presented rationally and scientifically. Therefore, the manuscript looks like a descriptive report rather than a scientific article. The article, in particular, section 2 has to be rewritten as its language is not suitable for a scientific article. The given explanations should be focus exactly to the subject of the actual work. It should be coherent, brief and precise. There is no reference cited in section 2. There are many sentences with vague meanings. It is better to describe previous definitions of performance by including their citations. Then the authors can define their definition for the phenomenon of performance and the method of its quantitative estimation. There are some analysis, calculations and graphs. But at least those aspects have to be explained in a dedicated section for methodology.
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Response: This Section 2 has been revised as per your instructions. --------------------------------------------------------------------
Specific comments Line 93: There is an issue of line numbers.
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Response: It is not clear to us what is meant for Line 93. --------------------------------------------------------------------
Line 108: The sentence may not be suitable for a scientific article. Do you want to suggest that the phenomenon of performance is subjective and it has to be defined in the context of the subject?
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Response: Section 2 has been revised.
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Line 109-100: It is better to define the phenomenon of performance quantitatively (mathematically). Authors definitions are qualitative and do not show any idea of the scale. --------------------------------------------------------------------
Response: A table with the different definitions of scale (mathematical definitions) has been inserted (Table 1). --------------------------------------------------------------------
Line 126: Loosely speaking? This terminology is not appropriate for a scientific article.
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Response: It has been revised
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Line 152: The sentence is vague and not clear.
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Response: The whole section 2 has been revised.
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Line 159: give examples for previous definitions with their citations.
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Response: Please see revised manuscript. New citations and figures have been inserted in the revised manuscript. --------------------------------------------------------------------
Line 162: Which criteria. Cite it.
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Response: Text has been revised. Lines 185-186.
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Line 166: By the same token… These phrases are not appropriate.
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Response: Text has been revised.
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Line 172: Fortunately, pure logic… remove such phrases.
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Response: Text has been revised.
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Line 176-183: Objectives given here are general objectives of the FRM4ALT strategy of ESA. Explain how these objectives are relevant to the objectives of this article. There is no explanation of the methodology of achieving 5 objectives given here. For instance, the methodology to conduct full data analysis is not explained nor there an indication of the exact number of variables that are to be analysed in this article.
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Response: Text has been revised. Please see lines 207-208
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Line 315: “have be revamped”?
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Response: Corrected. Thank you
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Line 391: Let us take a real-life example... rewrite
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Response: Corrected.
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Line 505: In the main text, explain how the GVD8 altitude daily solutions were obtained.
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Response: Text has been modified. --------------------------------------------------------------------
Line 518-510: Was figure 9 obtained by authors of this article or is it a reproduction from the cited article (29). If the authors created this graph, it should be explained properly. There should be a distinct section for methodology.
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Response: Caption of Fig.14 has been modified.
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Line 544: Explain how these figures were created or give citations if they are reproduced from other sources.
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Response: These are processed by this team.
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Line 849: This is not table 1. The table should be named as table 2. This table should be included in the before the conclusion. The methodology of calculating uncertainties should be given in a different section that is dedicated to explaining the methodology.
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Response: Tables have been renumbered. References have been added.
Round 2
Reviewer 3 Report
Authors have taken a good effort to address the concerns of reviewers. Thus, the manuscript was improved though it looks like a review paper. Therefore, I recommend accepting the manuscript in its present form.
