the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jul 2025
IASI Global Radiometric Uncertainty Budget
Abstract. The Infrared Atmospheric Sounder Interferometer (IASI) is a Fourier Transform Spectrometer onboard the Eumetsat MetOp (Meteorological Operational) polar orbit satellite series. The three MetOp satellites (A, B, and C) were launched in October 2006, September 2012 and November 2018, respectively. IASI-B and IASI-C are still operational, while IASI-A was decommissioned in November 2021. IASI’s mission is to provide accurate atmospheric spectra primarily for meteorological and climate applications, and as such a high measurement precision is required. Furthermore, the estimation of the measurement uncertainty is a valuable element, especially concerning climate studies and the establishment of long climate series. This study presents the global radiometric uncertainty budget as estimated for the IASI instruments. Four major contributors, the correction of the analog non-linearity, the black body characterization, the scan mirror reflectivity, and the background radiance instability, have been identified and their impact is presented alongside other minor contributors. For black body or earth view scenes under investigation, the global budget was found to be lower than 0.2 K for a reference temperature of 280 K, when all uncertainties were considered fully correlated. These estimates are expected to remain relatively stable throughout the instrument lifetime.
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RC1: 'Comment on egusphere-2025-2913', Sahar Dehnavi, 18 Aug 2025
This study provides a well-structured analysis of the global radiometric uncertainty budget for the IASI instruments, highlighting the main contributors such as analog non-linearity correction, blackbody characterization, scan mirror reflectivity, and background radiance instability. The results demonstrate that the overall uncertainty remains below 0.2 K at 280 K, ensuring long-term stability. Given the quality and relevance of this work—particularly in view of upcoming missions like IRS, IASI-NG, HIRS, and GIIRS—I strongly recommend this paper for publication.
Citation: https://doi.org/10.5194/egusphere-2025-2913-RC1 -
RC2: 'Comment on egusphere-2025-2913', Anonymous Referee #2, 21 Aug 2025
Review of "IASI Global Radiometric Uncertainty Budget" by Kilymis et al.
This paper presents the radiometric uncertainty budget of the IASI instruments by taking the leading error sources into account and applying a rigourous error propagation. The paper is clearly written and presents the estimated error for various scenes.
General comment:Although the impact of the various error sources is well elaborated, I am missing some details about the error sources themselves. I would like to learn more about the estimates of these errors or at least find a reference where more details about these estimates are given (see corresponding comments below).
Comments:Line 88:
"The quadratic elements have no impact in B1 and can therefore be neglected. The same goes for the cubic terms whose
contribution is also negligible."
I assume they do not contribute to the spectrum because they are spectrally out of band? If so, I would describe it like this. If not, please explain why the contribution is negligible.Line 99:
"Given that the lifetime focal plane temperature fluctuations are within 0.2 K for IASI-A and 0.05 K for IASI-B and IASI-C,
the analog non-linearity can be considered stable with respect to the temperature."
What about detector aging? Could this change the non-linearity over the lifetime? (Same question holds for line 220.)Line 102:
"Therefore, the uncertainty contribution lies essentially in the estimation of the coefficients A2 and A3, which have been characterized in flight using verification interferograms."
Here I would like to have some more information on what these "verification interferograms" are, how the coefficients have been characterised and where the standard errors come from. These standard errors seem to originate from a statistical determination (e.g., using several verification interferograms). Can you rule out systematic biases which are not reflected in these standard errors?Line 117:
"The uncertainties on this temperature including measurement uncertainty, the non-blackness and the non-uniformity of the
black body have been estimated during testing of the PFM detector aboard IASI-B and were found to be equal to 91 mK."
Again, I am missing some more information on how the non-blackness and the non-uniformity as well as the temperature uncertainty iteself have been estimated / determined and how these different sources of uncertainty contribute to the 91 mK.Line 145:
"A possible instability of the temperature of different instrument components during the calibration process may affect the intensity
of Earth View and Cold Space straylight."
Can you explain a bit more how temperature instabilities impact the straylight? Are the different instrument components themeselves the sources of straylight? Or does their temperature affect their behaviour of straying light from other sources? Please clarify.Figure 7:
Can you please indicate the temperature of the black body scenes in the figure caption?
Technical corrections / suggestions:Line 104:
"The calculated coefficients, ..."
I suggest to write:
"The calculated coefficients for each pixel PN1 to PN4,Line 105:
I suggest to delete "Therefore"Line 204:
Typo: "at al." -> et al.Line 206:
"stemming by" -> stemming fromCitation: https://doi.org/10.5194/egusphere-2025-2913-RC2 -
RC3: 'Comment on egusphere-2025-2913', David Tobin, 21 Aug 2025
This article by Dimitrios Kilymis et al. describing the uncertainty in the IASI radiances is both very important and comprehensive and well written, and somewhat overdue. The IASI radiances (combined with those from CrIS) are of great value for the primary purposes of medium range weather forecasting, thermodynamic profiling, and atmospheric composition, but also for establishing a record of hyperspectral infrared radiances which provide a wealth of information to characterize changes in the Earth’s climate. The paper provides information on the various sources and size of uncertainty in the calibrated radiances, which is the information needed to interpret trends in the observed radiance record … information which is much better and more useful than the overall sensor/contract spec of 0.5K at 280K. I suggest publishing as is.
Citation: https://doi.org/10.5194/egusphere-2025-2913-RC3
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