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.
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.