Abstract
Establishing the Moon as an absolute spectral irradiance reference source has the potential to improve on-orbit calibrations of earth-observing instruments, especially across gaps in the data record. The Moon is of comparable brightness to the Earth when viewed from orbit, its surface reflectance is exceptionally stable, and it has the advantage of not being obscured by the Earth's atmosphere. Observations of the Moon have been used to track changes in satellite sensor response at the sub-percent level, relying on the USGS ROLO model of lunar irradiance to predict time-dependent changes in lunar irradiance. The absolute scale of the ROLO model, however, is not known accurately enough to allow the Moon to specify an absolute scale for instrument response. NIST is currently engaged in an effort to improve the absolute calibration of lunar irradiance at reflected solar wavelengths from an accuracy of ~10 % to 1 %, first in the spectral range accessible with silicon detectors, then at near-infrared wavelengths in subsequent years. Our prototype observation platform is a 4-inch telescope with an integrating sphere in the optical train at a position where the sphere aperture is slightly larger than the imaged lunar disk. The sphere is fiber-coupled to a laboratory-calibrated spectrometer with a resolution of 3 nm. We currently derive the lunar spectral irradiance from each night's observations from a Langley analysis that also yields detailed information about atmospheric optical properties. We will present progress since the previous CALCON meeting, including an updated uncertainty budget for the instrument calibration, results from ground-based observations, and plans to complement ground-based observations with observations made above the Earth's atmosphere.
Progress Towards an Absolute Calibration of Lunar Irradiance at Reflected Solar Wavelengths
Establishing the Moon as an absolute spectral irradiance reference source has the potential to improve on-orbit calibrations of earth-observing instruments, especially across gaps in the data record. The Moon is of comparable brightness to the Earth when viewed from orbit, its surface reflectance is exceptionally stable, and it has the advantage of not being obscured by the Earth's atmosphere. Observations of the Moon have been used to track changes in satellite sensor response at the sub-percent level, relying on the USGS ROLO model of lunar irradiance to predict time-dependent changes in lunar irradiance. The absolute scale of the ROLO model, however, is not known accurately enough to allow the Moon to specify an absolute scale for instrument response. NIST is currently engaged in an effort to improve the absolute calibration of lunar irradiance at reflected solar wavelengths from an accuracy of ~10 % to 1 %, first in the spectral range accessible with silicon detectors, then at near-infrared wavelengths in subsequent years. Our prototype observation platform is a 4-inch telescope with an integrating sphere in the optical train at a position where the sphere aperture is slightly larger than the imaged lunar disk. The sphere is fiber-coupled to a laboratory-calibrated spectrometer with a resolution of 3 nm. We currently derive the lunar spectral irradiance from each night's observations from a Langley analysis that also yields detailed information about atmospheric optical properties. We will present progress since the previous CALCON meeting, including an updated uncertainty budget for the instrument calibration, results from ground-based observations, and plans to complement ground-based observations with observations made above the Earth's atmosphere.