ARCSTONE will provide more accurate lunar spectral reflectance measurements to establish an absolute lunar calibration standard for past, current, and future Earth observing sensors. The objective of this project is to demonstrate in-space validation of an approach for establishing the Moon as an accurate reference for on-orbit calibration of reflected solar instruments.
Calibration accuracy and long-term stability are the primary on-orbit performance metrics for all Earth observing sensors. The challenges in achieving the radiometric accuracy and stability levels over the long timescales that are needed for understanding complex systems such as Earth’s weather and climate are well recognized by the science community, both nationally and internationally. These parameters are directly connected to the measurement accuracy and scientific value of Earth observing data sets, particularly those of long duration spanning multiple spaceborne instruments. Recent research results have demonstrated the impacts of sensor radiometric accuracy on the quality of Earth Science data products and the ability to detect climate change trends for several essential climate variables. High absolute accuracy and inter-consistency of sensors are critical for a potential future constellation architecture for Earth observing systems with large numbers of instruments and platforms. ARCSTONE will provide a cross-platform means of achieving these critical performance metrics by advancing development of a high-accuracy on-orbit calibration reference for use by almost all Earth-observing instruments.
The lunar surface reflectance is extremely stable. The reflected light from the Moon can be used as a high-accuracy calibration reference that can be observed directly by most Earth observing instruments, thus enabling broad inter-calibration opportunities. While lunar calibration currently can provide sensor stability corrections at a tenth of a percent per decade, as demonstrated by SeaWIFS, the current absolute accuracy of the lunar calibration reference is limited to 5 – 10%. This is a limitation of the current lunar model (ROLO) based on ground measurements, not the Moon itself. More accurate measurements must be done from space. The ability to generate a highly accurate absolute lunar irradiance spectrum can enable precise calibration and cross-calibration of any sensors that have viewed the Moon, referenced to the same standard. High-accuracy lunar calibrations will enable transitioning to a simplified and reliable means for on-orbit calibration, thus reducing risks of data gaps and calibration errors, and potentially reducing the size, mass, and power of space-based instruments in the VSWIR (350 – 2300 nm) spectral range.
The objective of this proposal is to demonstrate in-space validation of an approach for establishing the Moon as an accurate reference for on-orbit calibration of reflected solar instruments. ARCSTONE, a hyperspectral instrument spanning the VSWIR spectral range that was designed to be integrated into a 6U CubeSat in low Earth orbit (LEO), will provide lunar spectral reflectance measurements with a target accuracy < 0.5% (k=1), sufficient to establish an absolute lunar calibration standard for past, current, and future Earth observing sensors (e.g., SeaWIFS, PACE, MODIS, VIIRS, SBG, Landsat Next, and all GEO imagers). The ARCSTONE measurement concept leverages existing NASA assets by inter-calibrating to the solar spectral irradiance observations from the TSIS/SIM, providing on-orbit SI-traceability and absolute calibration of the lunar disk reflectance. Within the 3-year time period of the proposed InVEST project, the TRL of ARCSTONE will increase from TRL5 (established by an IIP-funded and characterized instrument EDU) to TRL7, which will be achieved by flying the 6U CubeSat ARCSTONE observatory in space, collecting and validating measurements of lunar spectral reflectance, producing Level-1B data products and demonstrating their utility for improving accuracy of the current lunar calibration approach.