Level0 to Level1B processor for MethaneAIR

Conway, Eamon K.; Souri, Amir H.; Benmergui, Joshua; Sun, Kang; Liu, Xiong; Staebell, Carly; Chan Miller, Christopher; Franklin, Jonathan; Samra, Jenna; Wilzewski, Jonas; Roche, Sebastien; Luo, Bingkun; Chulakadabba, Apisada; Sargent, Maryann; Hohl, Jacob; Daube, Bruce; Gordon, Iouli; Chance, Kelly; Wofsy, Steven

doi:https://doi.org/10.5194/amt-17-1347-2024

Articles | Volume 17, issue 4

https://doi.org/10.5194/amt-17-1347-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-17-1347-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 4

Research article

|

29 Feb 2024

Research article |

| 29 Feb 2024

Level0 to Level1B processor for MethaneAIR

Eamon K. Conway, Amir H. Souri, Joshua Benmergui, Kang Sun, Xiong Liu, Carly Staebell, Christopher Chan Miller, Jonathan Franklin, Jenna Samra, Jonas Wilzewski, Sebastien Roche, Bingkun Luo, Apisada Chulakadabba, Maryann Sargent, Jacob Hohl, Bruce Daube, Iouli Gordon, Kelly Chance, and Steven Wofsy

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Final revised paper (published on 29 Feb 2024)
Preprint (discussion started on 23 Jun 2023)

Interactive discussion

Status: closed

RC1:
'Comment on amt-2023-111', Anonymous Referee #1, 03 Aug 2023

GENERAL COMMENTS
Atmospheric methane is one of the most important greenhouse gases. Most of the emissions come from point sources. The remote sensing by airplanes is a powerful tool.
(1) The CH₄ measurement with accuracy of “2-4 ppb on 2 km²” is very challenging. The accuracy by remote sensing technique with existing spectrometers using solar reflected light is about 10 ppb. The L1B product is usually defined as calibrated radiance spectra. What are required for characterization, calibration and level 1 products? How stable the wavelength or the instrument response function of the spectrometer should be during the flight to achieve accuracy of 2-4 ppb? How accurate are radiometrically calibrated (in Figure 2)?
Brief summary of the instrument specifications and requirement and contents for Level 1 products will help readers understanding.
(2) Chapters
The title of the Chapter 2 is “instrument”, but it describes the research flight. There is only “2.1” and no “2.2”.
Major revisions are needed before publication.

SPECIFIC COMMENTS
(1) Page 4, Line 87 “Are not monitored in the L0-L1B”
Does it mean “Laboratory-measured wavelength is not used. Wavelength is tuned during the trace gas retrieved”?

(2) Page 4, Figure 1
The description of the area such as Colorado, New Mexico etc. and legend of “RF **” will help readers’ understanding.

(3) Page 10, Line 258, “approximately 30 %”
Does it mean “difference in FWHM of ILS”?

(4) Page 10, Line 264, Page “in order to pinpoint the exact location”
How accurate the orthorectification should be from the cruising altitude?

(5) Page 11, Figure 4
It is difficult to distinguish between bold dots and asterisks.
There are more than one plots at indices of 500 and 1000. Why?

(6) Page 18, Line 425 “small increase in cabin temperature”
It is not clear how small the increase is. The actual temperature variation will help readers understanding.

TECHNICAL CORRECTIONS
(1) Page 4, Line 90, “for each sensor”
Does it mean CH₄ and O₂ spectrometers?

(2) Page 12, Line 281, “+/- 10 m file”
Is it “+/- 10 m”?

Citation: https://doi.org/10.5194/amt-2023-111-RC1
- AC1: 'Reply on RC1', Eamon Conway, 12 Sep 2023
  
  The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-111/amt-2023-111-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/amt-2023-111-AC1
RC2:
'Comment on amt-2023-111', Anonymous Referee #2, 09 Aug 2023

Please find the review commnets on the preprint in the attached PDF file.

Citation: https://doi.org/10.5194/amt-2023-111-RC2
- AC2: 'Reply on RC2', Eamon Conway, 12 Sep 2023
  
  The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-111/amt-2023-111-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/amt-2023-111-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Eamon Conway on behalf of the Authors (12 Sep 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (01 Oct 2023) by Meng Gao

RR by Anonymous Referee #1 (07 Oct 2023)

RR by Anonymous Referee #3 (07 Nov 2023)

Suggestions for revision or reasons for rejection

The Authors describe the L0 to L1B processing chain for the MethaneAIR airborne instrument, with a particular focus on wavelength calibration and image orthorectification and co-registration. The processor results on a number of test flights over the US are presented, and a good performance is documented. While the technical procedures presented appear sound, it was not always easy for me to understand what was actually done. Some of the terminology used throughout the paper will be only understandable to readers that are specialized in spectrometer calibration and geometric processing. Below my detailed comments. Line numbers refer to the revised manuscript.

- L181-182. Summarize some of the "various reasons" for straylight for the reader's benefit

- L185. What do you mean by "stray-light kernels"?

- L201. When you say "the parameterization of lambda with respect to spectral pixel index is assumed throughout" do you mean that lambda is the wavelength corresponding to the i-th spectral pixel but you omit the i subscript in the equation?

- Eq. 2. Are Doppler shifts due to Sun-Earth relative motion taken into account in computing the high-resolution solar spectrum? Or are they considered irrelevant for this purpose? I am asking because it is not uncommon that they are accounted for in GHG remote sensing from high-resolution SWIR spectrometers (e.g., O'Dell et al., 2012; Connor et al., 2016).

References:

O'Dell et al. (2012), "The ACOS CO2 retrieval algorithm - Part 1: Description and validation against synthetic observations", Atmos. Meas. Tech., 5, 99-121, 2012

Connor et al. (2016), "GFIT-2: an experimental algorithm for vertical profile retrieval from near-IR spectra"

- L305-309. A lot of prior knowledge is required from the reader to understand references to ECEF, ENU reference frames, etc., but I suspect not many readers of this journal have such knowledge. It may be useful to explain these reference frames in an appendix or provide a reference to where the reader can look them up.

- L344. If I understand correctly, A-KAZE looks for the same features in the MethaneAir and in the MSI images. Is the Hamming distance used to assess which feature in one image corresponds to which feature in the other image? This was not entirely clear to me.

- L346. Add a citation to a paper describing the RANSAC method

- Table 2. It is not exactly clear to me how you define a "success" and how you define a "failure"

- L365. "Necessitated our need to..." -> "necessitated us to"?

Hide

ED: Publish subject to technical corrections (22 Nov 2023) by Meng Gao

Please consider making following correactions:

The Authors describe the L0 to L1B processing chain for the MethaneAIR airborne instrument, with a particular focus on wavelength calibration and image orthorectification and co-registration. The processor results on a number of test flights over the US are presented, and a good performance is documented. While the technical procedures presented appear sound, it was not always easy for me to understand what was actually done. Some of the terminology used throughout the paper will be only understandable to readers that are specialized in spectrometer calibration and geometric processing. Below my detailed comments. Line numbers refer to the revised manuscript.

- L181-182. Summarize some of the "various reasons" for straylight for the reader's benefit

- L185. What do you mean by "stray-light kernels"?

- L201. When you say "the parameterization of lambda with respect to spectral pixel index is assumed throughout" do you mean that lambda is the wavelength corresponding to the i-th spectral pixel but you omit the i subscript in the equation?

- Eq. 2. Are Doppler shifts due to Sun-Earth relative motion taken into account in computing the high-resolution solar spectrum? Or are they considered irrelevant for this purpose? I am asking because it is not uncommon that they are accounted for in GHG remote sensing from high-resolution SWIR spectrometers (e.g., O'Dell et al., 2012; Connor et al., 2016).

References:

O'Dell et al. (2012), "The ACOS CO2 retrieval algorithm - Part 1: Description and validation against synthetic observations", Atmos. Meas. Tech., 5, 99-121, 2012

Connor et al. (2016), "GFIT-2: an experimental algorithm for vertical profile retrieval from near-IR spectra"

- L305-309. A lot of prior knowledge is required from the reader to understand references to ECEF, ENU reference frames, etc., but I suspect not many readers of this journal have such knowledge. It may be useful to explain these reference frames in an appendix or provide a reference to where the reader can look them up.

- L344. If I understand correctly, A-KAZE looks for the same features in the MethaneAir and in the MSI images. Is the Hamming distance used to assess which feature in one image corresponds to which feature in the other image? This was not entirely clear to me.

- L346. Add a citation to a paper describing the RANSAC method

- Table 2. It is not exactly clear to me how you define a "success" and how you define a "failure"

- L365. "Necessitated our need to..." -> "necessitated us to"?

Hide

AR by Eamon Conway on behalf of the Authors (15 Dec 2023) Author's response Manuscript

Short summary

The work presented here describes the processes required to convert raw sensor data for the MethaneAIR instrument to geometrically calibrated data. Each algorithm is described in detail. MethaneAIR is the airborne simulator for MethaneSAT, a new satellite under development by MethaneSAT LLC, a subsidiary of the EDF. MethaneSAT's goals are to precisely map over 80 % of the production sources of methane emissions from oil and gas fields across the globe to a high degree of accuracy.