The Infrared Absolute Radiance Interferometer (ARI) for CLARREO

Abstract

The Absolute Radiance Interferometer (ARI) is an infrared spectrometer designed to serve as an on-orbit radiometric reference with the ultra-high accuracy (better than 0.1 K 3‑σ or k = 3 brightness temperature at scene brightness temperature) needed to optimize measurement of the long-term changes of Earth’s atmosphere and surface. If flown in an orbit that frequently crosses sun-synchronous orbits, ARI could be used to inter-calibrate the international fleet of infrared (IR) hyperspectral sounders to similar measurement accuracy, thereby establishing an observing system capable of achieving sampling biases on high-information-content spectral radiance products that are also < 0.1 K 3‑σ. It has been shown that such a climate observing system with <0.1 K 2‑σ overall accuracy would make it possible to realize times to detect subtle trends of temperature and water vapor distributions that closely match those of an ideal system, given the limit set by the natural variability of the atmosphere. This paper presents the ARI sensor's overall design, the new technologies developed to allow on-orbit verification and test of its accuracy, and the laboratory results that demonstrate its capability. In addition, we describe the techniques and uncertainty estimates for transferring ARI accuracy to operational sounders, providing economical global coverage. Societal challenges posed by climate change suggest that a Pathfinder ARI should be deployed as soon as possible.

Comparison of the AIRS, IASI, and CrIS Infrared Sounders Using Simultaneous Nadir Overpasses: Novel Methods Applied to Data From 1 October 2019 to 1 October 2020

“…New in this paper, to the historically performed high spectral resolution IR sounder SNO comparisons, was the use of spatial sampling uncertainties as outlined in Tobin et al (2016) and Taylor et al (2020) and modified here. These uncertainties quantify how well the various IR sounders can be compared using the SNO methodology.…”

“…These terms are then used in the following equation, which defines a spatial variance, used as the SNO spatial sampling uncertainty: $$${\sigma }_{\mathrm{s}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{e}}^{2}=\left(1-\sfrac{{O}_{1}}{{M}_{1}}\right){\sigma }_{1}^{2}/{M}_{1}+\left(1-\sfrac{{O}_{2}}{{M}_{2}}\right){\sigma }_{2}^{2}/{M}_{2}$$$ where σ is the standard deviation of the sounder measurements within the big circle. This equation is derived such that the mean measurement over the big circle for each instrument is thought of as an area weighting of samples which overlap and samples which do not overlap (Taylor et al., 2020). Note that if there is no overlap, the equation gives the expected sum of the uncertainty variance for two independent random variables, and if all of the footprints overlap, there is no sampling error.…”

“…Taylor et al. (2020, Section 6) later iterated on the mathematical formulations laid out by Tobin et al. (2016) and provided an update with corrected limits as the number of smaller sounder footprints within the larger sounder footprint goes to zero or equals the number of effective footprints contained within the larger sized footprint.…”

“…A challenging but important aspect of comparing measurements from two instruments is ensuring the spatial and temporal mismatch is minimized and accounted for in an uncertainty estimate. The method adopted here is taken from Tobin et al (2016) and Taylor et al (2020), which discuss the theoretical uncertainties due to sampling differences between two satellite-based sounding instruments. The setup of the sounder comparisons they describe is somewhat different than the big-circle SNO method used here (defined previously in Section 3) but has an analogous comparison approach.…”

Radiometric nonlinearity and the correction strategies for infrared hyperspectral benchmark sounder

“…Ideally, the FTS measurement is linearly proportional to the incident spectral radiance. However, according to the instrumentation study and data analysis of some existing remote sensing IR FTS, such as MetOp/IASI [8], SNPP/CrIS [9], FY-3D/HIRAS [10], the signal chain nonlinearity is expected to be one dominant source of uncertainty associated with the measured spectra [11][12]. Generally, the measurement directly recorded on a FTS detector is called interferogram, which is converted by the inverse Fourier transform of the incident light spectrum.…”