Solar Spectral Irradiance Monitor (SIM)

The Spectral Irradiance Monitor (SIM) will measure the solar spectral irradiance between 0.2 mum to 2.0 mum to a relative standard uncertainty of 3 x 10(-4) (1 sigma) and with a noise level of <1 x 10(-4). The instrument will be launched in 2002 on the SORCE mission of the Earth Observing System (EOS). Sunlight passes through a 0.3 mm x 7 mm entrance slit and is modulated by a shutter at 0.1 Hz. Light then travels 400 mm to a Fery prism that disperses and focuses the light at the focal plane containing five separate exit slits, with an electrical-substitution radiometer (ESR) and four photodiode detectors. The ESR consists of two back-to-back 1 mm x 10 mm diamond bolometers blackened with nickel phosphorus. The bolometer bridge, electrical power replacement, ESR temperature regulation, and prism rotation are under digital signal processor control. The measurement equation and the uncertainty budget for the SIM instrument are presented along with plans for the pre-flight characterization of the instrument.