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.
Technical Description
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).
