A Mobile Triaxial Stabilized Ship-Borne Radiometric System for In Situ Measurements: Case Study of Sentinel-3 OLCI Validation in Highly Turbid Waters

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study developed a ship-borne system (“MTS-WRMS”) of measuring water-leaving reflectance and presented a case study of implementing this system in OLCI data validation in two turbid water regions. Although the manuscript indicated the challenges of vessel-based measurements in dynamic aquatic environments and mentioned the advantages of MTS-WRMS, the current manuscript is inappropriate to be published in this journal.

• The manuscript is irrelevant to water quality assessment even though the proposed system might be used for water quality monitoring, so that it does not meet the topics of the special issue.
• The manuscript talked about the challenges of vessel-based measurements but did not introduce and/or discuss any other existing underway measurement systems. Besides, it is notable that some observation systems can be deployed at fixed stations and ships, such as HyperSAS.
• The proposed MTS-WRMS is following the existing approach of measuring Lw, however, many relevant to the engineering of MTS-WRMS are unclear in the manuscript. For instance, all characteristics of MTS-WRMS are simply mentioned but without any proof shown, while MTS-WRMS seems not a mature, commercial solution.
• The validation work with the use of MTS-WRMS in field is a vital part of this study, but data quality control method is not mentioned at all. Additionally, the tested environmental conditions are not enough since the wind speed in all the experimental dates is below 5 m/s. Although this validation is only a case study and the results could be preliminary, the tested conditions should include broader dynamic environmental factors and the number of the matched satellite images should be more.
• Other issues related to the methods and results would be reconsidered after the above is addressed.

Comments on the Quality of English Language

N/A

Author Response

Thank you very much for your thoughtful and constructive comments on our manuscript. Your feedback has been invaluable in helping us improve the quality and clarity of the paper. We greatly appreciate the time and effort you have taken to review our work.

We have carefully addressed all of your comments and suggestions. Detailed responses to each point are provided in the attached response letter. We believe these revisions have strengthened our manuscript and we hope that the changes meet your expectations.

Once again, thank you for your thorough review and helpful input.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

1. General Comments

The study develops a novel shipborne triaxial stabilized radiometric measurement system (MTS-WRMS) for high-precision real-time acquisition of water color parameters, such as water-leaving radiance (Lw) and remote sensing reflectance (Rrs), in dynamic aquatic environments. Based on synchronous in situ measurements, it further validates the accuracy of Sentinel-3 OLCI products in highly turbid waters. The main contributions include: the development of an innovative system integrating a triaxial stabilized gimbal, wireless telemetry, and automatic azimuth adjustment, which significantly reduces angular deviations caused by vessel motion; the validation of the system using the "direct approach", demonstrating low systematic bias in the 440–720 nm range (WAPD = 4.42%); and the identification of systematic overestimation in Sentinel-3 OLCI Rrs retrievals in highly turbid waters (e.g., Gaoyou Lake and Zhuhai nearshore waters), with WPD ≈ 16% and WAPD ≈ 31%, particularly in the 400–443 nm range due to inadequate atmospheric correction. This research provides a high-precision tool for remote sensing validation in dynamic water environments and offers valuable insights for improving satellite data algorithms.

The main weaknesses include: insufficient comparative analysis with existing technologies (e.g., HyperSAS, HYPSTAR), as the study does not quantify the performance advantages of MTS-WRMS; a rather superficial discussion on the mechanisms of Sentinel-3 OLCI overestimation, lacking an in-depth analysis of the influence of atmospheric correction models or inherent optical properties (IOPs) in highly turbid waters; inadequate description of experimental design details, such as sampling point distribution and temporal match-up methodology.

Review Decision: Major Revisions. The study presents a clear innovation and is supported by adequate experimental data, but requires further comparative analysis, deeper mechanistic discussion, and corrections in formatting and logical consistency to meet publication standards.

2. Specific Revision Suggestions

(1) Comments on article structure

The manuscript describes the hardware components of the measurement system in the system introduction section but does not provide a detailed explanation of the control principles of the triaxial stabilized gimbal, particularly regarding how it maintains a stable measurement angle under varying sea conditions (Lines 91-94). It is recommended to supplement the discussion with details on the gimbal control algorithm, error range, and dynamic correction strategies, as well as how the system compensates for vessel movements.

In the results and discussion section, the analysis of Sentinel-3 OLCI data bias is relatively simple and does not delve deeply into the sources of error (Lines 375-395). It is suggested to expand the discussion on potential error sources, such as atmospheric correction errors, pixel-mixing effects, or temporal-spatial mismatch issues that may lead to inaccuracies.

The conclusion section does not sufficiently highlight the research contributions and lacks discussion on potential future improvements to the system. It is recommended to incorporate discussions on integrating this system with emerging technologies, such as UAV-based measurement systems or AI-based large models, to further enhance its applicability.

 (2) Rationality of research motivation

The manuscript does not fully compare MTS-WRMS with existing methods, such as HyperSAS and SeaPRISM (Lines 85-90). It is suggested to include a comparative table listing key parameters such as accuracy, stability, and applicable scenarios to better highlight the advantages of MTS-WRMS.

The study focuses exclusively on highly turbid waters, without discussing whether the system is also applicable to clear waters (e.g., open ocean environments) (Lines 88-89). It is recommended to add a section discussing the potential applicability of the system to different water types and suggest future testing in various aquatic environments.

(3) On article innovativeness

Although the system incorporates a triaxial stabilized gimbal, the manuscript does not provide experimental data to demonstrate its improvement in angular stability compared to other methods (Lines 161-186). It is suggested to include comparative tests with traditional shipborne measurement systems (e.g., WISP-3 or other shipborne instruments) and provide quantitative analysis of angular stability improvements.

The manuscript mentions that the system can be used for long-term fixed-point monitoring, but no specific case study is provided (Lines 152-160). It is recommended to supplement the study with comparisons to long-term monitoring stations (e.g., lake hydrological stations) and provide examples of long-term datasets collected using this system.

(4) Rationality of research methodology

The manuscript uses a fixed value of ρ = 0.028 for wind speed correction but does not explain whether this value is applicable to all measurement dates (Lines 343-345). It is suggested to include a discussion on wind speed variations and water surface reflectance correction parameters, referencing relevant literature to justify correction methods under different wind conditions.

Although the WPD and WAPD calculation methods are reasonable, the manuscript does not include statistical confidence interval analysis (Lines 364-368). It is recommended to add standard deviation or confidence intervals in the error calculation section to enhance the statistical significance of the results.

3. Detailed Review of the Manuscript

（1）In terms of semantic expression

The expression in Lines 223-224, "reduce angular deviation", is vague. It is recommended to give specific angular deviation control ranges to ensure measurement stability.

In Lines 497-499, the manuscript states that "Sentinel-3 OLCI overestimates R_rs in the 400–443 nm band", but no explanation for this overestimation is provided. It is suggested to include potential causes, such as atmospheric correction errors or sensor calibration inaccuracies, to support this claim.

（2）Citation support aspects

Some conclusions lack supporting references. For example, in Lines 455-456, the statement "all bands exhibit a positive bias" does not provide references related to atmospheric correction errors. It is recommended to cite relevant literature, such as Mobley et al. (1999) and Ruddick et al. (2019), which discuss remote sensing correction errors in highly turbid waters, to strengthen the argument.

（3）Chart formatting aspects

Figure 1 does not label each component, reducing its readability. It is recommended to add labels for each sensor, such as "L_w Sensor" and "E_d Sensor", and include arrows to clearly indicate their positions and functions.

Author Response

Thank you very much for your thoughtful and constructive comments on our manuscript. Your feedback has been invaluable in helping us improve the quality and clarity of the paper. We greatly appreciate the time and effort you have taken to review our work.

We have carefully addressed all of your comments and suggestions. Detailed responses to each point are provided in the attached response letter. We believe these revisions have strengthened our manuscript and we hope that the changes meet your expectations.

Once again, thank you for your thorough review and helpful input.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Title: A Mobile Triaxial Stabilized Ship-Borne Radiometric System for In-situ Measurements: Case Study of Sentinel-3 OLCI Validation in Highly Turbid Waters

 

Objective: It is to present the “Mobile Triaxial Stabilized Water-leaving Reflectance 13 Measurement System” (MTS-WRMS), a ship-borne radiometric system designed for high-14 precision acquisition of water-leaving radiance (Lw) and remote sensing reflectance (Rrs) in 15 mobile aquatic environments.

 

Strengths: The paper presents a versatile system with ship-borne applications that collect spatially distributed spectral data across different water areas, providing valuable insights into the spatial distribution of water quality. Alternatively, it can be deployed for long-term shore-based fixed-point measurements, capturing time-series data at a single location to observe temporal variations.

 

Observation points:

 

1) Introduction:

- lines 55 to 62 (equation 1): This line of argument is a methodological approach.

- lines 66 to 67: It is not necessary.

- lines 91 to 94: It is not necessary.

 

2) System Advantages

- Other types of sensors using hyperspectral spectrometers could be discussed.

- What are the system limits?

 

3) Results and Discussion

Understanding of the results remained scattered. If one of the products was the NDVI or NDWI image of the lake area, the sensitivity variation would be better defined. As a suggestion, the authors could prepare a synthesis board of data accuracy between the Ship-Borne Radiometric System MTS-WRMS and Sentinel-3 A/B OLCI Level-2 Water Full Resolution (WFR) for the two study areas.

Comments on the Quality of English Language

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Author Response

Thank you very much for your thoughtful and constructive comments on our manuscript. Your feedback has been invaluable in helping us improve the quality and clarity of the paper. We greatly appreciate the time and effort you have taken to review our work.

We have carefully addressed all of your comments and suggestions. Detailed responses to each point are provided in the attached response letter. We believe these revisions have strengthened our manuscript and we hope that the changes meet your expectations.

Once again, thank you for your thorough review and helpful input.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

This paper presents a ship-borne device to measure water-leaving radiance and reflectance in water bodies such as lakes and rivers. Authors evaluate the accuracy of the system by comparing its measurements to the spectrally calibrated Sentinel-3 satellite images and found that Sentinel data overestimating the reflectance

I have the following comments:

• In your direct approach validation (lines 187-225) you mention that the probe should be positioned about 3-5 cm below the surface. Can you quantify the actual dept variation observed during your measurements?
• Need to stronger support use of a constant p value of 0.028 for all measurements(lines 344-345). It looks like the conditions were diverse across the measurement sites, and more discussion about why this constant value works across all sites would be great.
• The scatter plots in Fig. 9 show poor correlations with r2 values less than 0.22. You briefly mentioned this in 447-447, but I think you should discuss this in more detail, how this affects the reliability of your conclusions.
• You’re mentioning in lines 144-148 about sigma pitch < 0.5 degrees and sigma roll <0.05 degrees. Could you explain in more details how you determined these thresholds and how they’re affecting measurement accuracy?

Author Response

Thank you very much for your thoughtful and constructive comments on our manuscript. Your feedback has been invaluable in helping us improve the quality and clarity of the paper. We greatly appreciate the time and effort you have taken to review our work.

We have carefully addressed all of your comments and suggestions. Detailed responses to each point are provided in the attached response letter. We believe these revisions have strengthened our manuscript and we hope that the changes meet your expectations.

Once again, thank you for your thorough review and helpful input.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

My comments are addressed in the revisions.

Comments on the Quality of English Language

n/a

Reviewer 3 Report

Comments and Suggestions for Authors

The authors had a look at the article and suggestions for improvement were taken into account.

Comments on the Quality of English Language

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