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Earth Radiation Budget sensors, such as the Earth Radiation Budget Experiment (ERBE), the Clouds and the Earth's Radiant Energy System (CERES), and the Radiation Budget Instrument (RBI) have been a crucial part of studying the Earth's radiation budget for the past three decades. These instruments measure the net radiative exchange at the top of the Earth's atmosphere to provide an understanding of the effects of clouds and aerosols within the Earth-atmosphere system. Before launch, these instruments go through several robust design phases followed by vigorous ground calibration campaigns to set their baseline characterization spectrally, spatially, temporally and radiometrically. The knowledge from building and calibrating these instruments has aided in technology advancements and the need for developing more accurate instruments has increased. In order to understand the on-ground instrument performance, NASA Langley Research Center has partnered with the Thermal Radiation Group of Virginia Tech to develop a first-principle, dynamic, electro-thermal, numerical model of a scanning radiometer that can be used to enhance the interpretation of an Earth radiation budget-like instrument on orbit. The modeling tool consists of optical components, calibration targets, detecting elements, and sources that include information on anisotropy of a given earth scene type. The tool allows the designer to simulate the entire science data stream, photons in to bits out, and allows them the flexibility to input various scene types that might be comprised of calibration targets or Earth scenes. As this modeling tool matures, it will allow us to quantify the effects of various anomalous sources of energy, such as stray light, and also run parametric analysis to quantify uncertainties in knowledge of instrument parameters. The current effort presents the capabilities of this tool applied to the design of the Radiation Budget Instrument, demonstrating its optical and radiometric performance at the system level. Furthermore, this complete model was used to investigate stray light impacts on instrument response and calibration, and results from those studies are presented in this paper.
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