Instrument Calibration and Characterization Ancillary Files

Radiometric Ancillary Data

The AIRS radiometric calibration employs a physical-based calibration equation where the terms in the calibration, or ‘coefficients’ represent physical properties of the instrument, including temperature and ‘effective’ emissivity of the On-Board Calibrator (OBC) and mirror polarization and temperature. Note: it was not possible in AIRS to represent nonlinearity as a physical property of the response of the detector and electronics so it is represented as a ‘system-level’ quantity. The philosophy of the radiometric calibration is to maintain SI traceability of the instrument to the external Large Area Blackbody (LABB), and Space View targets. As such, the properties can occasionally deviate from physical behavior (e.g. effective emissivity > 1 by 1 part in 1000), but this does not impact the calibration accuracy. The AIRS radiometric calibration equation and methodology for deriving the coefficients is discussed in detail in the following reference:

Pagano, T.S. et al., “Updates to the Radiometric Calibration of the Atmospheric Infrared Sounder (AIRS)”, Proc. SPIE 12685-19, San Diego, Ca (2023), http://dx.doi.org/10.1117/12.2676416

Measurements

Radiometric calibration and performance testing was performed under thermal vacuum conditions, at BAE SYSTEMS on October 29, and November 3, 1999. During testing, the AIRS viewed the LABB and Space View Source (SVS) alternately while scanning. The LABB was stepped from as low as 197 K to as high as 357 K over the course of 8+ hours to calibrate the AIRS radiometric response. The SVS was held at a temperature of 85 K. The radiance of the LABB is computed as a Planck blackbody radiance distribution at the reported control temperature of the LABB. The radiances are fit to a polynomial in the difference in signals obtained between the LABB and SVS. Polarization offset is calculated prior to the fit using the polarization product and phase as derived from the space views in orbit. This allows independent determination of the other radiometric coefficients from the polarization coefficients.

Radiometric Calibration Coefficients and Data

Calibration coefficients are presented graphically in the above reference paper and can now be found in the Version 8 L1B data product for each granule. For Version 8 all coefficients are represented for A, B and AB gain settings of the instrument detectors. The values in the L1B data file correspond to the A, B and AB gain setting at the time the granule was acquired. A complete list of calibration coefficients included in the L1B used to derive the radiances and invert them to counts, if desired, can be found in section 2.9 of the AIRS V8 L1B Test Report:

Pagano, T. et al., “AIRS Version 8 Level 1B Test Report”, May 22, 2024, https://disc.gsfc.nasa.gov/information/documents?title=AIRS%20Documentation

Residual errors between the AIRS and the LABB has been achieved to better than 100 mK in over 2000 of the AIRS 2378 channels over 14 tests (Pagano, 2023). No recalibration is required or recommended. Nevertheless, we provide the pre-flight data used to derive the coefficients here for information only. The data have been processed to extract just the essential information including signals and standard deviation of the Earth View and on-board targets. The data include 14 tests each representing a different A, B or AB side of the instrument at multiple scan angles and over 12 temperatures of the LABB. More information is provided in the above reference (Pagano, 2023).

Radiometric calibration coefficients including the polarization, OBC effective emissivity, and nonlinearity are included in the Version 8 L1B data files. The coefficients for Version 5 can be downloaded from the AIRS Documentation Page. The filenames are lab_coefs_v5.1.txt, obc_emis_v5.1.txt and pol_coefs_v5.1.txt.

A NetCDF file with the pre-flight radiometric response information identified in the table below can also be downloaded from the AIRS Documentation Page. The file name is AIRS_V8_L1B_Radiometric_Ancillary.nc. Data are ordered in the Product Generation Executable (PGE) ID, i.e. same as the Level 1B data product.

Name	Size	Bytes	Class	Comments
T_labb_c	14x12	1344	double	LABB Temperature (K)
T_obc	14x12	1344	double	OBC Temperature (K)
T_sm	14x12	1344	double	Scan Mirror Temperature (K)
qc_earth	14x12x2378	1598016	single	Flag Channels with Potentially Saturated Values
sig_earth	14x12x2378	3196032	double	LABB View – Space View Counts (dn)
sig_obc	14x12x2378	3196032	double	OBC View – Space View Counts (dn)
sig_pary	14x12x2378	3196032	double	Parylene View – Space View Counts (dn)
sig_space	14x12x2378	3196032	double	Space View 2 – Space View 1 Counts (dn)
std_earth	14x12x2378	3196032	double	Std Dev of LABB View – Space View Counts (dn)
std_obc	14x12x2378	3196032	double	Std Dev of OBC View – Space View Counts (dn)
std_pary	14x12x2378	3196032	double	Std Dev of Parylene View – Space View Counts (dn)
std_space	14x12x2378	3196032	double	Std Dev of Space View 2 – Space View 1 Counts (dn)
test_name	14x12	336	char	Test Name, e.g. ‘AB 40 deg ’
tests_id_lin	14x12	672	single	Test ID’s corresponding to the acquisition
wnum	2378x1	19024	double	Frequency (cm^-1)