Extending the spectral range of the RObotic Lunar Observatory(ROLO) model to climate science-relevant wavelengths

A critical aspect of monitoring and understanding climate change is maintaining a continuous Earth Radiation Budget (ERB) record. The radiation budget is the balance between absorbed solar radiation and emitted thermal IR radiation by the Earth-atmosphere system, and ERB sensors require shortwave (SW) broadband measurements from 0.2 5 m. Calibration accuracy, stability, and inter-consistency among different instruments are key to developing reliable composite long-term data records, but achieving sufficiently low uncertainties for these performance metrics poses a significant challenge. For this purpose, space-borne instruments commonly carry on-board references for calibration at various wavelengths, but these increase mass and mission complexity, and therefore cost, and are subject to degradation in the space environment. We present a new path towards accurate on-orbit calibration. The Moon can be considered a natural solar diffuser that can be used as a calibration target by reflected-solar radiometer instruments and can be viewed by most Earth orbiting sensors. Using the Moon as a high accuracy on-orbit reference for ERB sensors will help reduce the time it takes to detect trends in climate variables to better inform public policy and societal actions, and may allow for more robust gap-tolerant observing systems in the future. The Moon has an exceptionally stable visible to SWIR reflectance, but its brightness is continually changing. Predicting this changing brightness requires an analytical model, the most reliable of which is the RObotic Lunar Observatory (ROLO) model, which has been established at the 0.35 2.45 m spectral range. The lunar reflected UV radiation from 0.2 0.3 m and reflected/emitted NIR radiation beyond 2.45 m not accounted for by the ROLO model are small, but this additional uncertainty (on the order of a few percent) is enough to prevent using the Moon as an absolute calibration reference over the SW broadband range. We present a method for expanding the spectral range of the ROLO lunar calibration reference from the current 0.35 µm UV limit to 0.2 µm, and from the current 2.45 µm IR limit to 5.0 µm to fit the specific needs of the climate science community for absolute calibration. This expanded lunar reference will support past, present, and future ERB measurements in the SW broadband.