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.
Aperture area knowledge is a primary calibration in radiometric instruments. Corrections for edge effects, particularly
diffraction and scatter, must also be taken into account for high accuracy measurements. 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. In order to measure irradiance, the TIM instrument measures the total optical power that
passes through circular diamond-turned precision apertures. The geometric areas of the 8-mm diameter apertures are
measured to approximately 25 parts per million (ppm) at the National Institute of Standards and Technology . Due to
scatter and diffraction, not all light that passes through the geometric area of an aperture will enter the radiometer cavity
of the instrument, and corrections must be made for these edge effects. Diffraction effects are generally well understood
and are calculated from the instrument geometry. Scatter, on the other hand, is dependent on the microscopic edge
quality of each individual aperture, and so must be measured. This paper describes the measurement of aperture edge
diffraction and scatter for the precision apertures on NASA's Glory/TIM instrument.
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.