Spectral Calibration of the Spectrometer on Board the Colombian FACSAT-2 Satellite Mission

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript seems appropriate for publication as is. Thanks

Author Response

Thank you very much for your review, we appreciate your support.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors discuss the results obtained from a CubSat satellite for the measurement of greenhouse gases, CO₂ and H₂O (+O₂) in the region from 1000-1650 nm (secondary payload).  The satellite has been in orbit for about 1 year.  They present data relevant to the wavelength calibration of the secondary payload only and discuss their retrieval algorithm based on HITRAN simulations using Genspec software code and the required convolutions with the spectrometer resolution (6 nm) and the slit function.  Corrections are also considered for the field of view and the detector’s QE.

The introduction gives a fairly comprehensive overview of the former and current satellites for GHG retrievals and the ground based calibration facilities. 

The retrieval algorithm is discussed at length and follows the procedure reported for satellite retrievals of pollution over Canadia using a spectrometer made by the same manufacturer.  The authors discuss the validation procedure relative to the results from the TCCON database.

The content of the work is relevant to the Remote Sensing Journal.  However, after reading the paper, I have found several areas where clarifications and improvements are required as outlined in detail below.  Therefore, I find that major revisions are necessary before it is suitable for publication.

1. The HITRAN 2000 spectral database used in this analysis is rather dated.  The authors should justify their choice relative to the more recent HITRAN updates.
2. Line 339: The details about how the wavelength calibration curve shown in Fig. 7a should be given.
3. Line 353: “i.e., “feasible detectable the proportions …” does not make sense – please rewrite.
4. Line 382: It is not clear what the FWHM is and how it was used in the analysis.
5. Line 385: How was the “most optimal performance” determined – by eye?  Can this be compared with the TCCON data?
6. The spectra in Fig. 11 all pretty much look the same except for a change in the y-axis. Only Fig. 11a needs to be shown unless more is added as to their value.
7. Line 367: The authors mention that the reflectivity factor, the temperatures of the Sun and Earth can be modified (presumable fit) to minimize the residuals between the simulation and measured spectra.  Were these variables “adjusted” in the comparison of the data in Fig 12?
8. Line 392: The especially high “concordance” for the spectral region near CO₂ is not obvious.  The integrated absorption for the 1575 nm band looks significantly underestimated (by 2-fold or more) relative the higher energy band near 1600 nm.  The authors should expand this region and discuss the discrepancies between the data and model.
9. The primary goal will be to obtain a fitted concentration for the column from the data.  The uncertainty associated with the retrievals of CO₂ and H₂O should be discussed based on the data in Fig. 12.  Furthermore, additional discussions regarding areas where any expected improvements in the uncertainties should also be added.
10. Line 319: The authors indicate that additional spectra were obtained from the numerous locations show in Fig. 4.  These other data and fitted uncertainties from the different locations should therefore be shown and discussed to help assess the readiness and reliability of the instrument to quantify GHGs from LEO.

Comments on the Quality of English Language

The authors should break many of the very long sentences into shorter pieces. 

Author Response

• Comments 1: The HITRAN 2000 spectral database used in this analysis is rather dated.  The authors should justify their choice relative to the more recent HITRAN updates.  

Response 1 : Thank you for your comment. We agree that there are more updated releases of HITRAN database, but for the scope of this research approach, the limited spectral range and resolution of the ARGUS spectrometer, the spectral lines data available on the 2000 HITRAN database is enough.  

 We modified the paragraph as follows: 

“…Although the latest HITRAN databases are available, the GenSpect forward modelling code only works with the HITRAN 1996 and HITRAN 2000 spectral databases. The most recent was chosen”. 

• Comments 2: Line 339: The details about how the wavelength calibration curve shown in Fig. 7a should be given. 

Response 2: These details are included in the 2.6 section 

We have revised the section title to "In-Orbit Spectral Calibration." 

The title of the graph in Figure 8b was changed to "Comparison between in orbit and factory calibrations relative to absorption bands of H2O, O2 and CO2. Unlike, factory calibration, in orbit calibration properly matches the CO2 absorption bands” 

• Comment 3: Line 353: “i.e., “feasible detectable the proportions …” does not make sense – please rewrite. 

Response 3: Thank you for your comment. We agree and we deleted the sentence 

• Comment 4: Line 382: It is not clear what the FWHM is and how it was used in the analysis 

Response 4: Thank you for your comment. We agree and include a new explanation about the FWHM, please see the document attached

• Comment 5: Line 385: How was the “most optimal performance” determined – by eye?  Can this be compared with the TCCON data? 

Response 5: 

Thank you for your comment. Unfortunately, we omitted the criteria used to determine the best slit function. We include the next explanation: 

Figure 11 shows convolved spectra with each of slit functions used here. Figure 12 shows the results of the conversion of the radiance convolved spectra to Figure 12, energy spectra (Figure 12a) and counts detector spectra (Figure 12b). A Full Width at Half Maximum (FWHM) of approximately 1.5 times the average wavenumber spacing around 1600 nm region has been used. The Michelson and Rectangular slit functions have considerable noise along the spectrum for the chosen resolution. For each slit function, we computed the Mean Square Error in the 1570 - 1620 nm wavelength range, obtaining for Michelson, 0.0072, Gaussian, 0.0037, Triangular, 0.0038, Dispersion, 0.005, Rectangular, 0.0045 and Diffraction, 0.0035. The findings indicate that, in the context of CO₂ absorption, the diffraction function and the Gaussian function are observed to exhibit the most optimal performance. 

• Comment 6: The spectra in Fig. 11 all pretty much look the same except for a change in the y-axis. Only Fig. 11a needs to be shown unless more is added as to their value. 

Response 6: We agree with the referees; however, we believe it is necessary to show the noticeable differences in the spectrum profile when transforming from intensity to instrumental counts. Therefore, we have reduced Figure 11 to just two figures (Figure 12a and Figure 12b); the first shows the result of the conversion from radiance to energy, and the second from energy to counts. 

• Comment 7: Line 367: The authors mention that the reflectivity factor, the temperatures of the Sun and Earth can be modified (presumable fit) to minimize the residuals between the simulation and measured spectra.  Were these variables “adjusted” in the comparison of the data in Fig 12? 

Response 7: Yes, temperature values were modified to “real” values and reflectivity factor was adjusted, now in Figure 13.

• Comment 8: Line 392: The especially high “concordance” for the spectral region near CO2 is not obvious.  The integrated absorption for the 1575 nm band looks significantly underestimated (by 2-fold or more) relative the higher energy band near 1600 nm.  The authors should expand this region and discuss the discrepancies between the data and model. 

Response 8: We agree with the referee's comment; we have improved the fit between the simulated spectrum and the measured spectrum in the spectral region between 1600 and 1700 nm. Therefore, we have modified Figure 13. 

• Comment 9: The primary goal will be to obtain a fitted concentration for the column from the data.  The uncertainty associated with the retrievals of CO2 and H2O should be discussed based on the data in Fig. 12.  Furthermore, additional discussions regarding areas where any expected improvements in the uncertainties should also be added.

Response 9: We agree with your comment, the main objective of this project will be to obtain the CO2 concentration profile for an atmospheric column in specific regions, based on infrared spectra measured by FACSAT-2. However, in this article, we present an initial approach aimed at calibrating the spectrometer in orbit and obtaining a simulated spectrum as close as possible to the measured spectrum in the spectral region corresponding to CO2 absorption.

• Comment 10: Line 319: The authors indicate that additional spectra were obtained from the numerous locations show in Fig. 4.  These other data and fitted uncertainties from the different locations should therefore be shown and discussed to help assess the readiness and reliability of the instrument to quantify GHGs from LEO.

Response 10: We agree with the referee's suggestion; however, in this initial work, we want to showcase the setup of the SWIR spectrometer onboard the first Colombian satellite for the measurement of greenhouse gases.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The purpose of the article, according to the authors, is to describe the spectral calibration of a spectrometer in the SWIR. Unfortunately, there is little on the description of this activity, and what is present is very lacking in content and information. Moreover, given the lack of information, the innovative content of the paper and its scientific interest is very limited: The algorithm for calibration is not sophisticated and there is minor novelty in the procedure. The publication of the paper is to be rejected.

Going into detail about the shortcomings of the paper:

-        Paragraph 2.2. SWIR Spectrometer: information about the spatial resolution / spot observed on the ground is missing.

-        Paragraph 2.3. Integration of the spectrometer to the satellite:  

o   The authors should better explain the reasons for the choice of the selected spectrometer (for example by using data simulations and/or comparison with other spectrometers).

o   The authors must add information on spectrometer optics because it is totally missing. Why was that type of optics chosen? what are the relevant characteristics?

o   The authors must add information on PCB. Why was that type of optics chosen? what are the relevant characteristics?

-        Paragraph 2.5. Spectra geolocation. The paragraph mentions a software STK for simulating geolocation starting from quaternions. The software allows the determination of approximate coordinates of the observed pixel. However, no value is given for this approximation, details are missing

-        Paragraph 2.6. In-Orbit Spectral Calibration.

o   The authors state: “The ARGUS2000 spectrometer was calibrated in orbit to consider possible variations in the optics due to atmospheric conditions .. .“. The sentence is not exact. Spectrometer optics is not influenced by atmospheric conditions.

o   I suggest to show the spectral bands used for calibration, in particular it is important to show the central wavelength and the corresponding bandwidth.

-        Paragraph 2.7. Simulated Spectrum Algorithms. The authors state: “Genspect is used in this study to simulate the real atmosphere…”, and also thy state that scattering effects are not considered.  If scattering effects are  not considered the real atmosphere is not simulated.

-        Paragraph 2. 7.1 Spectral radiance to detector. The procedure is online outlined, but details or formulas describing the procedure are not given. The content of the paragraph is too poor and must be improved.

-      Paragraph 3.2. In-Orbit Spectral Recalibration. The description of the procedure for the spectrometer recalibration is completely absent. Figure 7 is shown but it is not explained how the authors reached such results.

Comments on the Quality of English Language

As for the use of the English language: Earth sometimes is written using lowercase, but above all, sometimes the syntax leaves something to be desired. For this reason, several sentences should be revised. For example, the ones at:

-        Page 2 lines 83 – 95

-        Page 2 lines 96 -98

-        Page 3 lines 118 – 120

-        Page 4 lines 161 - 163

Author Response

• Comment 1: Paragraph 2.2. SWIR Spectrometer: information about the spatial resolution / spot observed on the ground is missing.

Response 1: We agree to the referee's suggestion.  We modified the table to include the spatial resolution: 1.5 km at 500 km orbit 

• Comment 2: Paragraph 2.3. Integration of the spectrometer to the satellite:   

Comment 2a: The authors should better explain the reasons for the choice of the selected spectrometer (for example by using data simulations and/or comparison with other spectrometers). 

Response 2a: 

We agree to the referee's suggestion.  We modified the following paragraph: 

… The selection of the second payload for the satellite mission was a work carried out by the researchers of the project, alliance between the Colombian Air Force (COLAF) and ECOPETROL, where after a market study and technological surveillance on sensors for the analysis of GHG that were compatible with a 6U structure (it had to be the size of a half-U), and that had space inheritance (Argus was launched on aboard CanX-2 micro-satellite on the  2008 as part of a technology demonstration mission), it was determined that the ARGUS 2000 spectrometer was the only one that meet the requirements and restrictions of the proposed mission. 

Comment 2b:  The authors must add information on spectrometer optics because it is totally missing. Why was that type of optics chosen? what are the relevant characteristics? 

Response 2b: We agree to the referee's suggestion. However, the ARGUS 2000 spectrometer is a commercial spectrometer designed and developed by Thoth Technology Inc., the important features and optical characteristics are described in the article, based on the datasheet. 

Comment 2c: The authors must add information on PCB. Why was that type of optics chosen? what are the relevant characteristics? 

Response 2c: We agree to the referee's suggestion. The PCB was developed to supply the need for data control and power supply between the satellite bus and the ARGUS 2000 sensor, so it is not related to the optics of the sensor, it is related to its interfaces with the satellite bus. 

• Comment 3: Paragraph 2.5. Spectra geolocation. The paragraph mentions a software STK for simulating geolocation starting from quaternions. The software allows the determination of approximate coordinates of the observed pixel. However, no value is given for this approximation, details are missing.

Response 3: We agree to the referee's suggestion.  We modified the table to include the latitude and longitude for each scan 

• Comment 4 a: Paragraph 2.6. In-Orbit Spectral Calibration. 

The authors state: “The ARGUS2000 spectrometer was calibrated in orbit to consider possible variations in the optics due to atmospheric conditions .. .“. The sentence is not exact. Spectrometer optics is not influenced by atmospheric conditions. 

I suggest to show the spectral bands used for calibration, in particular it is important to show the central wavelength and the corresponding bandwidth

Response 4a: 

We agree the referee’s comment. We have corrected the sentence as follows:  

The ARGUS2000 spectrometer was calibrated in orbit to consider possible variations in the optics due to mechanical stresses caused by the launch. 

Comment 4b: I suggest to show the spectral bands used for calibration, in particular it is important to show the central wavelength and the corresponding bandwidth.

Response 4b: In section 2.6. Figure 3. We show the peaks used for calibration. 

• Comment 5: Paragraph 2.7. Simulated Spectrum Algorithms. The authors state: “Genspect is used in this study to simulate the real atmosphere…”, and also thy state that scattering effects are not considered.  If scattering effects are not considered the real atmosphere is not simulated. 

Response 5: We agree that not considering nonlinear phenomena introduces discrepancies between the real atmosphere and the simulated atmosphere. However, in this initial approach, we have used software that accurately adjusts the simulated spectrum to the real spectrum without the need to consider these phenomena. In any case, it is important to keep in mind that in modelling reality, it is very difficult to account for all natural phenomena. We hope to continue refining the simulation process to achieve better matches between the measured spectrum and the simulated spectrum

• Comment 6: Paragraph 2. 7.1 Spectral radiance to detector. The procedure is online outlined, but details or formulas describing the procedure are not given. The content of the paragraph is too poor and must be improved. 

Response 6: We agree. We have omitted technical mathematical details that could provide better understanding of the mentioned procedure and enhance the subsequent discussion. Please see attached. 

• Comment 7: Paragraph 3.2. In-Orbit Spectral Recalibration. The description of the procedure for the spectrometer recalibration is completely absent. Figure 7 is shown but it is not explained how the authors reached such results.

Response 7: Thank you, These details are provided in the 2.6 section 

• Comment 8: Page 2 lines 83 – 95 

Response 8: 

We agree the referee’s comment. We have corrected the sentence as follows: 

The OCO series (2 and 3) were designed for the mission to record global CO2 variations, OCO-2 was operational until 2019, at that point it was installed on the International Space Station, where it remains to this day. OCO-3 is still fully functional. The OCO series has recorded spectral data in three distinct infrared regions; 0.7576-0.7726 μm, 1.5906-1.6218 μm and 2.0431-2.0834 μm. Each spectral band is measured with a different spectrometer, equipped with HCT detectors.  

The SCIAMACHY instrument (operating on the ENVISAT satellite) is capable of measuring both incident and reflected solar radiation from the Earth over a broad spectral range of 240-2380 nm, encompassing ultraviolet (UV), visible and near-infrared (VIS-NIR) wavelengths, as well as shortwave infrared (SWIR) wavelengths. The absorption of various atmospheric gases, including CO2, can be recorded in this spectral region. The system comprises an optical assembly that enables the measurement of eight spectral bands using silicon and InGaAs photodetectors. 

• Comment 9: Page 2 lines 96 -98 

Response 9: 

We agree the referee’s comment. We have corrected the sentence as follows: 

Finally, ground-based or in situ systems comprise high-precision Fourier transform spectrometers (FTIR) situated at various points of interest on Earth (the poles, the ocean, etc.). The three most significant networks of this kind are as follows: The Total Carbon Column Observing Network (TCCON), the Network for the Detection of Atmospheric Composition Change (NDACC) and the Collaborative Carbon Column Observing Network (COCCON). The aim of these systems is to provide highly accurate and precise measurements of gas concentrations, including CO2, CH4, CO, N2O, and other species in the near-infrared (NIR) and mid-infrared (MIR) region. In addition, data from ground-based systems such as TCCON are used to calibrate satellite-based measurements [16]; In the Colombian context, for instance, bottom-up measurements conducted by ECOPETROL in certain operational regions, provide high-precision data that enhance the calibration of FACSAT-2 integrated sensor, thereby supporting research on climate change. This data is also used as a reference point. 

• Comment 10: Page 3 lines 118 – 120

Response 10: 

We agree the referee’s comment. We have corrected the sentence as follows: 

 …with a molar absorptivity σ(λ). From this equation, the concentration of the gaseous medium can be easily derived if the intensities before and after passing through the medium are measured and its molar absorptivity is known.  

• Comment 11: Page 4 lines 161 – 163 

Response 11: Colombian personnel participated in the design of the FACSAT-2 mission, through the planning and co-development of different phases of the life cycle, including the definition of objectives and requirements, mission characterization, critical design, and payload integration. After launch, the satellite is being operated by Colombian personnel from the Space Operation Center (SPOC), located in the city of Cali. From there, the tasks of the Launch and Early Orbit Phase and the in-orbit calibration of the payloads were carried out.  

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

See attached document.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

I have a few grammatical corrections/suggestions:

line 24-25 of the abstract is repeated in the abstract.

line 54 should read: atmospheric composition was limited

lines 83-85 was confusing perhaps this sentence better states what the authors are trying to say. "The OCO series (2 and 3) have the mission to record global CO2 variations, 83 OCO-2 operated until 2019, in that year the OCO-3 version was installed on the international space 84 station and is still functional until today.

line 89: incidents should be incident

line 92 would be clearer to refer back to the actual spectral domain. Now reading as 'In the SWIR, the absorption...'

Table 2.  Julian time not Juliam

line 303: proper should be property

 

Author Response

• Comment 1: The authors present a reasonable approach for on-orbit calibration of the spectrometer on the FACSAT-2 satellite. My only scientific concern is the use of spectral simulation software that does not account for scattering in the atmosphere. I used MODTRAN to quickly try to assess the importance of scattering in the 1000 to 1650 spectral region and the authors would probably find a better fit using a model which includes scattering. Here is a plot of the spectral radiance using a 5nm spectral resolution for aerosol loadings which correspond to 100, 23 and 10 km horizontal visibility. The importance of scattering is clearly shown for wavelengths shorter than 1350 nm. 

Response 1: Thank you for the comment; we understand the importance of scattering phenomena. However, in this initial approach, we used HITRAN because Genspect works with that database. Additionally, HITRAN is a freely available database.  

• Comment 2: line 24-25 of the abstract is repeated in the abstract

Response 2: We agree, we deleted the sentence 

• Comment 3: line 54 should read: atmospheric composition was limited

Response 3: 

We agree, thank you, we modified the sentence 

 … The development of satellite platforms in the past two decades have enhanced the capability to observe the Earth and its atmosphere on a global scale.  

• Comment 4: lines 83-85 was confusing perhaps this sentence better states what the authors are trying to say. "The OCO series (2 and 3) have the mission to record global CO2 variations, 83 OCO-2 operated until 2019, in that year the OCO-3 version was installed on the international space 84 station and is still functional until today. 

Response 4: 

We agree the referee’s comment. We have corrected the sentence as follows: 

The OCO series (2 and 3) were designed for the mission to record global CO2 variations, OCO-2 was operational until 2019, at that point it was installed on the International Space Station, where it remains to this day. OCO-3 is still fully functional. The OCO series has recorded spectral data in three distinct infrared regions; 0.7576-0.7726 μm, 1.5906-1.6218 μm and 2.0431-2.0834 μm. Each spectral band is measured with a different spectrometer, equipped with HCT detectors.   

• Comment 5: line 89: incidents should be incident 

Response 5: 

We agree the referee’s comment. We have corrected the sentence as follows: 

The SCIAMACHY instrument (operating on the ENVISAT satellite) is capable of measuring both incident and reflected solar radiation from the Earth over a broad spectral range of 240-2380 nm, encompassing ultraviolet (UV), visible and near-infrared (VIS-NIR) wavelengths, as well as shortwave infrared (SWIR) wavelengths. The absorption of various atmospheric gases, including CO2, can be recorded in this spectral region. The system comprises an optical assembly that enables the measurement of eight spectral bands using silicon and InGaAs photodetectors.

• Comment 6: line 92 would be clearer to refer back to the actual spectral domain. Now reading as 'In the SWIR, the absorption...' 

Response 6: 

We agree the referee’s comment. We have corrected the sentence as follows: 

 The SCIAMACHY instrument (operating on the ENVISAT satellite) is capable of measuring both incident and reflected solar radiation from the Earth over a broad spectral range of 240-2380 nm, encompassing ultraviolet (UV), visible and near-infrared (VIS-NIR) wavelengths, as well as shortwave infrared (SWIR) wavelengths. The absorption of various atmospheric gases, including CO2, can be recorded in this spectral region. The system comprises an optical assembly that enables the measurement of eight spectral bands using silicon and InGaAs photodetectors.   

• Comment 7: Table 2.  Julian time not Juliam 

Response 7: We agree the referee’s comment. We have corrected Juliam to Julian

• Comment 8: line 303: proper should be property 

Response 8:  

We agree the referee’s comment. We have corrected the sentence as follows: 

 …Accordingly, counts spectra are obtained by multiplying intensity synthetic spectra [W m-2] by the lens area [m2]and exposure time obtaining the energy spectra. Then, this spectrum is divided by the instrument joules/counts rate polynomial function.

• Comment 9: Finally, we have taken the editor's suggestion and have added the Discussion section

Response 9:

The successful integration, commissioning, and in-orbit calibration of the Argus 2000 SWIR spectrometer onboard the FACSAT-2 satellite represent a significant milestone in Colombia's space exploration efforts, particularly in the remote sensing of greenhouse gases (GHGs). The ability of the satellite to acquire consistent spectral signatures over a year in orbit demonstrates the robustness and reliability of the instrument.  

 One of the key aspects of this study involved the development of retrieval algorithms using the Genspect commercial software to generate synthetic spectra. By incorporating instrumental parameters such as, FOV angle, quantum efficiency, and slit function (and spectral resolution), the synthetic spectra were fine-tuned to align closely with the real spectra acquired from the satellite. The effectiveness of the Gaussian and diffraction slit functions in accurately simulating the experimental spectra is particularly noteworthy, as it highlights the importance of choosing appropriate slit functions in spectral analysis.   

 The comparison of the synthetic spectra with real data using TCCON concentrations as a benchmark provides a strong validation of the calibration and retrieval processes employed. The match between the synthetic spectra and the spectral signatures acquired by FACSAT-2 over Tenerife indicates that the methodologies used in this study are effective for accurate remote sensing of atmospheric gases. However, it also raises questions about the limitations and potential sources of error in the current approach.  

 One area that guarantees further exploration is the treatment of nonlinearities in the spectrometer's response. While the current approach has been successful without explicitly considering these phenomena, future work could investigate their impact on the accuracy of the retrieved spectra. In addition, global optimization processes could be explored to better tune the parameters that influence the simulated spectrum to see if they offer further improvements in the spectral match.   

 The outcomes of this work validate the operational capabilities of the FACSAT-2 Argus 2000 SWIR spectrometer and provide valuable insights into the challenges and opportunities associated with the remote sensing of GHGs from space. This study lays the groundwork for further regional advancements in this field, particularly in refining the retrieval algorithms, expanding the analysis to other atmospheric gases, and exploring new methods for spectral calibration and validation.  

 It can be concluded that the current work has achieved significant success, it also opens the door to numerous avenues for further research and improvement. By continuing to refine the methodologies and explore new approaches, the full potential of the FACSAT-2 satellite for GHG monitoring can be realized, contributing to both scientific understanding and global climate monitoring efforts. 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed my main concerns. 

Author Response

Thank you very much for all your support. 

Reviewer 3 Report

Comments and Suggestions for Authors

The paper titled “Spectral Calibration of the Spectrometer on Board the Colombian FACSAT-2 Satellite Mission” was re-submitted after a deep revision. The authors replied to all the comments, but some responses still do not fully answer to the observations made during the first review. The quality of the paper improved a lot, but it still needs some minor revisions.

Going into details:

1.       Page 3 Lines 101: replace “;” with “.”

2.       Page 3 Lines 105: replace “is” with “are”

3.       Page 5 Lines 199 - 202: “ All these are challenges allowed to have in space a sensor for the acquisition of data, attempting to provide the quantification of GHG, especially CO2, from different processes such as geolocation, calibration in orbit, simulation of synthetic spectra and conversion of accounts, among other processes that are described below”. The authors should revise the structure of the sentence that it is not synthetically correct. Presently the meaning of the sentence is not very clear.

4.       Page 6 Paragraph 2.5: Could you give more information on how much is the approximation? I mean can you give for example a percentage on the approximate values i.e. 1%, 5% , 10 % or so on.

5.       Page 7 Paragraph 2.6: The description of spectral calibration is still too generic and poor. The procedure for calibration is still not yet described.

6.       Page 8 line 275: Since in the simulation no aerosol was considered, it is not correct to state that the study simulates a real atmosphere. I suggest to remove the word "real".

 

7.       Page 13 Figure 8 - caption: “Unlike, factory calibration, in-orbit calibration properly matches the CO2 absorption bands”: this sentence is meaningless, please revise it.

Comments on the Quality of English Language

The paper titled “Spectral Calibration of the Spectrometer on Board the Colombian FACSAT-2 Satellite Mission” was re-submitted after a deep revision. The authors replied to all the comments, but some responses still do not fully answer to the observations made during the first review. The quality of the paper improved a lot, but it still needs some minor revisions.

Going into details:

1.       Page 3 Lines 101: replace “;” with “.”

2.       Page 3 Lines 105: replace “is” with “are”

3.       Page 5 Lines 199 - 202: “ All these are challenges allowed to have in space a sensor for the acquisition of data, attempting to provide the quantification of GHG, especially CO2, from different processes such as geolocation, calibration in orbit, simulation of synthetic spectra and conversion of accounts, among other processes that are described below”. The authors should revise the structure of the sentence that it is not synthetically correct. Presently the meaning of the sentence is not very clear.

4.       Page 6 Paragraph 2.5: Could you give more information on how much is the approximation? I mean can you give for example a percentage on the approximate values i.e. 1%, 5% , 10 % or so on.

5.       Page 7 Paragraph 2.6: The description of spectral calibration is still too generic and poor. The procedure for calibration is still not yet described.

6.       Page 8 line 275: Since in the simulation no aerosol was considered, it is not correct to state that the study simulates a real atmosphere. I suggest to remove the word "real".

 

7.       Page 13 Figure 8 - caption: “Unlike, factory calibration, in-orbit calibration properly matches the CO2 absorption bands”: this sentence is meaningless, please revise it.

Author Response

Comments 1: Page 3 Lines 101: replace “;” with “.” 

 Response 1: We agree, thank you, we modified the sentence. 
 

Comments 2: Page 3 Lines 105: replace “is” with “are” 

Response 2: We agree, thank you, we modified the sentence: These data are also used as a reference point. 

 

Comments 3: Page 5 Lines 199 - 202: “ All these are challenges allowed to have in space a sensor for the acquisition of data, attempting to provide the quantification of GHG, especially CO2, from different processes such as geolocation, calibration in orbit, simulation of synthetic spectra and conversion of accounts, among other processes that are described below”. The authors should revise the structure of the sentence that it is not synthetically correct. Presently the meaning of the sentence is not very clear. 

 Response 3:  We agree, thank you, we modified the sentence: These challenges enable the deployment of a space-based sensor for data acquisition aimed at quantifying greenhouse gases (GHG), particularly CO2, through various processes such as geolocation, in-orbit calibration, synthetic spectra simulation, and data conversion, among others. The following sections describe these processes in more detail. 

 

Comments 4: Page 6 Paragraph 2.5: Could you give more information on how much is the approximation? I mean can you give for example a percentage on the approximate values i.e. 1%, 5%, 10 % or so on. 

Response 4: Thank you. We agree the referee’s comment. 

The absolute error in the coordinates was calculated, and subsequently, an average value of 6.3 km was determined. 

We have corrected the sentence as follows: 

The quaternions obtained from the satellite are used in a simulation in the STK (System Tool Kit) software to yield an approximate latitude and longitude coordinate corresponding to the point over which the spectrum was captured with an uncertainty of 6.3 km. 

 

Comments 5:  Page 7 Paragraph 2.6: The description of spectral calibration is still too generic and poor. The procedure for calibration is still not yet described. 

 Response 5: Thank you for your feedback, the following generalities are added:  

 

The following is a description of the generalities of the process:  

1. The most prominent and resolved band wavelengths (control points) in the synthetic spectrum are selected before picking. 
2. The pixel values corresponding to the control points in the Argus raw spectrum are picked. 
3. The wavelength-pixel scatter is plotted. 
4. A polynomial fit is applied to the quasi-linear trend of the wavelength-pixel scatter. 
5. The wavelength pixel calibration is applied to both the raw and simulated spectra. 
6. The correlation between the Argus and simulated spectra is measured. 
7. If the correlation is unacceptable, the control point picking on the raw spectrum is repeated (step 2). 

Additionally, the detailed procedure for spectral calibration is described in Section 3.2 of the Results. We hope this section provides the clarity needed. 

Comments 6: Page 8 line 275: Since in the simulation no aerosol was considered, it is not correct to state that the study simulates a real atmosphere. I suggest to remove the word "real". 

Response 6: We agree the referee’s comment. We deleted the word “real” 

 

Comments 7:  Page 13 Figure 8 - caption: “Unlike, factory calibration, in-orbit calibration properly matches the CO2 absorption bands”: this sentence is meaningless, please revise it. 

Response 7: Thank you for your comment, we agree the referee’s comment. we have decided to explain the image within the previous paragraph to reduce the length of the title and improve the overall clarity of the text. We have added the sentence as follows:   

In contrast to factory calibration, in-orbit calibration more accurately aligns with the CO2 absorption bands. 

Author Response File: Author Response.pdf