Calibration of the absorptance cavities for the spaceflight solar radiometer TIM

摘要

The Total Irradiance Monitor (TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory mission launching in 2008. The primary sensors in TIM must absorb energy with accurately calibrated efficiency across the entire solar spectrum. To achieve high efficiency and good thermal conduction, the four sensors in each instrument are hollow conical silver cavities with a cylindrical entrance extension and a diffuse black nickel phosphorous (NiP) interior that converts absorbed incident radiation to thermal energy. A stable resistive heater wire embedded in the cone along with thermistors mounted on the cavity exterior are used in a temperature-sensing servo loop to measure the spectrally-integrated incident solar radiation. Characterization of the absorptance properties of the cavities across the solar spectrum is a dominant driver of instrument accuracy, and a dedicated facility has been developed to acquire these calibrations with uncertainties of approximately 50 ppm (0.005%). This paper describes the absorptance calibration facility, presents the preliminary cavity reflectance results for the Glory mission's TIM instrument, and details the uncertainty budget for measuring these cavity reflectances.