# Hyperspectral infrared nadir sounding *sensing . methodologies* High-spectral-resolution thermal infrared spectrometry of the atmosphere viewed at or near nadir, using Fourier-transform or grating spectrometers with more than a thousand channels to retrieve vertical profiles of temperature and humidity and columns of trace gases such as ozone, carbon monoxide and methane. The sounding backbone of numerical weather prediction. Orbit (polar or geostationary) and scan geometry are attributes, not separate methods. ## Specifications - **family**: Atmospheric spectroscopy - **requirements envelope**: {"kind":"spectral","spectral_range_nm":{"min":3600,"max":15500},"bands_min":1000,"calibration_tier_min":"radiometric","daylight_required":false,"cloud_tolerant":false} - **entity type**: methodology - **last verified date**: 2026-10-21 - **verified by**: agency-doc - **claim status**: unclaimed - **technology**: optical-spectral-sensing ## Editorial Hyperspectral infrared nadir sounding is used to retrieve vertical profiles of atmospheric temperature and humidity, and columns of trace gases such as ozone, carbon monoxide, ammonia and methane, from the thermal radiation the atmosphere emits.[^clerbaux-2009-iasi] The instruments are passive and measure emitted thermal radiation, so they work by day and by night.[^cnes-iasi-ng] The physics is summarised in [Wikipedia: Atmospheric sounding](https://en.wikipedia.org/wiki/Atmospheric_sounding). **Requirements.** The method depends on three things. First, spectral coverage in the thermal infrared. The class as a whole spans roughly 3.6 to 15.5 micrometres, but individual instruments cover narrower and sometimes discontinuous ranges: IASI (Infrared Atmospheric Sounding Interferometer) lists 3620 to 15500 nanometres,[^oscar-iasi] IKFS-2 begins near 5 micrometres,[^oscar-ikfs-2] and GIIRS (Geostationary Interferometric Infrared Sounder) and the Meteosat Third Generation Infrared Sounder (MTG-IRS) begin near 4.4 micrometres.[^oscar-giirs][^oscar-irs] Second, more than a thousand spectral channels; CrIS (Cross-track Infrared Sounder) has 1305 channels in normal mode and 2211 in full spectral resolution mode.[^oscar-cris] Third, acceptance of footprints of kilometres to several kilometres rather than fine imaging resolution; a 2023 conference presentation gives 4 km nadir sampling for MTG-IRS.[^ecss2023-irs-nowcasting] **Limits.** Cloud is the main constraint on an optical thermal infrared measurement. AIRS (Atmospheric Infrared Sounder) was designed to retrieve through partial cloud by cloud clearing with co-located microwave data,[^chahine-2003-airs-clouds] and the EUMETSAT IASI Level 2 temperature and humidity product likewise combines IASI with the AMSU-A and MHS microwave sounders.[^eumetsat-iasi-l2-catalogue] Membership of the WMO OSCAR cross-nadir infrared sounder class does not imply a hyperspectral instrument: that class has 54 instrument records,[^space-oscar-wmo-int] and the INSAT Sounder in it uses a filter wheel with 18 infrared channels and one visible channel.[^mosdac-insat-3dr-payloads] **Implementation.** AIRS is an echelle grating spectrometer,[^oscar-airs] while IASI and CrIS are Fourier transform instruments.[^oscar-iasi][^oscar-cris] Channel counts range from 2378 on AIRS[^oscar-airs] and 2670 on IKFS-2[^oscar-ikfs-2] to 8461 on IASI[^oscar-iasi] and 16921 on IASI-NG (IASI New Generation).[^oscar-iasi-ng] Geostationary members step and stare at a fixed disc: GIIRS covers 700 to 1130 and 1650 to 2250 reciprocal centimetres,[^nsmc-giirs] and a 2023 presentation on instability products gives a 30 minute repeat over Europe for MTG-IRS.[^ecss2023-irs-nowcasting] **Status as of 21 October 2026.** OSCAR lists IASI on Metop-B and Metop-C, with Metop-A ended in November 2021;[^oscar-iasi] CrIS on Suomi NPP, NOAA-20 and NOAA-21;[^oscar-cris] AIRS on Aqua;[^oscar-airs] HIRAS on FY-3D;[^oscar-hiras] GIIRS on FY-4A;[^oscar-giirs] GIIRS-2 on FY-4B, in service since September 2021;[^oscar-giirs-2] and IKFS-2 on Meteor-M N2-3 and N2-4.[^oscar-ikfs-2] Metop-SG-A1, carrying IASI-NG, launched on 13 August 2025,[^airbus-metop-sg-a1-launch] and CNES reports first light on 30 September 2025.[^cnes-iasi-ng-first-light] ESA reports that MTG-S1 commissioning closed on 2 July 2026,[^esa-mtg-s1-commissioning] and a repost of a EUMETSAT release states that pre-operational IRS data went to all users on 30 July 2026.[^eumetsat-irs-preop-repost] **Applications.** Published work covers temperature profile validation for IKFS-2 against radiosondes,[^ikfs2-temperature-validation] height-resolved methane retrievals from IASI,[^siddans-2017-iasi-ch4] global ammonia distributions from IASI,[^vandamme-2014-iasi-nh3] near-real-time outgoing longwave radiation estimated from CrIS radiances,[^zhang-2017-cris-olr] comparison of AIRS and MODIS cloud retrievals,[^nasiri-2011-airs-modis-clouds] and an accuracy assessment of IASI sea surface temperature against AVHRR (Advanced Very High Resolution Radiometer).[^ocarroll-2012-iasi-sst] A study of FY-4A GIIRS assimilates its radiances in a forecast of Typhoon Bavi.[^xiao-2022-giirs-bavi] ## Sources - [space-oscar-wmo-int] | WMO OSCAR Space instruments database (API listing) | https://space.oscar.wmo.int/api/v1/instruments | tier=agency-doc | accessed=2026-10-21 Source of the 54-instrument cross-nadir infrared sounder class count (1250 instrument records read 2026-10-21). - [oscar-iasi] | WMO OSCAR instrument record: IASI | https://space.oscar.wmo.int/instruments/view/iasi | tier=agency-doc | accessed=2026-10-21 - [oscar-cris] | WMO OSCAR instrument record: CrIS | https://space.oscar.wmo.int/instruments/view/cris | tier=agency-doc | accessed=2026-10-21 - [oscar-airs] | WMO OSCAR instrument record: AIRS | https://space.oscar.wmo.int/instruments/view/airs | tier=agency-doc | accessed=2026-10-21 - [oscar-hiras] | WMO OSCAR instrument record: HIRAS | https://space.oscar.wmo.int/instruments/view/hiras | tier=agency-doc | accessed=2026-10-21 - [oscar-giirs] | WMO OSCAR instrument record: GIIRS | https://space.oscar.wmo.int/instruments/view/giirs | tier=agency-doc | accessed=2026-10-21 - [oscar-giirs-2] | WMO OSCAR instrument record: GIIRS-2 | https://space.oscar.wmo.int/instruments/view/giirs_2 | tier=agency-doc | accessed=2026-10-21 - [oscar-iasi-ng] | WMO OSCAR instrument record: IASI-NG | https://space.oscar.wmo.int/instruments/view/iasi_ng | tier=agency-doc | accessed=2026-10-21 - [oscar-ikfs-2] | WMO OSCAR instrument record: IKFS-2 | https://space.oscar.wmo.int/instruments/view/ikfs_2 | tier=agency-doc | accessed=2026-10-21 - [oscar-irs] | WMO OSCAR instrument record: IRS (MTG-S1) | https://space.oscar.wmo.int/instruments/view/irs | tier=agency-doc | accessed=2026-10-21 - [oscar-aius] | WMO OSCAR instrument record: AIUS | https://space.oscar.wmo.int/instruments/view/aius | tier=agency-doc | accessed=2026-10-21 - [oscar-sounder-insat] | WMO OSCAR instrument record: INSAT Sounder | https://space.oscar.wmo.int/instruments/view/sounder_insat | tier=agency-doc | accessed=2026-10-21 - [mosdac-insat-3dr-payloads] | MOSDAC INSAT-3DR payloads (Sounder section) | https://www.mosdac.gov.in/insat-3dr-payloads | tier=agency-doc | accessed=2026-10-21 - [nsmc-giirs] | NSMC Geostationary Interferometric Infrared Sounder (GIIRS) | https://nsmc.org.cn/nsmc/en/instrument/GIIRS.html | tier=agency-doc | accessed=2026-10-21 - [cnes-iasi-ng] | CNES IASI-NG project page | https://cnes.fr/en/projects/iasi-ng | tier=agency-doc | accessed=2026-10-21 - [cnes-iasi-ng-first-light] | CNES: IASI-NG sees first light | https://cnes.fr/en/news/iasi-ng-sees-first-light-france | tier=agency-doc | accessed=2026-10-21 | published=2025-10-31 - [airs-jpl-instrument-suite] | NASA JPL AIRS instrument suite (CLIMCAPS science guide) | https://airs.jpl.nasa.gov/data/guides-docs/climcaps-science/instruments-data-overview-1/airs-on-aqua/copy_of_airs-guide/instrument-suite | tier=agency-doc | accessed=2026-10-21 - [chahine-2003-airs-clouds] | Chahine, Aumann and O'Callaghan: Infrared remote sensing in the presence of clouds with AIRS-AMSU-HSB | https://ntrs.nasa.gov/citations/20060030330 | tier=agency-doc | accessed=2026-10-21 | published=2003-02-03 - [clerbaux-2009-iasi] | Clerbaux et al.: Monitoring of atmospheric composition using the thermal infrared IASI/MetOp sounder | https://acp.copernicus.org/articles/9/6041/2009/ | tier=peer-reviewed | accessed=2026-10-21 | published=2009-08-28 Atmos. Chem. Phys. 9, 6041-6054, doi:10.5194/acp-9-6041-2009. - [vandamme-2014-iasi-nh3] | Van Damme et al.: Global distributions, time series and error characterization of atmospheric ammonia from IASI | https://acp.copernicus.org/articles/14/2905/2014/ | tier=peer-reviewed | accessed=2026-10-21 | published=2014-03-14 Atmos. Chem. Phys. 14, 2905-2922, doi:10.5194/acp-14-2905-2014. - [siddans-2017-iasi-ch4] | Siddans et al.: Global height-resolved methane retrievals from IASI on MetOp | https://amt.copernicus.org/articles/10/4135/2017/ | tier=peer-reviewed | accessed=2026-10-21 | published=2017-11-10 Atmos. Meas. Tech. 10, 4135-4164, doi:10.5194/amt-10-4135-2017. - [zhang-2017-cris-olr] | Zhang et al.: Estimation of near-real-time outgoing longwave radiation from CrIS radiance measurements | https://repository.library.noaa.gov/view/noaa/43919 | tier=peer-reviewed | accessed=2026-10-21 | published=2017-03-01 J. Atmos. Oceanic Technol. 34(3), 643-655, doi:10.1175/jtech-d-15-0238.1. - [nasiri-2011-airs-modis-clouds] | Nasiri et al.: Comparing MODIS and AIRS infrared-based cloud retrievals | https://espo.nasa.gov/solve2/content/Comparing_MODIS_and_AIRS_Infrared-Based_Cloud_Retrievals | tier=peer-reviewed | accessed=2026-10-21 | published=2011-05-01 J. Appl. Meteor. Climatol. 50, 1057-1072. - [ocarroll-2012-iasi-sst] | O'Carroll et al.: The accuracy of SST retrievals from Metop-A IASI and AVHRR using the EUMETSAT OSI-SAF matchup dataset | https://bibliotheque.meteo.fr/pub/DOC00025417-the-accuracy-sst-retrievals-from-metop-iasi-and-av.html | tier=peer-reviewed | accessed=2026-10-21 | published=2012-01-01 Journal not stated on the library page. - [cao-2021-aius-gf5] | Cao et al.: Assessment of spectra of the Atmospheric Infrared Ultraspectral Sounder on GF-5 and validation of water vapor retrieval | https://pmc.ncbi.nlm.nih.gov/articles/PMC7825146/ | tier=peer-reviewed | accessed=2026-10-21 | published=2021-01-06 Sensors 21(2), 325, doi:10.3390/s21020325. - [xiao-2022-giirs-bavi] | Assimilation of FY-4A GIIRS radiance observations in the forecast of Typhoon Bavi | https://www.frontiersin.org/articles/10.3389/feart.2022.1033474/full | tier=peer-reviewed | accessed=2026-10-21 Frontiers in Earth Science, doi:10.3389/feart.2022.1033474. Content taken from a search summary; page body not fetched. - [ikfs2-temperature-validation] | The Satellite Atmospheric Sounder IKFS-2: 2. Validation of the Temperature Sounding of the Atmosphere | https://journals.rcsi.science/0001-4338/article/view/148671 | tier=peer-reviewed | accessed=2026-10-21 Izvestiya, Atmospheric and Oceanic Physics. Content taken from a search summary; page body not fetched. - [hiras-itsc22] | Wu et al.: HIRAS status and calibration (ITSC-22) | https://itwg.ssec.wisc.edu/wordpress/wp-content/uploads/2023/05/2.04.Chunqiang-Wu_itsc22.pdf | tier=community | accessed=2026-10-21 Channel breakdown 778 LWIR, 433 MWIR, 159 SWIR taken from a search summary; page body not fetched. - [esa-mtg-s1-commissioning] | ESA: MTG-S1 Steering Committee closes commissioning | https://www.esa.int/ESA_Multimedia/Images/2026/07/MTG-S1_Steering_Committee | tier=agency-doc | accessed=2026-10-21 | published=2026-07-02 - [eumetsat-irs-preop-repost] | MTG Infrared Sounder data debut for European forecasters (repost of EUMETSAT release) | https://www.hjkc.de/_blog/27778-planet-erde-mtg-infrared-sounder-data-debut-for-european-forecasters/ | tier=third-party | accessed=2026-10-21 Credits EUMETSAT; pre-operational IRS data released to all users on 30 July 2026. The primary EUMETSAT page returned HTTP 403. - [ecss2023-irs-nowcasting] | ECSS 2023 presentation: MTG-IRS instability products | https://presentations.copernicus.org/ECSS2023/ECSS2023-168_presentation.pdf | tier=community | accessed=2026-10-21 IRS 4 km nadir sampling, 30 minute repeat over Europe, stability index continuity goal; taken from a search summary. - [eumetsat-iasi-l2-catalogue] | EUMETSAT Data Store: EARS IASI Level 2 temperature and humidity | https://user.eumetsat.int/catalogue/EO%3AEUM%3ADAT%3AMETOP%3AEARS-IASI-L2 | tier=agency-doc | accessed=2026-10-21 - [airbus-metop-sg-a1-launch] | Airbus: MetOp-SG A1 launch press release | https://www.airbus.com/en/newsroom/press-releases/2025-08-europes-new-era-of-weather-forecasting-begins-with-successful | tier=operator-press | accessed=2026-10-21 | published=2025-08-13 Launch 13 August 2025 on Ariane 6 from Kourou. --- Source: https://eo-atlas.org/topics/methodologies/hyperspectral-ir-nadir-sounding Publisher: SpectraWorks B.V. 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