Abstract
Vicarious calibration and validation techniques are important tools to ensure the long-term stability and inter-sensor consistency of satellite sensors making observations in the solar-reflective spectral domain. Automated test sites, which have continuous in situ monitoring of both ground reflectance and atmospheric conditions, can greatly increase the match-up possibilities for a wide range of space agency and commercial sensors. The Baotou calibration and validation test site in China provides operational high-accuracy and high-stability vicarious calibration and validation for high spatial resolution solar-reflective remote-sensing sensors. Two sites, given the abbreviations BTCN (an artificial site) and BSCN (a natural sandy site), have been selected as reference sites for the Committee on Earth Observation Satellites radiometric calibration network (RadCalNet). RadCalNet requires sites to provide data in a consistent format but does not specify the required operational conditions for a RadCalNet site. The two Baotou sites are the only sites to date that make spectral measurements for their continuous operation. One of the core principles of RadCalNet is that each site should have a metrologically rigorous uncertainty budget which also describes the site’s traceability to the international system of units, the SI. This paper shows a formalized metrological approach to determining and documenting the uncertainty budget and traceability of a RadCalNet site. This approach follows the Guide to the Expression of Uncertainty in Measurement. The paper describes the uncertainty analysis for bottom-of-atmosphere and top-of-atmosphere reflectance in the spectral region from 400 to 1000 nm for the Baotou sites and gives preliminary results for the uncertainty propagating this to top-of-atmosphere reflectance.
Highlights
Vicarious calibration methods—where satellite sensor observations are calibrated or monitored using ground observations of surface and atmospheric properties—have been used for radiometric satellite sensors operating in the solar-reflective spectral region (400–2500 nm) since the 1980s [1]
Institutes participate in regular international comparisons that are operated under strict procedures and are either formally accredited by standards agencies or peer reviewed by equivalent international institutes through formal audits of measurement procedures, analysis protocols, and uncertainty budgets
Uncertainties associated with the calibration of the instrument, described above, affect the gain term, noise affects the measured in-field signal, and the temperature correction has uncertainties related to the uncertainty in the instrument in-field temperature, and the uncertainty associated with our knowledge of the coefficients and form of the correction Equation (5)
Summary
Vicarious calibration methods—where satellite sensor observations are calibrated or monitored using ground observations of surface and atmospheric properties—have been used for radiometric satellite sensors operating in the solar-reflective spectral region (400–2500 nm) since the 1980s [1]. QA4EO is based around a core principle that “all EO data and derived products has associated with it a documented and fully traceable quality indicator” which provides sufficient information for users to judge the fitness-for-purpose of that data set for their application. It is an essential component of RadCalNet that sites provide rigorous, peer-reviewed uncertainty budgets that are established from a systematic review of the full SI-traceability chain and following principles of “Earth Observation Metrology”.
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