The TSI Radiometer Facility: absolute calibrations for total solar irradiance instruments

The total solar irradiance (TSI) climate data record includes overlapping measurements from 10 spaceborne radiometers. The continuity of this climate data record is essential for detecting potential long-term solar fluctuations, as offsets between different instruments generally exceed the stated instrument uncertainties. The risk of loss of continuity in this nearly 30-year record drives the need for future instruments with <0.01% uncertainty on a absolute scale. No facility currently exists to calibrate a TSI instrument end-to-end for irradiance at solar power levels to these needed accuracy levels. The new TSI Radiometer Facility (TRF) is intended to provide such calibrations. Based on a cryogenic radiometer with a uniform input light source of solar irradiance power levels, the TRF allows direct comparisons between a TSI instrument and a reference cryogenic radiometer viewing the same light beam in a common vacuum system. We describe here the details of this facility designed to achieve 0.01% absolute accuracy.