Calibration of the Solar Position Sensor on GOES-R as a proxy for Total Solar Irradiance I: modeling the SPS bandpass

Context: The Geostationary Environmental Operational Satellites R series (GOES-R) includes an instrument that measures visible light from the Sun at high cadence: the Solar Position Sensor (SPS). SPS is part of the Extreme ultraviolet and X-ray Irradiance Sensors (EXIS) instrument package. The visible wavelength range observed by SPS includes the peak power of the solar spectrum. Tracking the solar input to the climate system is important at all timescales. Aims: This article is the first in a series that will describe using the SPS data as a high-cadence proxy for Total Solar Irradiance (TSI), and as an input to the spectral model used in the NOAA Climate Data Record for solar irradiance. Methods: We describe the design of the SPS instrument and create a model of its output using the solar spectrum measured by the Total and Spectral Irradiance Sensor-1 (TSIS-1) on the International Space Station. We apply the bandpasses of the SPS components to the daily TSIS-1 Solar Irradiance Monitor (SIM) spectrum and integrate over wavelength to simulate the SPS measurement. Results: After applying the appropriate SPS filter transmittances and diode responsivity to the TSIS-1/SIM spectrum, we compare the integrated irradiances from the full SIM spectrum and the SPS model to the TSI measurement from TSIS-1. These comparisons of daily averages show that the integrated SPS model reproduces TSI with an uncertainty of 53 parts per million. We also show a preliminary comparison of the SPS data to the high-cadence TSI measurements from the Digital Absolute RAdiometer (DARA) on the FY-3E satellite. Conclusions: The modeled SPS spectrum shows excellent agreement with TSI on a daily cadence. Once all the instrument calibrations have been established, the SPS data will be a viable operational high-cadence proxy for TSI.