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

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. The SRCA has two adjacent exit slits: 1) Main slit and 2) Calibration slit. The output from the main slit is measured by a reference silicon photo-diode (SiPD) and then passes through the MODIS. The output from the calibration slit passes through a piece of didymium transmission glass and then it is measured by a calibration SiPD. The centroids of the sharp spectral peaks of a didymium glass are utilized as wavelength standards. After normalization using the reference SiPD signal to eliminate the effects of the illuminating source spectra, the calibration SiPD establishes the relationship between the peaks of the didymium spectra and the grating angle; this is accomplished through the grating equation. In the grating equation the monochromator parameters, 13(half angle between the incident and diffractive beams) and 0off (offset angle of the grating motor) are determined by matching, in a least square sense, the known centroid wavelengths of the didymium peaks and the calculated centroid grating angles from the calibration SiPD signals for the peaks. A displacement between the calibration SiPD and the reference SiPD complicates the signal processing. The response of the MODIS bands to the SRCA output is also normalized by the reference SiPD signal to eliminate the effect of the source spectrum. That response differs from what is measured by MODIS using the Spectral Measurements Assembly (SpMA), a laboratory double-monochromator, due to the wider slit width of the SRCA. The SRCA slit function, calculated using measurements by the SpMA and the SRCA at the same MODIS temperature, will be used to recover the MODIS spectral response using the SRCA data measured on-orbit. A series of test results, at different temperatures, indicate that the SRCA is stable and is capable of detecting MODIS wavelength shift on-orbit. 1. Goddard Space Flight Center, Greenbelt, MD 20771, [email protected] 2. Swales Aerospace, Beltsville, MD 20705, [email protected] 3. Research and Data System Co., Greenbelt, MD 20771,[email protected] 4. Raytheon STX, Lanham, MD 20706, [email protected]