Progress with the NISTAR On-Orbit Absolute Radiometric Scale

Abstract:

The NIST Advanced Radiometer (NISTAR) that resides on the Deep Space Climate Observatory (DSCOVR) has been measuring the irradiance from the sun-lit earth in 3 bands for more than 3.5 years while orbiting the L1 Lagrange point between the Earth and Sun. The bands measure the outgoing total and reflected-solar radiation from the earth at a limited range of solar angles. These measurements assist in answering questions of the earth radiation imbalance and future climate change. The NISTAR absolute radiometric uncertainty goal is less-than 1.5%, which is very challenging and requires a rigorous ground calibration and determination of the on-orbit degradation of the instrument. To this end, the NISTAR radiometers were designed for maximum stability and were calibrated and characterized extensively in the laboratory on the ground at state-of-the art facilities—in 2010 at NIST and most recently in 2013 at L-1 Standards and Technology. Questions have been raised concerning the accuracy of the short-wave (SW) channel due a difference between it and CERES derived models. The difference is outside the mutual uncertainties (k=2) of the CERES modeled product and NISTAR SW, but only by a few percent. A small shift in the magnitude or uncertainties of either the NISTAR scale and/or the modeled anisotropy factors needed compare the CERES Synoptic SW flux with NISTAR would bring the two into agreement. Current analysis indicates that the NISTAR SW is radiometrically very stable, but that the on-orbit offset measurements have more variability than the expected measurement noise. An investigation to better understand and characterize offset variability and bias is under way.