Performance and polarization response of slit homogenizers for the GeoCarb mission

Crowell, Sean; Haist, Tobias; Tscherpel, Michael; Caron, Jérôme; Burgh, Eric; Moore III, Berrien

doi:https://doi.org/10.5194/amt-16-195-2023

Articles | Volume 16, issue 1

https://doi.org/10.5194/amt-16-195-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-16-195-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 1

Research article

|

17 Jan 2023

Research article |

| 17 Jan 2023

Performance and polarization response of slit homogenizers for the GeoCarb mission

Sean Crowell, Tobias Haist, Michael Tscherpel, Jérôme Caron, Eric Burgh, and Berrien Moore III

Download

Final revised paper (published on 17 Jan 2023)
Preprint (discussion started on 11 Jul 2022)

Interactive discussion

Status: closed

RC1:
'Comment on amt-2022-146', Anonymous Referee #1, 28 Aug 2022

<General Comments>

Imaging spectrometer with high spectral resolution is a key technology for greenhouse gases and their related species monitoring by remote sensing. Acquired image provides large emission sources and plume information. Very narrow spectral width needs moderate optical throughput to achieve high signal to noise ratio. Inhomogeneity within the footprint distorts the instrument spectral response function (ISRF). Gratings are conventionally used, but they have high polarization. Measurement techniques acquiring solar lights reflected by the Earth’s surface and scattered by thin cloud and aerosols should care input light polarization, too. The topics discussed here is challenging and important. My question is which is more critical for GeoCarb: polarization sensitivity or spatial inhomogeneity? Both OCO-2 and GeoCarb are using high resolution imaging spectrometer technology, but they have different swath, spectral coverage, and throughput, The OCO-2 and OCO-3 on orbit already achieved low bias and reduce random errors in CO₂ retrieval without using state-of-art slit homogenizer. Are there a critical angle and/or spectral band, and footprint size, where the distortion in ISRF becomes critical for CO2 retrieval? What is the main difference between OCO-2 and GeoCarb?

Slit design, method, and results of characterization test in the laboratory are well described. I recommend minor revision before publication.

<Specific Comments>

(1) Page 4. MODIS data

If it is a real data, the location and date of the observation should be described.

(2) Page 10, Fig. 6 The right figure

Is “analyzer” a “polarizer” in the middle figure? Description of analyzer is needed.

(3) Page 10, Line 187 “noise”

Is it random electrical noise?

(4) Page 13, Figure 8, “different level of illumination”

More detailed explanation such as definition of “level”, will help readers’ understanding.

<Technical Corrections>

There are no specific comments.

Citation: https://doi.org/10.5194/amt-2022-146-RC1
- AC1: 'Reply on RC1', Sean Crowell, 28 Oct 2022
  
  <General Comments>
  Imaging spectrometer with high spectral resolution is a key technology for greenhouse gases and their related species monitoring by remote sensing. Acquired image provides large emission sources and plume information. Very narrow spectral width needs moderate optical throughput to achieve high signal to noise ratio. Inhomogeneity within the footprint distorts the instrument spectral response function (ISRF). Gratings are conventionally used, but they have high polarization. Measurement techniques acquiring solar lights reflected by the Earth’s surface and scattered by thin cloud and aerosols should care input light polarization, too. The topics discussed here is challenging and important. My question is which is more critical for GeoCarb: polarization sensitivity or spatial inhomogeneity? Both OCO-2 and GeoCarb are using high resolution imaging spectrometer technology, but they have different swath, spectral coverage, and throughput, The OCO-2 and OCO-3 on orbit already achieved low bias and reduce random errors in CO₂ retrieval without using state-of-art slit homogenizer. Are there a critical angle and/or spectral band, and footprint size, where the distortion in ISRF becomes critical for CO2 retrieval? What is the main difference between OCO-2 and GeoCarb?
  Thanks for your question. Indeed, the ISRF shape is very important for GeoCarb as it is for the OCO missions. We have done some experiments that are in a paper in preparation that show we can mitigate much of the retrieval error by introducing an ILS stretch parameter into the state vector of the retrieval algorithm. We will add some text to the manuscript to this effect. Note that the OCO missions mitigated the effects of scene inhomogeneity through reduction in image quality. This, combined with the much smaller footprint, have effectively gotten rid of this as a challenge. This was checked with our ILS fitting tests applied to the OCO-2 data where we found almost no change.
  Slit design, method, and results of characterization test in the laboratory are well described. I recommend minor revision before publication.
  Thank you!
  <Specific Comments>
  (1) Page 4. MODIS data
  If it is a real data, the location and date of the observation should be described.
  Agreed. We will add this in the revised manuscript.
  (2) Page 10, Fig. 6 The right figure
  Is “analyzer” a “polarizer” in the middle figure? Description of analyzer is needed.
  Yes, that's the case. We will correct this label in the revised manuscript.
  (3) Page 10, Line 187 “noise”
  Is it random electrical noise?
  The variations in values in the image are driven primarily by shot noise and read noise.
  (4) Page 13, Figure 8, “different level of illumination”
  More detailed explanation such as definition of “level”, will help readers’ understanding.
  We will clarify in the revised manuscript.
  
  Citation: https://doi.org/10.5194/amt-2022-146-AC1
RC2:
'Comment on amt-2022-146', Anonymous Referee #2, 16 Sep 2022

Please find the comments in the attached pdf document.

Citation: https://doi.org/10.5194/amt-2022-146-RC2
- AC2: 'Reply on RC2', Sean Crowell, 28 Oct 2022
  
  General Comments
  
  The paper is well structured and written. The investigation of effects of polarized scenes on slit homogenizer was missing so far. To our knowledge neither measurements nor simulation exist. In this respect this study is a novelty. While the result on polarization dependence performance is of general interest also the second goal of the study is very interesting, i.e. the dependence on the different devices – even with the same geometry but from different vendors. The result is very helpful for future missions.
  
  Just a remark: The performance of a scene homogenizer for a step and stare system like GeoCarb Mission is more important than for a push broom like Sentinel-5 (due to smearing effects of the moving scene). So it is not understandable why the slit homogenizer has been removed from GeoCarb instead adding a polarization scrambler. Another option would have been fibre-based 2D-slit homogenizer concepts. Maybe this technology was not mature enough by the time of design freeze of GeoCarb, wasn’t it? Some comments in this direction would be helpful.
  
  This is a good point that we will elaborate on in the revised manuscript. We examined the GeoCarb design early on and determined that inclusion of a scrambler was not feasible given the design constraints we were working with. Additionally, the fibre-based 2D-SH concepts were not quite ready for implementation into our system at the time we had to make the decision, in particular with respect to radiation testing.
  
  The conclusion from the results seem to be not consistent. In the abstract (line 3f, 9f) one reads that a slit homogenizer is recommended but polarisation should be controlled. While in Discussion you reveal that the SH is not used at all for several technical reason.
  
  We will make this clearer in the revised manuscript. We were able to modify the instrument in a very straightforward way to take the homogenizer out, while adding a scrambler or some other mitigation would have constituted a significant design change.
  
  Throughout the full paper one gets the impression, that the use of a SH is a mistake even in other missions like Sentinel-5. Only in the last paragraph this impression is corrected. Other missions (like Sentinel-5) may differs from GeoCarb in many aspects, which have a significant impact on the SH performance. As you pointed out these may be different f# at the slit plane or the observation geometry (pushbroom) or the use of a polarization scrambler. The comparison with Sentinel-5 should be neutral and more elaborated. The differences must be given already at the first occurrence of Sentinel-5 in the paper to avoid a misleading understanding.
  
  We fully agree with the comments made by the reviewer. We will more carefully moderate the discussion of Sentinel 5 in the revised manuscript.
  
  We also agree with the other comments and will make the necessary changes. These changes will be carefully addressed and documented in our response to the editor.
  
  Citation: https://doi.org/10.5194/amt-2022-146-AC2

Peer review completion

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

AR by Sean Crowell on behalf of the Authors (06 Nov 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (21 Nov 2022) by Ulrich Platt

AR by Sean Crowell on behalf of the Authors (02 Dec 2022) Manuscript

Download

The requested paper has a corresponding corrigendum published. Please read the corrigendum first before downloading the article.

Article (2599 KB)
Full-text XML

Short summary

Variations in brightness in radiance measurements cause errors that can be mitigated with hardware that scrambles the pattern of the incoming light. GeoCarb took this route to minimize this source of errors, but lab testing determined that the solution chosen was too sensitive to the the polarization of the incoming light. Modeling found that this was a predictable result of using gold coatings in the design, which is typical of spaceflight optical instruments.