Concept & Operation
SIM measures the absolute solar spectral irradiance twice per day throughout the mission. There are two major science modes of operation for SIM. The first is an ESR step/stop/integrate mode where the wavelength drive is parked at a wavelength determined from a table, and the shutter is actuated at 0.1 Hz for a period of 100 seconds. The drive is then stepped to the next wavelength in the table and the process is repeated. The second mode of operation is a fast scan mode where the drive is scanned over the full spectral range of the instrument at a rate of 1/6 of a slit width per step. This permits the acquisition of a properly sampled solar spectrum in a time span of 24 minutes. Throughout the mission the radiant sensitivity of the focal plane photodiodes are calibrated at the ESR table locations.
Other measurement modes include field-of-view mapping of the instrument to track changes in the responsivity of the spectrometer as a function of time. This maneuver is performed every 6 months. Cruciform alignment maneuvers are used to track possible changes in solar pointing over time. An ESR scan mode is also used to measure the solar spectrum between 1.6 and 2.4 µm to extend the operation of the instrument into the infrared. Finally, there are two in-flight calibration modes that will be discussed in the next section.