In-flight characterization of the nonlinearity and instrumental noise of the AIRS

The Atmospheric Infrared Sounder (AIRS) was launched on May 4, 2002 on the NASA Earth Observing System (EOS) Aqua spacecraft. AIRS measures the hyperspectral upwelling spectral radiance of the Earth in the infrared from 3.7- 15.4μm in 2378 channels. Instrument raw digital counts are converted to radiances using a hybrid physical, empirical approach that maintains high measurement accuracy with SI traceability. Before launch, a comprehensive On-board Calibration Plan was developed that configures the AIRS instrument into certain modes that enable characterization and/or validation of various performance parameters. Ten of the eleven on-board tests have been performed on-orbit with varying frequencies. This paper addresses three of the tests that focus on the radiometric performance of the instrument including instrumental gain and noise, non-linearity, non-Gaussian noise and stray light effects on the On-Board Calibrator (OBC) Blackbody and Space View (SV), respectively. The Guard Test, C1, uploads A only and B only detector redundancy gain tables that allow characterization of the noise and gain in normal operational mode. The Space View Noise Test, C8, stops the scan mirror while AIRS views space, providing a long, continuous, measurement of a cold target for characterization of instrumental noise and drift. The OBC Float Test, C5, turns off the OBC blackbody to provide a range of observational temperatures to the instrument for calibration of the nonlinearity. All tests show excellent stability of the instrument response, albeit regular non-automated adjustments to the detector redundancy has been required throughout the mission due to normal instrumental aging and radiation hits. Despite the changes, the AIRS Calibration Team has managed to maintain the number of active channels to the values experienced shortly after launch for the life of the mission.