Development of the Moon as an Absolute SI-traceable Celestial Calibration Target to Support Earth Remote Sensing Instrument Measurement Requirements

To support measurements relevant to climate observables, emphasis has been placed on the development of space-based Climate Observatories, e.g. CLARREO Pathfinder (CPF), that can maintain SI-traceability on-orbit with uncertainties that meet observation requirements. The transfer of radiometric scales from Climate Observatories to the suite of sensors on-orbit at any given time anticipates using Simultaneous Nadir Overpasses (SNOs) of terrestrial calibration targets, requiring a continuous train of Climate Observatories be maintained on-orbit. The cost and complexity of Climate Observatories warrants examination of alternate approaches to the in situ calibration of climate sensors. The uncertainties in the transmittance through the atmosphere is one of the larger uncertainty components in terrestrial vicarious calibration. It makes sense, therefore, to consider exo-atmospheric calibration targets such as the sun, the moon, and stars. As a general calibration target for use by a wide range of sensors, the sun is too bright while stars are too dim, leaving the moon to evaluate as a possible calibration target supporting climate studies. The moon’s radiometric properties in the reflected solar regime, while dependent on illumination and viewing geometries, are predictable to within one part in 103. Observations of the moon using the same geometry have been extremely valuable for trending the temporal response of on-orbit sensors. Empirical models such as the USGS’s RObotic Lunar Observatory (ROLO) model are used to predict the lunar spectral irradiance given an illumination and viewing geometry. The estimated uncertainties in current models are too large to support climate measurement requirements and, in most cases, are not traceable to the SI. Further development of the Moon as an absolute celestial calibration target for radiometric applications warrants a refinement of the ROLO model or replacement with a new empirical model. In either case, new measurements are required. Air-LUSI is a program currently underway making low uncertainty, SI-traceable measurements of the lunar irradiance. To reduce the uncertainty in atmospheric transmittance, a dominant uncertainty component in ground-based measurements, air-LUSI makes measurements of lunar irradiance from a NASA aircraft flying above 95% of the atmosphere. In this talk, implications of results from an air-LUSI Demonstration Flight campaign toward establishing the moon as an absolute SI-traceable calibration target are discussed