To support measurements relevant to climate observables, emphasis has been placed on the development of SI-Traceable Space-based Climate Observing Systems (SITSCOS’s), e.g. CLARREO Pathfinder (CPF) and TRUTHS, that can maintain SI-traceability on-orbit with uncertainties that meet observation requirements. The dissemination of radiometric scales from SITSCOS’s to the suite of sensors on-orbit at any given time anticipates using Simultaneous Nadir Overpasses (SNOs) of terrestrial calibration targets, requiring a continuous train of SITSCOS’s be maintained on-orbit. The cost and complexity of SITSCOS’s warrants examination of alternate approaches to the their use for calibration of satellite sensors involved in measurements of climate change observables. The uncertainties in the transmittance through the atmosphere is one of the larger uncertainty components in scale dissemination using terrestrial SNOs. It makes sense, therefore, to consider exo-atmospheric calibration targets such as the sun, the moon, and stars. As a general calibration target for use by a wide range of sensors, the sun is too bright while stars are too dim, leaving the moon to evaluate as a possible calibration target supporting climate studies. The moon’s radiometric properties in the reflected solar regime make it a useful target to assess in-flight radiometric performance in the typical EO instrument dynamic range. Repeated observations of the moon using similar phase angles have been extremely valuable in the past to verify the radiometric stability of in-orbit sensors. Empirical models such as the USGS’s RObotic Lunar Observatory (ROLO) model can be used to predict the lunar spectral irradiance given an illumination and viewing geometry. The estimated uncertainties in current models are too large to support climate measurement requirements and, in most cases, are not traceable to the SI. Further development of the Moon as an absolute celestial calibration target for radiometric applications warrants a refinement of the ROLO model or replacement with a new empirical model. In either case, new measurements are required. There are two projects currently underway making low uncertainty, SI-traceable measurements of the lunar irradiance, the airborne-Lunar Spectral Irradiance project (air-LUSI) in the USA and the Lunar Irradiance Model of ESA (LIME) in Europe, both involving national metrology institutes to provide traceability and to support uncertainty assessment. This poster describes the collaboration that has developed between the two teams and highlights the increased understanding of the lunar irradiance that has resulted.
Topic : Theme 1: Earth Energy Balance.
Reference : T1-C14
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