Calibration of the total irradiance monitor

Abstract

Launching on the SORCE (SOlar Radiation and Climate Experiment), the total irradiance monitor (TIM) will measure the total solar irradiance to a relative standard uncertainty of 10−4 and a noise level of 2×10−6 each 400 s. Because of the outstanding brightness, uniformity, stability and collimation of the Sun, this ambient temperature primary radiometer achieves an absolute accuracy comparable to that of a cryogenic radiometer. The major paradigm shift from previous solar radiometers is the use of phase-sensitive detection at the shutter fundamental. We describe the equivalence between replacement power and radiant power as a complex number, the ratio of the (complex) thermal impedances. Our aperture measurements are provided by NIST, with added corrections for diffraction and scattering. We measure the cavity reflection losses versus wavelength using laser scans. We have verified methods for measuring, in flight, the servo gain, cavity reflection changes and shutter-modulated infrared offset (dark signal).