The Spectro-Radiometric Calibration Assembly (SRCA) is one of the on-board calibrators for the MODIS instrument.
The SRCA is operated in three modes: spectral, spatial, and radiometric. The spatial mode is used to track the changes
in band-to-band registration both along-scan (band and detector) and along-track (band) and the MTF in the along-scan
direction for all 36 MODIS bands over the MODIS lifetime. In the SRCA spatial mode, a rectangular knife-edge reticle,
located at the focus of the SRCA collimator, is imaged onto four MODIS Focal Plane Assemblies (FPA). The reticle is
illuminated by a spherical integration sphere and a glow-bar so that all bands can have an appropriate signal level. When
the MODIS scan mirror rotates, the illuminated knife-edge scans across the bands/detectors. In addition, there are five
electronic phase-delays so that the sampling spacing is reduced to 1/5 of the detector size, which results in dense data
points. After combining detector responses from all phase-delays, a combined bell-shaped response profile is formed.
The derivative of the detector response to the knife-edge is the Line Spread Function (LSF). In the frequency domain,
the Modulation Transfer Functions (MTF) are calculated from the normalized Fourier transform of the LSF. The MTF
results from the SRCA are validated by the pre-launch results from the Integrated Alignment Collimator (IAC) and a
SRCA collection performed in the Thermal Vacuum (TV). The six-year plus on-orbit MTF trending results show very
stable responses in the VIS and NIR FPAs, and meet the design specifications. Although there are noticeable MTF
degradations over the instrument lifetime in bands 1 and 2, they are negligible with the large specification margins. In
addition, a similar relationship is found between the band locations in the VIS and NIR FPAs versus MTF values.