Calibration of the MODIS PFM SRCA for on-orbit, cross-track MTF measurement

The SpectroRadiometric Calibration Assembly (SRCA) instrument performs spatial, radiometric, and spectral in-flight calibration of the MODIS PFM instrument on the TERRA satellite. In spatial mode, the SRCA is intended to characterize focal plane registration by measuring cross-track spatial shifts of individual detectors (channels), and in-track band centroid shifts. In this paper, we investigate the suitability of the SRCA to evaluate the MODIS MTF on-orbit. Using the SRCA to evaluate the MODIS MTF requires information on the optical quality of the SRCA itself, particularly since it is not designed specifically for MTF measurements. Because the SRCA illumination fills only 1/4 of the optical aperture of the MODIS system, the illumination conditions are significantly different from those of normal MODIS imaging. We characterize the SRCA’s spatial performance by estimating its MTF from the pre-launch SRCA and Integration and Alignment Collimator (IAC) datasets, the IAC being assumed to be of significantly higher quality than the SRCA. This analysis shows that the SRCA, after calibration by comparison to the IAC, may serve as one source for on-orbit MTF measurements of MODIS PFM.