The Space-Viewing Analog Module (SAM) performs a number of
functions that help keep MODIS data accurate, including reading
and processing scene data from Bands 1-30, converting and
sending data to the format engine, and ensuring perfect image
registration in the event of a mechanical shift. A full list
of the SAM's functions is provided below.
Technical Description
The SAM reads and processes analog scene values from Bands
1-30 using custom-designed readout chips, which use Capacitive
Tran-Impedance Amplified (CTIA) architecture. These chips
provide a total of seven outputs to the processing electronics,
with bands apportioned to the outputs to achieve approximately
equal data rates from each output.
The SAM also linearly converts the processed data into 12-bit
numbers, and then outputs them to the format engine.
The SAM periodically calculates and updates correction coefficients
using data from the space and blackbody views, even though
the detector gain offsets are expected to be quite stable
over the life of each MODIS instrument.
The SAM also provides 14 low-noise clocks. The low-noise
is important to keep from interfering with normal MODIS data
collection.
The SAM enables and biases voltages to each Focal Plane.
Some of these voltages can be reprogrammed if necessary.
Should any unexpected mechanical shifts occur, such as those
due to launch vibrations, the SAM advances and retards sample
timing from each Focal Plane in small increments to restore
perfect image registration between Focal Planes in the scan
direction.
Finally, the SAM controls an auxiliary charge injection circuit
that is incorporated into the Focal Plane readouts. This circuit
generates extremely linear calibration ramp signals, which
provide in-flight verification of continued A/D converter
linearity.
