The Electronics System runs all of the functions that the Main Electronics Module (MEM) does not. It was designed to provide the maximum operational versatility and performance combined with long life. Most operations, including calibration, are performed under internal processor control in response to time-tagged commands from the spacecraft. The electronics are highly redundant, such that no single-point of failure can disable the data from more than one of the 490 MODIS detectors. The processors can be reprogrammed from the ground, which allows modification of all formats, operational sequences, and algorithms. Since MODIS will encounter high-energy subatomic particles when flying through the South Atlantic Anomaly, the electronics have been designed throughout using components that are immune to single-event latchup damage, and which have a high threshold for single-event data upsets.
Electronics Systems functions include:
• Providing clocks and bias voltages to all four Focal Planes
• Detecting, preprocessing, converting (into 12-bit digital words), and formatting Focal Plane analog output voltages (representing scene radiances) into CCSDS packets
• Controlling Scan Mirror rotation to an accuracy of approximately 10 microradians rms
• Controlling the motion of eleven other mechanisms, ranging from simple aperture and cooler doors to microradian-accurate diffraction gratings in the calibrators
• Controlling temperatures on the Focal Planes and in the calibrators to stabilize detector performance
• Receiving and executing commands and providing telemetry via the MIL-STD 1553B interface
The electronics subsystem consists of four packages, plus some auxiliary Printed Wiring Boards (PWBs) in the calibration subsystem. The Space-viewing Analog Module (SAM) and the Forward-viewing Analog Module (FAM) perform, respectively, the analog processing and analog-to-digital conversion functions for Bands 1-30 and 31-26. However, the preamplifiers for Bands 31-36 are contained in the Cooler-Located Analog Module (CLAM) situated very close to the source of these low-level signals. All other electronics functions take place within the Main Electronics Module (including power conversion, instrument control, timing, formatting, command decoding, and telemetry formatting).