The Main Electronics Module (MEM) controls the format engine,
the instrument, and the Scan Mirror’s rotation. Brief
explanations of how the MEM accomplishes its tasks is provided
below.
Technical Description
The format engine, which performs storage, re-ordering, and
averaging functions in real time (990 words every 333µsec),
operates under the control of a 12-MHz MIL-STD 1750A computer,
or “format processor”. The format engine is fully
reprogrammable in-flight, which is a critical capability since
the timing of the format engine input will change if it is
necessary to modify the FPA timing to correct vibrational
misregistration launch effects.
MODIS specifications require that data representing a particular
scene-pixel from all 36 bands must be contained within the
same CCSDS packet, which requires processing by the format
engine. Because the bands are separated on the Focal Plane
Assemblies (FPAs) by up to 30 instantaneous fields of view
(IFOVs), 30 km worth of in-scan data must be accumulated before
data from the first kilometer can be fully formatted. Additionally,
several bands are represented by a number of detector arrays
whose outputs are averaged to reduce noise. Following processing
by the format engine, the formatted data are sent to two 15-megabit
memories. At any time one of these memories is reading out
data at a constant rate while the other accepts data at a
higher rate during the active portion of the scan. Thus, the
two memories together act as a FIFO (first in, first out)
data buffer to provide a constant instrument output data rate.
In addition to controlling the format engine, the format processor
generates CCSDS headings in real time, formats engineering
telemetry information, and calculates individual detector
offsets for each scan based on space-view data.
Instrument control and telemetry are provided by a second
12-MHz MIL-STD 1750A computer based on commands from the spacecraft,
which are received via a MIL STD-1553 data link. A total of
160 bi-level and 16 serial command functions provide the ground
with total versatility in instrument control (although most
instrument operations are normally performed via stored command
sequences activated by individual commands). The telemetry
and command (T&C) processor functions include the control
of power to select redundant hardware for all circuits except
the T&C processor itself and the power supplies. Power
Supply ON/OFF and T&C processor selection are performed
by external spacecraft bi-level commands, which eliminates
single-point failures in these critical functions.
Smooth and accurate rotation of the MODIS Scan Mirror is
critical to both image fidelity and band-to-band registration.
The mirror control system uses a 14-bit incremental optical
encoder phase-locked to system timing. To provide smooth torque
when transferring from one motor winding to another, the characteristics
of the motor are stored in memory and the optimum power transfer
characteristic is provided for each transition. The mirror
control system allows synchronization of instrument data-taking
operations to mirror rotation within 0.05 pixel, and even
corrects for the few microradians of “wedge” nonparallelism
in the two-sided Scan Mirror.
