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Laboratory for Atmospheric and Space Physics

Concept & Operation

SIM Instrument

The TSIS-1 SIM measures SSI (units of W m-2 nm-1) incident on a plane surface at the top of the atmosphere that is normal to the Sun two times per day from 200 to 2400 nm at variable (~2-45 nm) spectral resolution. The integrated irradiance contribution measured by SIM is approximately 96% of the total solar irradiance measured by the TIM instrument. The SIM is a follow-on to the SORCE SIM instrument with improvements in instrument design, relative accuracy (0.2% over the full spectrum), long-term stability (to better than 0.05% per year), and relative precision (0.01%). These improvements are key to understanding the contributions from varying spectral irradiance on climate. Long-term relative accuracy is maintained by duty cycling three independent spectrometers and directly measuring the prism transmission for each. Details on planned instrument operation and degradation correction are provided in the TSIS ATBD.


Full discussion of the SIM measurement geometry, design elements, including the detector design and operation, are provided in the TSIS ATBD, in addition to a summary of the SIM observation modes and a discussion of the SIM measurement uncertainties.

SIM instrument downlink telemetry nominally consists of the ESR and three diode detectors measurements. During nominal observing mode, TSIS-1 SIM will generate 258 bits-per-second of data (246 bps for science data and 12 bps for housekeeping telemetry). On a typical day, this corresponds to approximately 1.8 Mbytes of SIM science data.