The Infrared Atmospheric Sounding Interferometer (IASI) forms the main infrared sounding component of the European Organisation for the Exploitation of Meteorological Satellites's (EUMETSAT's) Meteorological Operation (MetOp)-A satellite (Klaes et al. 2007), which was launched in October 2006. This article presents the results of the first 4 yr of the operational IASI mission. The performance of the instrument is shown to be exceptional in terms of calibration and stability. The quality of the data has allowed the rapid use of the observations in operational numerical weather prediction (NWP) and the development of new products for atmospheric chemistry and climate studies, some of which were unexpected before launch. The assimilation of IASI observations in NWP models provides a significant forecast impact; in most cases the impact has been shown to be at least as large as for any previous instrument. In atmospheric chemistry, global distributions of gases, such as ozone and carbon monoxide, can be produced in near–real time, and short-lived species, such as ammonia or methanol, can be mapped, allowing the identification of new sources. The data have also shown the ability to track the location and chemistry of gaseous plumes and particles associated with volcanic eruptions and fires, providing valuable data for air quality monitoring and aircraft safety. IASI also contributes to the establishment of robust long-term data records of several essential climate variables. The suite of products being developed from IASI continues to expand as the data are investigated, and further impacts are expected from increased use of the data in NWP and climate studies in the coming years. The instrument has set a high standard for future operational hyperspectral infrared sounders and has demonstrated that such instruments have a vital role in the global observing system.

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The Joint Polar Satellite System (JPSS) Cross‐track Infrared Microwave Sounder Suite (CrIMSS) is an advanced operational satellite sounding system concept comprised of the Cross‐track Infrared Sounder and the Advanced Technology Microwave Sounder. These are synergistically designed to retrieve key environmental data records (EDR), namely atmospheric vertical temperature, moisture, and pressure profiles. CrIMSS will serve as the low‐Earth orbit satellite sounding system, starting with the Suomi National Polar‐orbiting Partnership (S‐NPP) satellite and spanning the JPSS‐1 and JPSS‐2 satellites. This paper organizes the general paradigm for validation of atmospheric profile EDR retrieved from satellite nadir sounder systems (e.g., CrIMSS) as a synthesis of complementary methods and statistical assessment metrics. The validation methodology is ordered hierarchically to include global numerical model comparisons, satellite EDR intercomparisons, radiosonde matchup assessments (conventional, dedicated, and reference), and intensive campaign “dissections.” We develop and recommend the proper approach for computing profile statistics relative to correlative data derived from high‐resolution in situ data (viz., radiosondes) reduced to forward model layers. The standard statistical metrics used for EDR product assessments on “coarse layers” are defined along with an overview of water vapor weighting schemes and the use of averaging kernels. We then overview the application of the methodology to CrIMSS within the context of the JPSS calibration/validation program, with focus given to summarizing the core data sets to be used for validation of S‐NPP sounder EDR products.