Greenhouse Gases Monitoring Instrument on GaoFen-5 Satellite-II: Optical Design and Evaluation

Round 1

Reviewer 1 Report

The manuscript proposed a new optical scheme for the quantitative remote sensing of atmospheric greenhouse gases, and the design results of the core unit of the optical system are given. The effectiveness of the optical scheme is verified by the main performance indicators, and further, the detection capability of Greenhouse gases Monitoring Instrument on GF-5 satellite-II (GMI-II) for CO₂ and CH₄ was evaluated. GMI-II alleviates the problem of inadequate spectral resolution and signal-to-noise ratio of traditional Spatial Heterodyne Spectroscopy(SHS) and has important application value, which is a novel and brilliant idea. The new method and experimental results proposed in this paper provide a feasible technical approach for atmospheric remote sensing. The novelty and impact of the manuscript meet the inclusion criteria of Remote Sensing.

 

The manuscript can be accepted, but some minor issues still need to be addressed.

 

1. SNR is the most important design input condition for optical system design. What is the relationship between interference data SNR and spectral SNR? What special coefficients affect the spectral SNR?

2. How to ensure the consistency of the field of view of the four channel optical system? How does the field-of-view match between the three main observation channels (B2, B3 and B4) and the auxiliary observation channel (B1) affect the data accuracy of GHG?

3. How to ensure that the performance of the optical system meets the requirements of the space environment in orbit?

4. The B2 channel spectrometer works in the short-wave infrared band, does it have the background noise caused by temperature? If so, how to suppress relevant noise?

5. It is suggested to supplement the composition description of spectral scaling uncertainty in Table 7.

6. It is suggested to supplement the research results of the in-orbit application of GMI-II as references to prove the effectiveness of the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In this manuscript, Luo et al. present the greenhouse gases (GHS) monitoring instrument that will be installed on the GF-5 observatory. The instrument uses a novel high-resolution spatial heterodyne spectrometer (SHS) to monitor atmospheric greenhouse gases. In the manuscript, the principles of SHS spectroscopy, the instrument's optical design, the fabrication and alignment of the optics, the performance parameters, the calibration procedures and the results are presented in detail.

Reporting the specifications, calibration and testing of the GHG monitoring instrument of the GMI-II observatory is essential for future data interpretation during the observatory operation. In this way, the instrument's credibility is assured to the scientific community. Therefore, the current manuscript is considered crucial and conforms with the objectives of the Remote Sensing journal.

While the manuscript is well-structured, the methodology employed by the authors is systematic and the arguments presented are supported adequately, there are a few areas for improvement. The references cited could be expanded and the equation and figure numbering have to be corrected. Moreover, some images and graphs do not provide additional information and thus could be removed without affecting the quality of the text.

Furthermore, a short discussion related to the advantages and disadvantages of satellite observatories compared to ground-based GHG monitoring devices should be included in the introduction section. All these suggestions and comments are included in the attached version of the manuscript.

Finally, even though the manuscript presents the instrument development, design and testing of the optical components in detail, the authors do not include any preliminary and ground testing measurements of CO2 and CH4. To validate the instrument's capabilities, it is essential to include tests on the actual detection of these gases.

Additionally, the authors claim that the spectrometer has an improved spectral resolution of up to 0.263 1/cm. The FWHM of the CO2 and CH4 absorption peaks at 1572 and 1650 nm respectively are smaller than the spectrometer resolution (as can be derived from the HITRAN database). The authors should explain how they address this limitation and include validation methods such as ground testing or detailed simulations in the manuscript.

Overall, these improvements would enhance the credibility of the instrument to the scientific community, and make the manuscript suitable for publication in the Remote Sensing journal.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

General comments

Although it might be revised later by the academic editors, I recommend authors to revise blank spaces related to symbols/units. A blank space must separate the numbers and the symbols (units or %), however, in the manuscript sometimes that rule is followed but other times not. Similarly, the blank spaces between words should be revised. Sometimes there are a lot of unnecessary blank spaces that can be found along the manuscript (examples: table 2, table 5, last paragraph from page 8, etc.). Many other times, there is a clear lack of those necessary blank spaces.

Authors also revise the references to figures and equations in the text, since the sentence ‘Error! Reference source not found’ can be continuously found along the manuscript.

Introduction: the sentence ‘The latest research result indicated that if current emission ….’ should be rewritten as is not clear enough. On the other hand, the sentence related to ‘The results demonstrate that the performance of all functional components and the main ….’, placed in the last paragraph of the Introduction, it seems to be out of place in that paragraph where the study, developed for the paper, has been presented.

Sections 2 and 3 appear to be a technical specification booklet rather than a research article section, however the title and abstract of the manuscript clearly explain that it is an article related to the instrument design and calibration. In addition, there is still no enough information in neither webpage regarding this instrument, which justifies the presence of the sections in the article.

Conclusion: this section should be consistent with the results presented in the manuscript. Therefore, any mention to subsequent papers, or new results not mentioned in the previous section (such as ‘operating temperature’) should be avoided in this section. Besides, the comment related to the research group it is also out of place in the ’Conclusion’ section.

 Specific comments

Page 5: the last equation is not numbered

Page 9: indicate the meaning of θ_L and W symbols used in the equation

Page 9: rewrite the paragraph following equation 8. Instead of repeating equation 8 in the paragraph (0.5Nxδσ, 0.25Nxδσ) and mentioning a value of k, when k is not appearing until equation 9, rewrite the paragraph to make it clear.

Page 10: the sentence ‘Under the condition that the effective pixel number N of the detector … is effectively improved’ seem to be a result as it is said ‘is effectively improved’. Therefore, if it really is a result of the experiment, it should be place in Section 4, or clarify the sentence if it really belongs to the design section.

Page 11: a reference could be included regarding the comment ‘It is worth noting the accuracy requirement …’.

Page 11: ILS mentioned in this page is defined in page 20. Revise it

Figure 9 -10: specific caption of each figure ((a), (b), (c)) should be placed following the general caption of the figure, not separately

Figure 15: if there is no and specific explanation to the used initial limit of the X-axis (0.50), maybe a new higher limit let avoid so many blank space in the graph. Same for Figure 10 (c)

Page 21: remove the words ‘below’ and ‘above’ when mentioning Table 7.

Figure 22: there is no explanation to why only two values for the wavelength are represented in the graphs of Figure 22. Moreover, an explanation should be included regarded to the two values given for the slope and two values given for the intercept that appear in the tables in those graphs. These results could be better analysed since they are only presented in the manuscript but no interpretation is made of them.

Table 9: an average SNR value is presented in the table, but there is no explanation of why it is calculated from both SNR values (required radiance and statistical radiance), or even the difference between both SNR displayed in Figure 23 (figure for which there appears no reference in the manuscript).

Table 10: explain the meaning of ZPD in the table

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors successfully addressed all the issues raised during my review. I have no further edits beyond the improvement of figure 4. The vertical axis legend should be "Transmittance" instead of "Reflectivity". Furthermore, additional improvements in the figure, such as reducing the figure size, will help the reader to understand the instrument design.
I am confident that the authors will proceed with the minor changes without requiring an additional review session.