Vicarious Calibration of the TROPOMI-SWIR module over the Railroad Valley playa
Abstract. The SWIR module of the TROPOMI instrument on board ESA's Sentinel-5p mission has been very stable during its five years in orbit. Calibration was performed on-ground, complemented by measurements during inflight instrument commissioning. The radiometric response and general performance of the SWIR module are monitored by onboard calibration sources. We show that after five years in orbit, TROPOMI-SWIR has continued to show excellent performance with degradation of at most 0.1 % in transmission and having lost less than 0.3 % of the detector pixels. Independent validation of the instrument calibration, via vicarious calibration, can be done through comparisons with ground-based reflectance data. In this work, measurements at the Railroad Valley Playa are used to perform vicarious calibration of the TROPOMI-SWIR measurements, using both dedicated measurement campaigns, as well as automated reflectance measurements through RADCALNET. As such, TROPOMI-SWIR is an excellent test case to explore the methodology of vicarious calibration applied to infrared spectroscopy. Using methodology developed for the vicarious calibration of the OCO-2 and GOSAT missions, the absolute radiometry of TROPOMI-SWIR performance is independently verified to be stable down to ~ 6–10 % using the Railroad Valley, both on the absolute and, thus, relative radiometric calibration. Differences with the onboard calibration originate from the BRDF effects of the desert surface, the large variety in viewing angles, and the different sizes of footprints of the TROPOMI pixels. However, vicarious calibration is shown to be an additional valuable tool in validating radiance-level performances of infra-red instruments such as TROPOMI-SWIR in the field of atmospheric composition.
Tim Anton van Kempen et al.
Status: open (until 01 Apr 2023)
- RC1: 'Comment on egusphere-2023-89', Anonymous Referee #1, 10 Mar 2023 reply
Tim Anton van Kempen et al.
Tim Anton van Kempen et al.
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Review of "Vicarious Calibration of the TROPOMI-SWIR module over the
Railroad Valley Playa" by Tim A. van Kempen et al., submitted to AMT, 2023
This paper describes both the results of the on-board calibration/monitoring of the TROPOMI SWIR module, and of independent vicarious ground-based calibration (or should that be verification) of the (absolute) radiometric accuracy and stability. It is shown from the on-board diagnostics that the SWIR module is extremely stable and suffers very little pixel loss. In a second theme, the ground-based vicarious calibration is described with details about the various required correction steps to make ground-based and satellite data truly comparable, although several limits are shown to be unavoidable (at this location, with this satellite sounder). In spite of the care taken to address these corrections, the vicarious method is shown to provide an independent but much less stringent assessment of the radiometric stability of the SWIR module.
The paper is very well written: clear language, logical structure, clear supporting figures (with too small lables) and the underlying work is comprehensive and to-the-point. The part on vicarious calibration using the Railroad Valley Playa is also a first for the TROPOMI mission and as such innovative. Besides a few minor comments listed below, my only gripe is with the conclusions in the abstract regarding the vicarious calibration: I think it may be made more clear that the vicarious calibration following the current state-of-the-art has severe limitations for an instrument such as TROPOMI and cannot compete with onboard calibration. Or phrased differently: onboard tools to allow the latter are still a must (as you state in the conclusions section).
As said, please find below some more minor remarks/questions that arose during my 1st reading of the paper (some may be answered further on in the paper, but that wasn't clear at 1st sight). I list these as they may help the authors to fine-tune the reading experience.
- You speak about vicarious calibration, but is it actually calibration, i.e. are correction factors derived and applied, or is it only verification?
- The last sentence about added value in the vicarious calibration w.r.t. the onboard calibration is very vague. Probably because your end conclusion is actually rather the opposite: that there is hardly any added value (if there are on-board calibration means)?
- line 11: perhaps specify that this is about the SWIR detector as the numbers (in particular the resolution on the ground) are different for the UVN module.
- some unexplained acronyms (RADCALNET, JPL LED), also happens in other locations in the paper.
- line 1: from on-ground measurements -> from pre-flight on-ground measurements
- Transmission: this is meant as both transmission along the optical path and detector sensitivity?
- line 1 on p8: pixels -> pixel's?
- is this poorer spectral resolution of RADCALNET (10nm) still fine enough to really probe only the continuum you're aiming for?
- You write: "This variation is dominated by the varying viewing angles, represented by the Bidirectional Reflection Distribution Function (BRDF) of the
non-Lambertian desert surface." How do you know? Is there a strong correlation with viewing angles but not solar angles or e.g., aerosol load?
- Aerosols are not considered in RemoteC?
- Please justify the use of a linear extrapolation (I guess this is what you expect for Mie scattering at these wavelengths)?
- I'm not sure the full mathematical description is of the model is needed here, but if you provide it, it would be nice to have some description of what the different parameters physically mean (e.g. r_0,k,b), if they can be given an intuitive meaning.
- What causes the remaining variability, besides the SZA evolution?
- Fig. 9: I don't think I understand the units of the y-axis, the range seems huge for a multiplicative correction factor. How is this correction factor applied?
- General comment on figures throughout the paper: many labels are rather small. E.g., in Figure 12.
- Maybe I missed it, but would it not be an interesting exercise to apply this (probably too strict) TROPOMI CH4 cloud filter? You'll lose a lot of data, but at least cloud contamination should be minimal.
- The first sentence sounds a bit overconfident. I guess you mean "For TROPOMI-SWIR, vicarious calibration cannot be improved upon from the RRV analysis presented in this work."
- 1st paragraph: so a more detailed mapping of the BRDF over the entire RRV (allowing you to drop the homogeneity assumption) would be a great step forwards? Then again, looking at Figure 2, it seems many pixels even reach outside the valley itself.
- At some point, true horizontal sensitivity over the pixel will probably also be an issue (i.e., it not being top-hat like but rather a super-Gaussian of some sort?).