the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Feb 2025
The Atmospheric Sounder Spectrometer by Infrared Spectral Technology (ASSIST): Instrument design and signal processing
Abstract. The Atmospheric Sounder Spectrometer by Infrared Spectral Technology (ASSIST) is a Fourier-transform spectrometer designed, fabricated, and sold by LR Tech Inc., which operates in the thermal infrared. When attached to its automated radiometric calibration module, it functions as an infrared spectroradiometer (IRS) that passively measures the absolute spectral radiance within a 46 mrad full field of view and over the 525 to 3300 cm-1 (3 to 19 μm) spectral range. For atmospheric studies, the ASSIST IRS is integrated into a mobile enclosure enabling autonomous, ground-based operation under harsh conditions. It is typically configured for downwelling radiance measurements (zenith view) at 0.5 cm-1 bin spacing, 0.6 cm-1 resolution, and 4 min-1 sampling rate, closely replicating the behavior of the Atmospheric Emitted Radiance Interferometer (AERI, in rapid-sampling mode), a similar but older IRS. Atmospheric variables affecting the shape of the downwelling thermal infrared radiance spectrum at ground level can be retrieved from the ASSIST's high-resolution measurements using dedicated inversion algorithms. This includes the properties of some aerosols and simple clouds, the mixing ratios of trace gases, and the vertical distribution of temperature and water vapor (thermodynamic profile) in the lower troposphere above the instrument. Due to the form of the radiative transfer equation, thermodynamic profiles can only be retrieved with low to moderate vertical resolution, but with sufficient accuracy and temporal resolution to help fill the current boundary layer observational gap. This paper provides a detailed description of the ASSIST's design and near real-time processing algorithm producing the calibrated radiance spectra that are useful in a variety of applications.
Competing interests: All authors except D. D. Turner are employed by or consult for the company that manufactures and markets the instrument described in this manuscript.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3617', Anonymous Referee #3, 16 Mar 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-RC1-supplement.pdf
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AC1: 'Reply on RC1', Vincent Michaud-Belleau, 10 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-AC1-supplement.pdf
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AC1: 'Reply on RC1', Vincent Michaud-Belleau, 10 May 2025
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RC2: 'Comment on egusphere-2024-3617', Anonymous Referee #1, 13 Apr 2025
This paper describes the ASSIST instrument, a Fourier transform spectrometer operating in the infrared and designed for ground-based measurements of downwelling radiances. The paper is clearly written, and the instrument as well as the data processing are well described. I have a few questions/comment, though, which are listed below.
Line 83:
You describe the beamsplitter as "nominally flat". Don't you have problems with multiple reflections at parallel surfaces?Fig. 3 Figure caption:
"For simplicity, rays are traced at 550 and 1650 cm-1 for only one arm of the interferometer and from two field points located at infinity: zero and 21 mrad relative to the optical axis (blue and red, respectively)"
With this description I'd expect four beams: 550 cm-1 with zero and 21 mrad and 1650 cm-1 with zero and 21 mrad. How do I know which rays belong to 550 cm-1 and which rays belong to 1650 cm-1?Line 206:
"a NIST-traceable in-house calibration procedure"
Can you spend a few words on this calibration procedure? What is the reference?Line 215: "Section 33.3" must be "Section3.3"
Line 221:
"a modular air conditioning unit that can maintain a steady-state internal temperature for external
temperatures up to at least 40◦C."
which steady-state internal temperature is used?Line 304, Section 3.2:
Do you have any idea if the non-linearity may change over the lifetime of the instrument due to detector aging?
Could the modulation efficiency change over time? Are laboratory characterization measurements for the non-linearity coefficients
foreseen from time to time in order to monitor and to account for such possible changes?Line 370:
"Figure 13 shows the typical responsivity (magnitude of the complex gain)
in the 500 to 3000 cm-1 range for the forward scan direction."
Just out of curiosity: I understand that the phase of the gain is sweep direction dependent,
but is the magnitude the same for both sweep directions?Figure 13:
How do you treat the absorption lines in the response curves?
I assume that these absoprtion lines lead to erroneous radiances in the calibrated data?
The same question holds for the atmospheric lines in the instrument offset (Figure 14).Figure 16 (a):
What are the red dots in the curve?Line 428:
"is proportional the cosine"
should read: "is proportional to the cosine"Fig. 21:
The red line turns into yellow around 1750 cm-1 and becomes white between 1825 and 1900 cm-1 on my screen. This is a bit irritating.Citation: https://doi.org/10.5194/egusphere-2024-3617-RC2 -
AC2: 'Reply on RC2', Vincent Michaud-Belleau, 10 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Vincent Michaud-Belleau, 10 May 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3617', Anonymous Referee #3, 16 Mar 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-RC1-supplement.pdf
-
AC1: 'Reply on RC1', Vincent Michaud-Belleau, 10 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Vincent Michaud-Belleau, 10 May 2025
-
RC2: 'Comment on egusphere-2024-3617', Anonymous Referee #1, 13 Apr 2025
This paper describes the ASSIST instrument, a Fourier transform spectrometer operating in the infrared and designed for ground-based measurements of downwelling radiances. The paper is clearly written, and the instrument as well as the data processing are well described. I have a few questions/comment, though, which are listed below.
Line 83:
You describe the beamsplitter as "nominally flat". Don't you have problems with multiple reflections at parallel surfaces?Fig. 3 Figure caption:
"For simplicity, rays are traced at 550 and 1650 cm-1 for only one arm of the interferometer and from two field points located at infinity: zero and 21 mrad relative to the optical axis (blue and red, respectively)"
With this description I'd expect four beams: 550 cm-1 with zero and 21 mrad and 1650 cm-1 with zero and 21 mrad. How do I know which rays belong to 550 cm-1 and which rays belong to 1650 cm-1?Line 206:
"a NIST-traceable in-house calibration procedure"
Can you spend a few words on this calibration procedure? What is the reference?Line 215: "Section 33.3" must be "Section3.3"
Line 221:
"a modular air conditioning unit that can maintain a steady-state internal temperature for external
temperatures up to at least 40◦C."
which steady-state internal temperature is used?Line 304, Section 3.2:
Do you have any idea if the non-linearity may change over the lifetime of the instrument due to detector aging?
Could the modulation efficiency change over time? Are laboratory characterization measurements for the non-linearity coefficients
foreseen from time to time in order to monitor and to account for such possible changes?Line 370:
"Figure 13 shows the typical responsivity (magnitude of the complex gain)
in the 500 to 3000 cm-1 range for the forward scan direction."
Just out of curiosity: I understand that the phase of the gain is sweep direction dependent,
but is the magnitude the same for both sweep directions?Figure 13:
How do you treat the absorption lines in the response curves?
I assume that these absoprtion lines lead to erroneous radiances in the calibrated data?
The same question holds for the atmospheric lines in the instrument offset (Figure 14).Figure 16 (a):
What are the red dots in the curve?Line 428:
"is proportional the cosine"
should read: "is proportional to the cosine"Fig. 21:
The red line turns into yellow around 1750 cm-1 and becomes white between 1825 and 1900 cm-1 on my screen. This is a bit irritating.Citation: https://doi.org/10.5194/egusphere-2024-3617-RC2 -
AC2: 'Reply on RC2', Vincent Michaud-Belleau, 10 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3617/egusphere-2024-3617-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Vincent Michaud-Belleau, 10 May 2025
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Vincent Michaud-Belleau
Michel Gaudreau
Jean Lacoursière
Éric Boisvert
Lalaina Ravelomanantsoa
David D. Turner
Luc Rochette
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(9322 KB) - Metadata XML