Abstract
Typical lunar calibration applications utilize spectral irradiance for comparing sensors’ lunar measurements against a reference. Because the Moon’s brightness continuously changes with time and the sensor location, reference lunar irradiance values are generated for the particular Sun-Moon-viewer geometry of the instrument’s observations by computing a numerical model. Nighttime aerosol optical depth (AOD) retrievals using ground-based photometer measurements and the Langley method require accounting for changes in the Moon’s brightness over the Langley data collection period, typically 1-2 hours. Models used for lunar calibration provide the exo-atmospheric irradiances needed for the Langley analyses. The success of nighttime AOD retrievals is indicated by continuity with daytime AOD measurements. To achieve this day-night consistency, scale offsets have been found necessary to be applied to the outputs of current lunar irradiance models.
A recent AOD measurement campaign conducted at Izana observatory (28.3 deg N, 16.5 deg W, 2401 m.a.s.l.) collected lunar measurements with a Precision Filter Radiometer (PFR) developed at PMOD and calibrated at the TULIP facility at PTB. Using the USGS ROLO model to predict the nightly relative lunar irradiance variations, the Langley analyses and AOD retrievals have produced top-of-atmosphere lunar irradiance measurements with expanded relative uncertainty under 0.5%. These measurements provide reliable, quantitative evaluations of the uncertainty and bias in the outputs of ROLO and other lunar irradiance models. They will be used guide advanced lunar modeling toward higher accuracy and an improved lunar calibration reference.
Assessing Bias in Lunar Irradiance Model Outputs Using Nighttime Aerosol Optical Depth Retrievals
Typical lunar calibration applications utilize spectral irradiance for comparing sensors’ lunar measurements against a reference. Because the Moon’s brightness continuously changes with time and the sensor location, reference lunar irradiance values are generated for the particular Sun-Moon-viewer geometry of the instrument’s observations by computing a numerical model. Nighttime aerosol optical depth (AOD) retrievals using ground-based photometer measurements and the Langley method require accounting for changes in the Moon’s brightness over the Langley data collection period, typically 1-2 hours. Models used for lunar calibration provide the exo-atmospheric irradiances needed for the Langley analyses. The success of nighttime AOD retrievals is indicated by continuity with daytime AOD measurements. To achieve this day-night consistency, scale offsets have been found necessary to be applied to the outputs of current lunar irradiance models.
A recent AOD measurement campaign conducted at Izana observatory (28.3 deg N, 16.5 deg W, 2401 m.a.s.l.) collected lunar measurements with a Precision Filter Radiometer (PFR) developed at PMOD and calibrated at the TULIP facility at PTB. Using the USGS ROLO model to predict the nightly relative lunar irradiance variations, the Langley analyses and AOD retrievals have produced top-of-atmosphere lunar irradiance measurements with expanded relative uncertainty under 0.5%. These measurements provide reliable, quantitative evaluations of the uncertainty and bias in the outputs of ROLO and other lunar irradiance models. They will be used guide advanced lunar modeling toward higher accuracy and an improved lunar calibration reference.