The Instrument Simulator for Infrared Sounder onboard Chinese SI-Tracable Satellite

In order to utilize satellite observations to address the climate change concerns, a concept of benchmark measurement is defined, and finally lead to the SI-Traceable Satellites (SITSat) missions. Traceability refers to the ability to track a measurement to a known standard unit (such as the Système Internationale (SI) standards) within a given measurement uncertainty. The SI-traceable observations can better withstand measurement-data gaps, and reduce uncertainties in long-term instrument calibration drifts while in orbit. Besides, The SITSat can serve as a space metrology lab to calibrate other space instruments and convert them into a climate benchmarking system with excellent global coverage. Now, there are several SITSat missions are under development by some space agencies, including the TRUTHS developed in ESA, and the CLARREO developed in NASA. In 2014, China Ministry of Science and Technology initiated and funded the Chinese Spaced-based Radiometric Benchmark (CSRB) project, with the ultimate goal of launching a flight unit of SITSat named LIBRA.

As a part of the LIBRA mission, an infrared sounder (LIBRA-IRS) based on a Michelson interferometer is designed to have a spectral range from 600-2700 cm^-1, with a spectral sampling of 0.5 cm^-1. To maintain the SI traceability of IR radiance, a high emissivity blackbody source is used as the onboard absolute calibration source, which uses multiple phase-change cells to provide an in-situ standard with absolute temperature accuracy.

In the other hand, achieving ultra-high accuracy of 0.1 K (k=3) also depends on a well-designed instrument (IRS) and an accurate absolute calibration model. In order to identify and evaluate the uncertainty contributions in calibrated radiance, and thereby improve the traditional calibration approach, an end-to-end instrument simulator is developed in conjunction with IRS instrument development and testing.

The simulator is a computer software written in MATLAB, and can be regarded as a numerical abstraction of the physical sounder. It takes atmospheric or calibration scene radiance as well as instrument parameters as inputs, then converts them into interferograms through Fourier transformation and adds errors and noise. Finally, it generates sampled interferograms through an analog-to-digital converter (ADC). The atmospheric radiance is calculated by the Line-By-Line Radiative Transfer Model (LBLRTM) with a spectral sampling less than 0.01 cm^-1. As for the instrument model, it includes all FTS relevant optical, mechanical, electronic and thermal physics such as: optical transmittance, interferometer modulation, moving mirror speed fluctuations and time-dependent tilt, polarization of optics, background thermal flux, self-apodization due to the extension of field of view, optical and electronics noise, detector spectral responsivity and response non-linearity, sampling laser wavelength, electronic signal chain and ADC quantization, etc. Subsequently, the simulated interferogram data of atmospheric and calibration scenes are input into the radiometric calibration model to produce the calibrated radiance. This simulator is helpful for understanding the instrument, analyzing the system performance, improving the instrument design through end-to-end error analysis, and providing proxy data for calibration algorithms and software development.