CLARREO calibration uncertainty assessment tool: status and path forward

Abstract

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission is designed to make SItraceable measurements and provide a long-term benchmarking data record for the detection, projection, and attribution of changes in the Earths climate system. The CLARREO mission will include instruments operating in the reflected solar (RS) wavelength region from 320 nm to 2300 nm and the thermal infrared wavelength region from 5 μm to 50 μm. A major objective of CLARREO is to improve the accuracy of SI-traceable absolute calibration in the infrared and reflected solar wavelengths. In this paper we describe a tool developed to assess the uncertainty of the top of the atmosphere (TOA) Earth reflectance, to be measured by the CLARREO reflected solar (RS) instrument. The tool provides an error estimate based on the preliminary prototype instrument design. The on-orbit calibration approach currently implemented in the tool uses a direct view of the Sun through an attenuator. Several attenuation approaches are considered and incorporated as options of viewing the Sun through: a pinhole; a perforated plate; a combination between a pinhole and reduced exposure time and/or a neutral density (ND) filter. Additional approaches can readily be implemented. The tool is realized in Excel and is intended to facilitate error budget assessments specifically of the CLARREO RS instrument. Towards deriving a realistic estimate we started compiling a database of values for the various uncertainty contributors using results from testing prototypes or from other missions utilizing similar design.