The TSIS-2 TIM Instrument Pre-Flight Calibration and Uncertainty

The total solar irradiance (TSI) is Earth’s primary source of energy, and accurate knowledge of its value and variability is crucial for understanding Earth’s climate and variability. In order to continue the existing 44 year data record of TSI measurements from space, NASA is developing the Total and Spectral Irradiance Sensors (TSIS) -2 mission. TSIS-2 consists of the Total Irradiance Monitor (TIM) and Spectral Irradiance Monitor (SIM) on a free flyer satellite, with an anticipated launch in the latter half of 2024. The TSIS-2/TIM is the latest iteration of the TIM instrument, prior versions of which flew onboard the SORCE, TCTE and TSIS-1 missions, and a direct rebuild of the TSIS-1 instrument. We present the pre-flight ground calibration of the TSIS-2/TIM instrument and its uncertainties. A key difference between the calibrations of the TSIS-1 and TSIS-2 instruments is the use of a novel low noise ambient temperature radiometer for TSIS-2 that significantly reduces the uncertainty in validating the component level calibrations through an end-to-end measurement. We compare component level (e.g. aperture area, detector reflectance, etc.) measurements and uncertainties for TSIS-2 with those from TSIS-1, and focus on areas where the uncertainty analysis differs from that applied to TIM instruments on prior missions and the implications of these differences.