Abstract
In remote sensing of the Earth system, accuracy is key for developing reliable, long-term environmental data records built from data collected by multiple instruments, for quickly detecting trends in the quantities being observed, and for determining the confidence with which conclusions can be drawn from those data. For sensors measuring light in the reflected solar wavelengths from Earth orbit, measurement accuracy has historically been limited by the techniques used in calibration (e.g., using onboard lamps and/or solar diffusers that increase instrument mass/complexity and degrade over time, or viewing ground sites through the Earth’s atmosphere, requiring an additional correction, etc.). The Moon can be used as a stable calibration standard, enabling intercalibration between sensors, even those that do not overlap temporally. Though the Moon’s surface reflectance is effectively timeinvariant, its apparent brightness is constantly changing due to orbital mechanics. Lunar models have been developed to predict the brightness that an instrument would sense when the Moon is observed from Earth orbit over a range of orbital geometries, but these have been limited in accuracy. To date, lunar radiometric models largely have been built from measurements taken by ground-based telescopes, and the Earth’s atmosphere is a source of error in these measurements. Arcstone is the first instrument dedicated to collecting the data needed for lunar model development from a space-based platform, where atmospheric weather does not interfere with the measurements, allowing consistent and frequent lunar sampling. Arcstone was developed as a project funded by the NASA Earth Science Technology Office’s In-Space Validation of Earth Science Technologies (InVEST) program that features a compact spectrometer integrated into a 6U cubesat bus. It is designed to sequentially measure the signals of reflected light from the Moon and light emitted from the Sun to enable highly accurate lunar disk reflectance calculations across the 350 nm to 2300 nm spectral range. Arcstone does this using the same optical system to observe both the Sun and Moon directly, viewing the Sun for orders of magnitude less time than the Moon to account for the very different brightnesses. Since its launch on June 23, 2025, Arcstone has successfully demonstrated this measurement concept and the mission is now in extended operations. The team is working to create reflectance data products with an unprecedented accuracy goal of < 0.5% (k=1). The Arcstone data will be made available to facilitate advancement of lunar calibration techniques for all Earth-orbiting sensors that have viewed the (near-side) full disk of the Moon in the reflected solar wavelengths: past, present, and future. Instrument performance and preliminary data will be presented by the Arcstone team.
Arcstone: Calibration of Lunar Spectral Reflectance from Space
In remote sensing of the Earth system, accuracy is key for developing reliable, long-term environmental data records built from data collected by multiple instruments, for quickly detecting trends in the quantities being observed, and for determining the confidence with which conclusions can be drawn from those data. For sensors measuring light in the reflected solar wavelengths from Earth orbit, measurement accuracy has historically been limited by the techniques used in calibration (e.g., using onboard lamps and/or solar diffusers that increase instrument mass/complexity and degrade over time, or viewing ground sites through the Earth’s atmosphere, requiring an additional correction, etc.). The Moon can be used as a stable calibration standard, enabling intercalibration between sensors, even those that do not overlap temporally. Though the Moon’s surface reflectance is effectively timeinvariant, its apparent brightness is constantly changing due to orbital mechanics. Lunar models have been developed to predict the brightness that an instrument would sense when the Moon is observed from Earth orbit over a range of orbital geometries, but these have been limited in accuracy. To date, lunar radiometric models largely have been built from measurements taken by ground-based telescopes, and the Earth’s atmosphere is a source of error in these measurements. Arcstone is the first instrument dedicated to collecting the data needed for lunar model development from a space-based platform, where atmospheric weather does not interfere with the measurements, allowing consistent and frequent lunar sampling. Arcstone was developed as a project funded by the NASA Earth Science Technology Office’s In-Space Validation of Earth Science Technologies (InVEST) program that features a compact spectrometer integrated into a 6U cubesat bus. It is designed to sequentially measure the signals of reflected light from the Moon and light emitted from the Sun to enable highly accurate lunar disk reflectance calculations across the 350 nm to 2300 nm spectral range. Arcstone does this using the same optical system to observe both the Sun and Moon directly, viewing the Sun for orders of magnitude less time than the Moon to account for the very different brightnesses. Since its launch on June 23, 2025, Arcstone has successfully demonstrated this measurement concept and the mission is now in extended operations. The team is working to create reflectance data products with an unprecedented accuracy goal of < 0.5% (k=1). The Arcstone data will be made available to facilitate advancement of lunar calibration techniques for all Earth-orbiting sensors that have viewed the (near-side) full disk of the Moon in the reflected solar wavelengths: past, present, and future. Instrument performance and preliminary data will be presented by the Arcstone team.