Lunar surface reflectance is considered extremely stable. This property has been used to validate the stability of satellite instrument calibration for reflective solar bands (RSB), such as Channels 1-6 of the Advanced Baseline Imager (ABI) onboard the Geostationary Operational Environmental Satellite (GOES). A common method is to compare the measured and modeled lunar irradiance over time. An early lunar irradiance model, the Robotic Lunar Observatory (ROLO) model, was released by the Global Space-based Inter-Calibration System (GSICS) as GSICS Implementation of the ROLO (GIRO) model. Another lunar irradiance model, the Spectral Lunar Irradiance Model Effective wavelength methodology (SLIM), was published recently. In this study, we evaluate these two models using regularly collected ABI lunar observations, with special attention to their dependence on lunar phase angle in the visible bands for B01-B02 (0.47 – 6.4 μm) and near-infrared bands for B03-B06 (0.86 – 2.3 μm). It was found that GIRO model performs well for 0.47 – 0.9 μm range but is biased for images of small lunar phase angle, and the bias increases with wavelength. SLIM model substantially corrected these biases, and the residual bias may be further reduced empirically. The SLIM model consistently predicts higher, and closer to ABI, irradiance values than the GIRO model across all ABI visible and nearinfrared (VNIR) channels.
A model of lunar spectral irradiance incorporates data from multiple spacecraft and surface telescopic observations. Using 12 data sources derived from 90,000 lunar images, models that are smooth across both geometry and wavelength and involve only about 35 derived coefficients are found with a mean weighted residual of <0.5 % . An irradiance libration model derived from lunar orbiter observations is used to reduce the number of coefficients required. Derivation uses iterative assignment of a single scaling factor for each band in each instrument, which is effectively the long-term lunar calibration coefficient for that band. Calibration of 26 instruments and two other published models reveals that although eight instruments agree within about 3% over 400 to 840 nm, some large biases exist. The model provides a sensitive assessment of instrument response trends.
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