AIRS Calibration
The AIRS instrument measures the upwelling spectrum of the atmosphere (spectral radiances) in 2378 infrared channels and 4 Vis/NIR channels with nearly global daily coverage. Like a digital camera, the raw data from AIRS is a stream of numbers, or ‘digital counts’. Calibration is the process of converting these numbers to geophysical units with accurate radiance and wavelength, and geolocating them on Earth and providing sufficient ancillary information to characterize the instrument response.
There are different stages of calibration. Calibration initially depends on data taken before launch ("pre-flight") that include characterizing the radiometric, polarimetric, spectral and spatial response of the instrument. For calibration after launch (“in-flight”), the AIRS includes an On-Board Calibrator (OBC) Blackbody for the infrared channels, a set of lamps for the Vis/NIR channels for regular radiometric calibration updates, and a Parylene coated mirror to check the spectral response of the infrared channels. AIRS also uses views to deep space for radiometric calibration and spectral features in the upwelling spectrum of the atmosphere for spectral calibration. Instrument performance changes with time due to launch vibrations, aging, radiation, and thermal cycling makes calibration an ongoing process. The AIRS calibration team and AIRS science team perform validation of the AIRS spectral radiances. More about all of these subjects is provided below.
Products - AIRS Calibrated Radiance Data Products and Data Sets
The raw data from AIRS that is packetized and sent to the ground is called Level 0. Level 1A data are uncalibrated digital counts and telemetry from the instrument. The first product produced from AIRS is the Level 1B (L1B). L1B contains the radiometrically and spectrally calibrated and geolocated radiances with metadata. Level 1C (L1C) processes the L1B data to a fixed spectral grid (since AIRS frequencies shift slightly with time and certain mission events). L1C also substitutes dead and noisy channels and fills the gaps in the AIRS spectrum with a Principal Component (PC) reconstruction. The AIRS Level 1 products are ingested into the Level 2 processing system to generate temperature and water vapor profiles and other AIRS geophysical products. For more information on the AIRS data products see:
Level 1 | About the Products – AIRS (nasa.gov)
Pre-Flight Calibration
Prior to delivery of the AIRS instrument, a complete complement of tests were performed to calibrate the instrument. Tests included characterization of the AIRS radiometric, spatial, spectral, and polarimetric response including functionality tests to ensure safe operation and longevity in space.
More information on these tests can be found in the literature:
General Calibration References:
Kenneth Overoye, Hartmut H. Aumann, Margaret H. Weiler, George W. Gigioli Jr., William Shaw, Ed Frost, Thomas McKay, "Test and calibration of the AIRS instrument," Proc. SPIE 3759, Infrared Spaceborne Remote Sensing VII, (6 December 1999); https://doi.org/10.1117/12.372674
Pagano, T.S., H. Aumann, S. Broberg, C. Canas, E. Manning, K. Overoye, R. Wilson, “SI-Traceability and Measurement Uncertainty of the Atmospheric Infrared Sounder Version 5 Level 1B Radiances”, Remote Sens. 2020, 12, 1338; https://doi.org/10.3390/rs12081338
Thomas S. Pagano, Evan M. Manning, Steven E. Broberg, Hartmut Aumann, Robert C. Wilson, Ken Overoye, "Updates to the radiometric calibration of the Atmospheric Infrared Sounder (AIRS)," Proc. SPIE 12685, Earth Observing Systems XXVIII, 126850K (4 October 2023); https://doi.org/10.1117/12.2676416
L.L. Strow, S.E. Hannon, M. Weiler, K. Overoye, S.L. Gaiser, H.H. Aumann “Prelaunch spectral calibration of the atmospheric infrared sounder (AIRS)”, IEEE Transactions on Geoscience and Remote Sensing, Vol 41, Issue 2, 2003
Margaret H. Weiler, L. Larrabee Strow, Scott E. Hannon, Steven L. Gaiser, Rudolf A. Schindler, Kenneth Overoye, Hartmut H. Aumann, "Spectral test and calibration of the atmospheric infrared sounder," Proc. SPIE 4483, Earth Observing Systems VI, (18 January 2002); https://doi.org/10.1117/12.453471
In-Flight Calibration Tests
A complete set of tests have been developed to calibrate and characterize the AIRS instrument in-orbit. The tests involve use of the on-board calibrators and views of space for radiometric calibration (see Pre-Flight Pagano et al., 2020) and use of the upwelling Earth radiance for spectral calibration, see:
S. L. Gaiser, H. H. Aumann, L. L. Strow, S. E. Hannon and M. Weiler, "In-flight spectral calibration of the Atmospheric Infrared Sounder," in IEEE Transactions on Geoscience and Remote Sensing, vol. 41, no. 2, pp. 287-297, Feb. 2003, https://doi.org/10.1109/TGRS.2003.809708
Although AIRS continuously takes data of the Earth, data collection is occasionally interrupted to take data required for calibration. For a complete description of the in-flight test, additional references, and a log of tests performed to date on the AIRS instrument, please see:
List and Log of in-flight calibration tests
Specific Tests
AIRS-C2, Guard Test: The guard test is run monthly to evaluate the response of the IR detectors under controlled conditions: first, only the “A-side” detectors for each dual-redundant channel are turned on, then only the B-side detectors. This gives the calibration team the information needed to give optimal weights to the A- and B-side for each channel.
AIRS-C5, OBC Cool: The OBC Cool, or OBC Float Test, measures instrument non-linearity with temperature by turning off the on-board calibrator (OBC) and taking data during cool down.
Reference: Thomas S. Pagano, Evan M. Manning, Steven E. Broberg, Hartmut H. Aumann, "Checking AIRS nonlinearity in flight," Proc. SPIE 11127, Earth Observing Systems XXIV, 1112717 (9 September 2019); https://doi.org/10.1117/12.2530156
AIRS-C7, Space View Noise: The IR detectors are exposed to space or to the onboard blackbody by stopping the scan mirror.
AIRS-C8, Radiation Circumvention: This test is the same as AIRS-C7 above, but also tests the effectiveness of the radiation circumvention circuitry.
AIRS-C9, Scan Profile: The AIRS nominal scan profile is shifted to allow the slow part of the scan to view either the space view or the combined onboard blackbody/Parylene view. This allows characterization of any stray light sources.
AIRS-C10, Lamp Operations: The photometric calibration test, run monthly, turns on each of the three calibration lamps in succession to measure the response of the Visual/Near Infrared (VIS/NIR) detectors.
Reference: Steven E. Broberg, Hartmut Aumann, Evan Manning, Thomas S. Pagano, "AIRS visible light channels: calibration using deep convective clouds," Proc. SPIE 12685, Earth Observing Systems XXVIII, 126850M (6 October 2023); https://doi.org/10.1117/12.2677742
Validation
The AIRS project and science team validate the AIRS Level 1 products by comparing the calibrated radiances to well known independently calibrated geophysical features such as sea surface temperatures, polar surface temperatures, atmospheric spectral features, models and other instruments. More information on the validation of the AIRS products can be found in the literature:
Aumann, H. H., Broberg, S., Manning, E., & Pagano, T. (2019). Radiometric Stability Validation of 17 Years of AIRS Data Using Sea Surface Temperatures. Geophysical Research Letters, 46, 12504–12510. https://doi.org/10.1029/2019GL085098
H. H. Aumann, Denis Elliott, Evan Manning, "Comparison of the AIRS, IASI, and CrIS 900 cm-1 channel for Dome Concordia," Proc. SPIE 9972, Earth Observing Systems XXI, 997209 (19 September 2016); https://doi.org/10.1117/12.2235945
Loveless, M., Knuteson, R., Revercomb, H., Borg, L., DeSlover, D., Martin, G., et al. (2023). 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. Earth and Space Science, 10, e2023EA002878. https://doi.org/10.1029/2023EA002878
Wang, L., Wu, X., Li, Y., Goldberg, M., Sohn, S.-H., & Cao, C. (2010). Comparison of AIRS and IASI Radiances Using GOES Imagers as Transfer Radiometers toward Climate Data Records. Journal of Applied Meteorology and Climatology, 49(3), 478–492. http://www.jstor.org/stable/26173759
Strow, L. L., S. E. Hannon, S. De-Souza Machado, H. E. Motteler, and D. C. Tobin (2006), Validation of the Atmospheric Infrared Sounder radiative transfer algorithm, J. Geophys. Res., 111, D09S06, doi:10.1029/2005JD006146.
Strow, L. L. and DeSouza-Machado, S.: Establishment of AIRS climate-level radiometric stability using radiance anomaly retrievals of minor gases and sea surface temperature, Atmos. Meas. Tech., 13, 4619–4644, https://doi.org/10.5194/amt-13-4619-2020, 2020.
D. T. Gregorich and H. H. Aumann, "Verification of AIRS boresight accuracy using coastline detection," in IEEE Transactions on Geoscience and Remote Sensing, vol. 41, no. 2, pp. 298-302, Feb. 2003, doi: 10.1109/TGRS.2002.808311.
Thomas S. Pagano, Steve E. Broberg, "Recent checks on the radiometric and spatial calibration of AIRS in-orbit," Proc. SPIE 9972, Earth Observing Systems XXI, 997208 (19 September 2016); https://doi.org/10.1117/12.2238765
Instrument Calibration and Characterization Ancillary Files
As part of the calibration of the AIRS instrument, measurements are made of the instrument spectral, spatial, radiometric, and polarimetric response and the instrumental noise. We provide below links to pages that describe each of these data sets. Please also look in the AIRS Level 1B and Level 1C data products for real-time metrics for instrumental noise behavior and calibration coefficients used to produce the radiances for each granule.
Spatial Response Function Ancillary Data
Correlated Noise Ancillary Data
Additional Information
Link to scan head assembly, describing the on-board calibrator system
Calibration Bibliography
An extensive bibliography of AIRS calibration papers published by the AIRS project is available at:
