The algorithm for MODIS wavelength on-orbit calibration using the SRCA

To the best of the authors' knowledge, this is the first time that a monochromator is in orbit to monitor the spectral characteristics of a radiometer on-orbit. The Spectro-Radiometric Calibration Assembly (SRCA) provides on-orbit spectral calibration of the MODerate resolution Imaging Spectroradiometer (MODIS) reflected solar bands, and this paper describes how it is accomplished. On-orbit, the SRCA will measure the band center wavelength shifts over the whole mission, and will monitor the spectral response variations for some specific bands intraorbit. A unique feature of the SRCA spectral mode is its wavelength self-calibration capability. A stable didymium glass with known transmission peaks is equipped to establish the relationship between grating step and wavelength when the grating is located at a series of positions. The measured MODIS band/channel responses versus grating step number is then scaled to wavelengths. A reference silicon photo-diode (SiPD) is used to normalize the didymium signal as well as the MODIS response signals so that the affect of the light source spectrum can be eliminated. A series of prelaunch tests at different temperatures in a thermal vacuum (TV) chamber has been completed. The test results show that the SRCA is stable and is capable of detecting MODIS wavelength shifts on-orbit.