The Spectral Irradiance Monitor (SIM) for the SORCE mission

The Spectral Irradiance Monitor (SIM) will measure the solar spectral irradiance from 0.2- to 2.0 muM with an accuracy of 300 ppm (lo) and a precision of 100 ppm. The SIM will be launched in 2002 on the EOS SORCE mission (SOlar Radiation and Climate Experiment). This instrument is a Fery prism spectrometer with an Electrical Substitution Radiometer (ESR) as the primary detector, and 4 additional photodiodes detectors. SIM incorporates the following design features. 1) Two independent spectrometer channels coupled with a periscope/calibrator mechanism to monitor changes in prism transmission in-flight. 2) A closed-loop wavelength drive provides precise position knowledge in the spectrometer focal plane. 3) An ESR to maintain the long-term absolute calibration of the instrument. The ESR consists of back-to-back 1x10-mm(2) diamond bolometers blackened with nickel phosphorous and mounted at the center of a spherical cavity to increase the effective bolometer blackness. A shutter located in front of the spectrometer entrance slit modulates the solar signal at 0.1 Hz; the ESR synchronously detects Light at the shutter fundamental. The bolometer bridge control, electrical power replacement, ESR temperature regulation, prism rotation, and shutter actuation are all under digital signal processor control.