On-Orbit Wavelength Calibration Error Analysis of the Spaceborne Hyperspectral Greenhouse Gas Monitoring Instrument Using the Solar Fraunhofer Lines
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn general I believe that this work shows merit in the area of wavelength calibration for hyperspectral payloads. My primary concerns are as follows: 1) The authors need to define what the "spacebourne hyperspectral greenhouse gas (GHG) monitoring instrument" is. This needs a brief description and a reference(s) provided. What type of instrument are we talking about? what are its characteristics? .... 2) It should be stated that calibrating a real instrument against interpolated spectra poses it own problems. The data provide are not any better than their measured/published values. Interpolation provides smooth data that are well behaved when trying to fit a smooth function to. This may not be the case in reality. 3) It should be made clear that the major problem may be in the radiometric arena and small perturbations in spectra wavelength may not be the major source of errors in the retrieval of GHGs, 4) To this end, the real impact should be expressed as a function of retrieved GHGs and not observed wavelengths. Do these error impact the retrieval of GHGs? What is this impact? How large an error in radiometric calibration is needed to insure that these values should be corrected and to what precision?
I have made a number of specific suggestions/comments in the attached version of the manuscript to hopefully aid in the review/publication process. I do think and editorial pass would be advised to enhance its overall readability/presentation. Some attention should be paid to the conclusion sections, and some of it may need to be reworded.
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis study presents a solar Fraunhofer lines based on-orbit wavelength calibration method and its error budget. Kurucz solar irradiance spectra and simulated hyperspectral data in the near infrared and short-wave infrared spectrum were used. While this study is potential valuable to the hyperspectral remote sensing calibration community, significant improvements are needed before it can be further reviewed.
1. Section 1: (2) Please review existing studies that use solar Fraunhofer lines for on-orbit wavelength calibration. (2) please clarify what are the unique contributions of the method present in this study. (3) Reference #28 seems irrelevant to the solar Fraunhofer lines.
2. Line 98-100 and Table 1: Please clarify the design parameters of which existing spaceborne GHG instrument was used in this study. Or is it a GHG instrument currently under-design/development?
3. Table 1 and Section 2: (1) It is confusing to use FWHM, which is more relevant to channel bandwidth, to represent spectral resolution. (2) The spectral sampling resolutions (0.00, 0.001, 0.0025 nm) seem given in lines 155-156. Please list them in Table 1. How many channels does each of the three bands (O2A, WCO2, and SCO2) have? (3) Please clarify which on-orbit GHG instrument, or which instrument currently under development, has such fine spectral resolutions. (4) What are the designed spatial resolution, swath width, and daily data volume of this instrument?
4. The reviewer strongly suggest that the authors use observations from an existing hyperspatial GHG instrument, instead of simulated data, in this study. Large uncertainties exist in the simulated data. Validation/verification results using data observed by an actual GHG instrument will be more convincing to the readers. One concern is that GHG instruments are designed to observe Earth targets instead the Sun. Therefore, their detectors usually saturate when they directly view the Sun. However, this factor is not considered in the error budget analyses. Another concern is SNR usually decreases as spectral resolution increases. Please demonstrate that the spectral resolutions, FWHM, and SNRs used in this study are reasonable.
Comments on the Quality of English Language
The English needs to be further smoothed.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsWhile some of my concerns have been answered, and some needed clean up has been implemented, there still seems to be some that are still unanswered. The 4 important outstanding ones that remain to be answer are: 1) what is the GHG Monitor instrument? A very sparce/vague description of it seems to match that of any of the existing instruments out there. What makes this different or the same. What instrument are we talking about on lines 33 and 34? what design parameter on 110 and 111 are we talking about? I think the authors need to remove and define a generic instrument, or describe the instrument that is being simulated, 2) if this is a real design, does it have a solar diffuser or does it expect to use ground observation to capture Fraunhofer line data? 3) What is the impact of this knowledge on retrieved GHGs does it even matter? 4) Is the data used in this study availability or is it the author's intent to make it available?. That is - is the data used in this study publicly available. I have add other specific comment to that attached file
Comments for author File: Comments.pdf
needs editoral scrub
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsPlease see the attached file.
Comments for author File: 
Comments.docx
The English in this manuscript is fine to me.
Author Response
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Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have attempted to clarify some of the points noted in the prior review cycle. They still have a very vague description of the generic instrument in question, and under which design parameters do these analyses apply, e.g. will this work with/without a solar diffuser? There is still little evident that the wavelength calibration accuracies defined in this work impact the retrieval of XCO2/CO2/Surface Pressure given the SNR provided. The use of Fraunhofer lines as a calibration reference source is not new, and I believe it applicability in this situation should to be shown as a function of its impact on the end product, not as a generic error term. Finally I think the Editor needs to weight in on the appropriateness of the data availability statement with respect to Journal policy.
Comments for author File: Comments.pdf
still need an editoral scrub
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authors1. Large uncertainty may exist in the simulated data used in this study, please discuss the limitations of using such data instead of actual GHG sensor observations in the text, and explicitly state the limitations in the Abstract and Conclusion sections.
2. For the general GHG instruments, what are the minimum requirements for spectral resolution, sampling rate, SNR, and radiometric calibration accuracy, for which the results from this study can be applied?
3. Response #9: “The length of the intercepted sample points is described in lines 510-513 of Section 3.7”:
Cannot locate the corresponding text pointed out by the author. What is “length of the intercepted sample points”? What are the relationships/differences among FWHM, sampling rate, sampling point, and the sensor field of view (FOV). In actual satellite observations, FOV is usually the finest data available for post-launch (on-orbit) analyses. For some instrument/channels, neighboring FOVs may partially overlap spatially, what may blur the Fraunhofer line features.
Author Response
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Author Response File: Author Response.pdf
