Abstract
The Moon is a demonstrated useful target for radiometric calibration at reflected solar wavelengths. The distinctive properties of the Moon as a spherical diffuse reflector of sunlight stipulate that lunar calibration reference values must be generated for the particular circumstances of illumination and viewing geometry of an instrument's Moon observations. Thus the lunar reference standard takes the form of an analytic model, rather than a physical measurement, although the model is based on a set of physical measurements. Currently, the USGS model for lunar spectral irradiance is considered the international reference standard for calibration against the Moon. But the Moon is rarely used as an absolute reference, primarily due to the uncertainty in the USGS model's absolute scale. The exceptional stability of the lunar surface means that the reflectance properties of the Moon potentially can be known with very high accuracy. Therefore the current limitations on calibration uses of the Moon are with the model-based reference, not with the Moon itself. Redeveloping the lunar radiometric reference will require collecting a new set of characterization measurements, with particular attention paid to absolute accuracy and SI traceability. The experience gained with the USGS Robotic Lunar Observatory (ROLO) project has led to the development of key requirements for new lunar measurements, such as the need for continuous spectral coverage, a long-term ground-based campaign at a high altitude observing site and a flight component for above-the-atmosphere measurements. Efforts toward acquiring a new, high-accuracy lunar characterization dataset are progressing on several fronts independently, but there are common concerns regarding agency support of these projects. This paper will discuss these concerns, the current status of ongoing and potential future efforts in this area, the key measurement requirements and the benefits of an improved lunar calibration reference at reflected solar wavelengths.
Toward a New Lunar Calibration Reference Standard
The Moon is a demonstrated useful target for radiometric calibration at reflected solar wavelengths. The distinctive properties of the Moon as a spherical diffuse reflector of sunlight stipulate that lunar calibration reference values must be generated for the particular circumstances of illumination and viewing geometry of an instrument's Moon observations. Thus the lunar reference standard takes the form of an analytic model, rather than a physical measurement, although the model is based on a set of physical measurements. Currently, the USGS model for lunar spectral irradiance is considered the international reference standard for calibration against the Moon. But the Moon is rarely used as an absolute reference, primarily due to the uncertainty in the USGS model's absolute scale. The exceptional stability of the lunar surface means that the reflectance properties of the Moon potentially can be known with very high accuracy. Therefore the current limitations on calibration uses of the Moon are with the model-based reference, not with the Moon itself. Redeveloping the lunar radiometric reference will require collecting a new set of characterization measurements, with particular attention paid to absolute accuracy and SI traceability. The experience gained with the USGS Robotic Lunar Observatory (ROLO) project has led to the development of key requirements for new lunar measurements, such as the need for continuous spectral coverage, a long-term ground-based campaign at a high altitude observing site and a flight component for above-the-atmosphere measurements. Efforts toward acquiring a new, high-accuracy lunar characterization dataset are progressing on several fronts independently, but there are common concerns regarding agency support of these projects. This paper will discuss these concerns, the current status of ongoing and potential future efforts in this area, the key measurement requirements and the benefits of an improved lunar calibration reference at reflected solar wavelengths.