A Concept of 2U Spaceborne Multichannel Heterodyne Spectroradiometer for Greenhouse Gases Remote Sensing

Round 1

Reviewer 1 Report

Accept in present form

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

see enclose pdf file

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

MDPI Review 1236409

A concept of 2U spaceborne multichannel heterodyne spectroradiometer for greenhouse gases remote sensing with the ground validation of expected results

by Sergei Zenevich et al.

Overview

The manuscript gives an overview of a 2U CubeSat project of a spaceborne multichannel heterodyne spectroradiometer (MLHS) with capabilities of vertical profiling of CO2, CH4, H2O and O2. It presents the mission objectives, ground based measurements and analysis, the future payload and gives an overview of the possible geometry of solar occultation measurements on board a LEO platform such as the international space station.

Overall its very interesting topic, with many small but important details on recent instrumental developments of the last years, combined and implemented in a nice small and lightweight instrument setup. The manuscript connects technical details and developments with current questions in climate research, but it would have been interesting to see more details on the data processing chain and in depth analysis of the spatial distribution in vertical and horizontal directions. That and the very short discussion and conclusion section are week points of the manuscript and could be improved.

Minor comments:

27: tomographic picture is not well described thru out the manuscript

33-35,40: First sentence.

What is “carbon balance instrumental control technology”?

Evaluation of GHG emissions is clear in the sense of natural and anthropocentric sources, but absorption?

What is a technogenic landscape?

58-62: The upcoming satellite project Merlin with a CH4 Lidar or other future or planed CO2 and CH4 projects could be mentioned. How important are GHG retrievals of CO2, CH4 and H2O and O2 in heights >30km, how much is the variation compared e.g. to land surface sources and sinks and what accuracy is needed? This project can provide what accuracy in which heights?

97: What about Tropomi on S5p?

232: Where does the Fabry-Perrot etalon comes from? Typical time of a single spectra 2min? This is not clear.

331: expected? When?

372: Are GHG of CO2, CH4 and O2 retrievals in subarctic regions in 10-55km height one of the most interesting areas on Earth? Can you provide a reference?

413: Figure 13, Are 8 bit ADC for the IF receiver sufficient and why?

482-488: Why is this part canceled?

503: Figure 15 subpart text a) b) c) hard to see

Green points and red curves hard to see

537-540: Where does this Figure come from?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

General

Basically the paper proposes the application of the MLHS method to be used on a CubeSat platform. However, the major technical part of the paper focuses on the ground based prototype. Then, in a second part, the paper theoretically handles how a solution could be realised as a 6U CubeSat. Therefore, the paper does not present so much a scientific approach, but merely a technical report.  

The proposed system would finally measure only stratospheric GHGs (20-80 km) using an tangential measure regime. It is mentioned that the ground base system is capable of measuring vertical columns. Because of that the question arises to how much extent this knowledge can be translated into the aimed tangential measurement mode. 

In the introduction section it’s motivated that the: “column abundances, and their vertical distributions on a global scale from the tropopause to the mesopause is crucial for the understanding of climate forcing processes” (lines 46-48). This, however, might be, but the major sources and sinks of GHG are still located near to the surface. It remains unclear how the change in methodology (tangential vs horizontal columns) and the domain measured (above 20 km) will reflect the claim (where vertical column abundances are the thing to capture) made?

Another question is that for CH4, the major sink is the tropospheric OH. Stratospheric photochemical removal plays a minor role, beside other tropospheric sinks like methanotrophic bacteria in soils and oxidation by chlorine radicals in the marine boundary layer, the latter both are also located in the troposphere.

Given these two concerns, I would suggest to the authors to rethink how they want to motivate their measurements utilising the CubeSat platform. 

Some Problems: 
Stratospheric measurements only, means that acc. std atmosphere the density is 7.2 %
at 20 km and 0.001 % at 80 km. While ground level measurements have 100 % density. This raises the question how much the SNR will shrink. AT 100 % column density you had SNR 50, how much will it be at 
7.2 % or even at 0.001 % column density? 

From your description I got that you need to carry a gas filled ICOS or single pass cell for standard or base signal purposes with you. Do you have any grading how much the thermal effects in space where no atmosphere buffers the radiation impact may impact your baseline providing system? Because in vacuum no energy dissipation by heat transfer is possible large temperature jumps may occur on short time intervals impacting the satellite and it’s payload. Thermostabilisation of that part e.g. needs extra energy.

Detailed

Line 188: Maybe introduce abbreviations like signal-to-noise ratio once at least. 

Figure 2: Does the unit (a.u.) also denote arbitrary power units here like later denoted in Fig 4.?

Figure 12: Only CH4, no CO2, what means HF here, is it hydrogen fluoride? Later used as well but acronym never explained.

Line 469ff: You “estimate” from the ground based measurements the SNR to be 50 at 1 second integration time. Please explain how this is done. Your data presented in Fig. 3 and the explaining text in lines 212-214 tell that a SNR 50 was obtained by integrating over a 2 min time interval. 

Technical notes

The reference style in the text is numbers but in the reference list there are none. That makes it quite hard to track what is cited. The Reference list is obviously ordered somehow by that numbers because it is not alphabetically sorted. However, without numbering that is not of any value.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

General comments

Give the meaning of all abbreviations used in the manuscript the first time they are referred. e.g. NIR, MIR meanings are missing, FPGA is given in line 391 instead of line 180 (Fig. 1).

As tropopause is rarely found at the height of 20 km, or even higher, I suggest to authors they do not use this term, tropopause, in order to define the range of observations heights of their instrument.

Some photos of the ground based prototype and 6U cubesat would improve the quality of the paper.

References should be given according to journal instructions

“Author Contributions” part of the manuscript is not given.

Specific comments

Lines 72 and 86: ENVISAT was the name of satellite and space mission while SCIAMACHY was the name of one of the instruments onboard ENVISAT. Authors should use the term ENVISAT in line 72 and remove the term SCIAMACHY from line 86.

Line 73: ENVISAT instead of SCIAMACHY.

Figs 5 and 8: Use wavelength or wavenumber both in captions and in figures. The use of both terms makes the manuscript difficult to the readers.

Figs 7a,b: According to authors figures are based on data from figs 5a and b. As I understand in figs 5a and b are presented simultaneous observations of CO2 and CH4. If this is right, why the reanalysis wind profiles are different in figs 7a and b?  I expect differences in the wind profiles obtained using CO2 and CH4 observations but why there are differences in the reanalysis wind profiles?

Line 148: I can not understand the term “stratosphere collapse”. What authors try to say here?

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx