Pre-Launch and On-Orbit Calibration of the Spectral Irrandiance Monitor (SIM) on SORCE

The Spectral Irradiance Monitor (SIM) aboard the Solar Radiation and Climate Experiment (SORCE) satellite measures the solar spectral irradiance from 200 - 2700 nm. SIM is a Féry prism spectrometer that uses an electrical substitution radiometer (ESR) to monitor the absolute irradiance calibration throughout operation. The two SIM instruments on SORCE are mirror images of each other, mounted side-by-side in the same package. Since the prism transmission coefficient is a term that appears directly in the instrument's measurement equation, laboratory measurements are required for data analysis. Ratiometric laboratory measurements of the prism transmission as a function of wavelength and polarization will be presented, along with experiment descriptions. Furthermore, on-orbit experiments monitor changes in prism transmission to maintain the pre-launch calibration. Each SIM can calibrate the other. During an on-orbit transmission measurement, one SIM directs monochromatic light into the second via a periscope mechanism. The second SIM then uses internal optics to measure its prism transmission ratiometrically. Preliminary results from the ongoing prism degredation analysis will also be presented.