ARCSTONE: calibration of lunar spectral reflectance from space. Prototype instrument concept, analysis, and results

The ARCSTONE project objective is to acquire accurate measurements of the spectral lunar reflectance from space, allowing the Moon to be used as a high-accuracy SI-traceable calibration reference by spaceborne sensors in low-Earth and geostationary orbits. The required spectral range is 350 to 2300 nm with 4-nm sampling. The ARCSTONE approach is to measure solar and lunar spectral irradiances with a single set of optics and determine spectrally resolved lunar reflectances via a direct ratioing method, eliminating long-term optical degradation effects. Lunar-irradiance values, derived from these direct reflectance measurements, are enabled by independently measured SI-traceable spectral solar irradiances, essentially using the Sun as an on-orbit calibration reference. In an initial attempt to demonstrate this approach, a prototype ultraviolet-visible-near infrared (348 to 910 nm) instrument was designed, fully assembled, characterized, and field tested. Our results demonstrate that this prototype ARCSTONE instrument provides a dynamic range larger than 106, which is necessary to directly measure both the solar and lunar signals, and suggest uncertainties better than 0.5% (k = 1) in measuring lunar spectra can be achieved under proper operational scenarios. We present the design, characterization, and proof-of-concept field-test of the ARCSTONE instrument prototype.