Achieving Climate Quality Inter-Calibration with Accurate Spectral Solar Irradiance (SSI) Measurements from NASA's SORCE and TSIS Missions

Spectral solar irradiance (SSI) plays a critical role in calibrating the radiances of UV-VIS-SWIR bands on weather satellites. Although reflectance factor measurements (upwelling radiance normalized by SSI) are less affected by the SSI accuracy, radiance measurements (e.g., Geostationary observations) and inter-sensor calibration are sensitive to errors in the irradiance spectrum used. Modern reflective solar sensors are able to detect 1-2% errors in radiometric calibration and demand a better (<1%) accuracy of SSI measurements. In this study we analyzed 9 widely used SSI spectra for radiometric calibration evaluation.  We found that differences in the so-called “calibrated radiances” could reach as large as +/-2% in VIS-SWIR bands and +/-7% in UV bands, depending on what SSI spectrum is used. Convolution of a higher resolution SSI reference spectrum to a broader remote sensing measurement band can also introduces some uncertainty. Such a large uncertainty has been a major challenge not only for achieving climate-quality inter-calibration but also for making accurate SSI measurements directly. NASA’s SORCE (Solar Radiation and Climate Experiment, 2003-present) and future TSIS (Total and Spectral Solar Irradiance Sensor) missions are aimed to provide climate-quality SSI measurements. While the SORCE SSI accuracy is ~2% at present, TSIS-1 (to be launched to International Space Station in late 2017 or early 2018) is tasked to improve the SSI accuracy to 1% or better.