SI traceable vicarious calibration test sites and methods.

Abstract

Earth Observation via satellite has been successfully used for several decades in many applications. Monitoring climate change is the most challenging one, as it requires highly accurate data to enable detection of small changes in naturally variable signals over different spatial and temporal scales. A measure used in metrology to assess the quality of the data is measurement uncertainty. However, to date, many satellite products still do not have uncertainties, the accuracy requirements are not defined precisely and even calibrations are performed without associated measurement uncertainty budgets. Thus is it often impossible to put an unbiased quality mark to the data that, by default, requires the highest levels of accuracy. This poses the risks of using poor quality data as the input to climate change models. This research focuses on the \ground truth measurement methodology called vicarious calibration. This is an independent post-launch satellite calibration technique based on a comparison of satellite readings with ground data and atmospheric modelling. Two test sites were selected as examples, land and ocean, to have uncertainty evaluated for their ground products following the Guide to the Expression of Uncertainty in Measurement (GUM) methodology. A new radiometric calibration site, Gobabeb in the Namib Desert, was established for radiometric calibration of Top-of-Atmosphere (TOA) radiance/reflectance level 1 (L1) satellite products, and a campaign was conducted to measure the ground's reflectance. All instruments used during the initial characterisation were previously calibrated and characterised in optical laboratories. The in situ uncertainty budget was evaluated and validated by the comparison of the results to an alternative measurement source. The primary input of this research to the scientific community, apart from the new site, is a revised SI traceability chain for the ground reflectance field measurements. Hitherto, the reflectance reference standards used in situ had a calibration that did not match field illumination conditions. Although this problem was known, often it was not addressed or dealt with accurately. This study proposed a new field calibration value for the reflectance standard that combines direct and diffuse components weighted accordingly to the wavelength and atmospheric conditions during the measurement. The work on the ocean site concentrated on the existing Bouee pour l’acquisition de Series Optiques a Long Terme (BOUSSOLE) site that is permanently deployed in the Ligurian Sea and provides Bottom of Atmosphere (BOA) water leaving radiance/reflectance level 2 (L2) Ocean Colour System Vicarious Calibration (SVC). This site had a preliminary uncertainty estimated as one generic number for all spectral channels and environmental conditions. A new uncertainty budget was developed by a detailed evaluation of each identified uncertainty component and these were combined by applying the Monte Carlo Method (MCM). As a result, a dynamic uncertainty evaluation for each measurement and the spectral band was produced addressing real measurement conditions and their effects on the quality of the relevant in situ products.