Abstract
On-orbit calibration requirements for a space-based climate observing system include long-term sensor response stability and reliable inter-calibration of multiple sensors, both contemporaneous and in succession. The difficulties with achieving these for reflected solar wavelength instruments are well known. The Moon can be considered a diffuse reflector of sunlight, and its exceptional photometric stability has enabled development of a lunar radiometric reference, manifest as a model that is queried for the specific conditions of Moon observations. The lunar irradiance model developed by the Robotic Lunar Observatory (ROLO) project has adequate precision for sensor response temporal trending, but a climate-quality lunar reference will require at least an order of magnitude improvement in absolute accuracy. To redevelop the lunar calibration reference with sub-percent uncertainty and SI traceability requires collecting new, high-accuracy Moon characterization measurements. This paper describes specifications for such measurements, along with a conceptual framework for reconstructing the lunar reference using them. Three currently active NASA-sponsored projects have objectives to acquire measurements that can support a climate-quality lunar reference: air-LUSI, dedicated lunar spectral irradiance measurements from the NASA ER-2 high altitude aircraft; ARCSTONE, dedicated lunar spectral reflectance measurements from a small satellite; and Moon viewing opportunities by CLARREO Pathfinder from the International Space Station.
Highlights
To detect the subtle signatures of climate change requires continuous observations of Earth environmental variables over decadal time periods
The differences seen in comparisons of lunar irradiance measurements made by sensors against Robotic Lunar Observatory (ROLO) model results need to be better understood to exploit the full potential of the Moon as a radiometric reference, such as absolute lunar calibration and inter-calibration
As moonlight polarization is an inherent property of the Moon, the capability to predict it must be a component of a lunar reference; the instrument-dependent nature of its impact means this capability likely will need to be developed and utilized separately from the radiometric reference
Summary
To detect the subtle signatures of climate change requires continuous observations of Earth environmental variables over decadal time periods. Since the positions of the Sun, Moon and an observer (and the observation geometry) typically can be known with high accuracy, the Moon’s brightness can be predicted given a photometric specification for the lunar surface reflectance and the solar irradiance. This prediction capability is the foundation for building a radiometric reference from moonlight. Inter-calibration can be achieved with high precision using the Moon as a common target without the need for simultaneous observations; a model must be employed to predict the differences between the sensors’ lunar measurements, since each view of the Moon has a unique observation geometry. The final section provides a conceptual framework for methodologies to utilize these new measurements to refine the existing lunar reference and for advanced lunar modeling to redevelop the reference
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