SOLar STellar Irradiance Comparison Experiment II (SOLSTICE II) for the NASA Earth Observing System's Solar Radiation and Climate Experiment mission

The SOLar STellar Irradiance Comparison Experiment II (SOLSTICE II) is a component the NASA Earth Observing System's Solar Radiation and Climate Experiment mission. SOLSTICE II will use a pair of diffraction grating spectrometers to measure solar irradiance from 115 nm to 320 nm with a spectral resolution of 1 nm and a cadence of 6 hours, with an absolute accuracy of 5%, and with a relative accuracy of 0.5% per year. We will achieve an initial 5% absolute accuracy by calibrating the instrument's radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Md. Once the instrument is on-orbit, we will track changes in its sensitivity with irradiance measurements of an ensemble of bright, stable, main-sequence B-A stars. SOLSTICE II is an evolution of the SOLSTICE I instrument that is currently operating on the Upper Atmosphere Research Satellite. In this paper we review the basic SOLSTICE concept and describe the characteristics, operating modes, and anticipated performance of the SOLSTICE II instrument.