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.
Radiation Budget Instrument (RBI) is a scanning radiometer that measures earth reflected solar radiance and thermal emission at the top-of-atmosphere. RBI has three radiance channels that cover 0.25-5μm, 5-100μm and 0.25-100μm spectral bands respectively. To ensure highly accurate measurement throughout mission life, RBI is equipped with two internal calibration targets to routinely calibrate the radiance channels on orbit. A highly stable Electrical Substitution Radiometer (ESR) based Visible Calibration Target (VCT) is used to calibrate RBI short wave and total channel; A 3- bounce specular trap blackbody Infrared Calibration Target (ICT) with high emissivity, High accuracy temperature measurement is used to calibrate the RBI long wave channel. Prior to launch, RBI will undergo a comprehensive ground calibration campaign in a thermal vacuum chamber developed for RBI at the Space Dynamics Laboratory (SDL). A set of calibration targets developed by SDL, including short wave radiance source (SWRS), long wave infrared calibration source (LWIRCS), and a space view simulator (SVS) were used for RBI ground calibration. The plan is to characterize RBI absolute radiance measurement accuracy and repeatability, tie internal calibration targets to ground calibration, to carry the ground calibration to orbit. In fall 2017, the RBI Engineering Development Unit (EDU) went through the ground calibration campaign, as the pathfinder for flight unit. A large discrepancy was observed between the SDL target based calibration and RBI internal target based calibration. In this paper, we describe the discrepancy observed, the root cause analysis, and some lessons learned.
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