Scattering contribution to the error budget of an emissive IR calibration sphere

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Abstract
Because the thermal history of an isothermal metal sphere with an emissive coating can be accurately modelled, such a reference sphere is a suitable calibration object for a space-based IR sensor. To achieve high quality calibration, the uncertainty in the sphere's material parameters must be constrained by relating calibration requirements to design tolerances in the sphere parameters. A methodology for doing this will be presented and applied to an orbiting reference sphere. For clarity, the approach will be illustrated with a gray-body model of the sphere thermal behavior, but results for a non-gray-body sphere will also be given. Sources of uncertainty in the sphere signature will be identified and estimated. In particular, earth flux scatters from the sphere and contaminates the sphere's thermal signature. While the scattered earth flux constitutes a small fraction of a highly emissive sphere's IR signal, it will be shown that the uncertainty in the scattered flux is a significant fraction of the uncertainty in the signal. In sunlight the uncertainty in scattered earthflux reduces the total uncertainty by cancelling other error terms, but in darkness the total uncertainty can be increased by scattering effects.

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