The instrumental responsivity effect to the calibrated radiances of infrared hyperspectral benchmark sounder

Abstract

The space-borne infrared (IR) hyperspectral sounder is one important part of benchmark instruments for detection of the tiny change of long-term global climate. The IR sounder should provide irrefutable benchmarking records by measuring the infrared radiance with an ultra-high accuracy of 0.1 K (k=3, or 99% confidence), and tracing it to the Système Internationale (SI) standard for the Kelvin through the Planck function theory. Besides, the IR sounder would also constitute a reference standard, or calibration observatory, in space to inter-calibrate the international fleet of IR sounders onboard weather satellites, especially those are not as well calibrated. The measurement needs to be well-calibrated with the instrument features being eliminated, and it is critical to investigate the possible error sources associated with the sounder design and its radiometric calibration. One calibration error that arises in Fourier Transform Spectrometers (FTS) has been found associated with the spectrally variable instrument responsivity. According to the theoretical analysis of the current radiometric calibration, this error is an intrinsic feature of the FTS instruments, but it will lead to the measurements no longer being served as the standard radiances because of a radiance error introduced in the calibrated spectrum. In this paper, the radiometric errors result from the instrument responsivity effect are revealed by numerical simulations based on the spectral responses for ideal and close-to-real instruments.