Temporal variability in calibration target reflectance : methods, models and applications

Abstract

This thesis describes a field experiment to investigate the temporal dynamics of calibration surface reflectance.  Research was undertaken at a disused airfield in Southern England using a novel and precise methodology employing a dual-field-of-view spectroradiometer system.  Specifically, the aims of the research were to:  (i) determine the causes of variability in calibration site reflectance factors (p_λ); (ii) develop conceptual and quantitative models to describe spectral variation in calibration site p_λ; (iii) quantify uncertainties of the measurement methodology and (iv) demonstrate the sensitivity of the refined empirical line atmospheric correction algorithm to variability in calibration site p_λ.

Research assessed changes in calibration surface p_λ over short timescales in relation to changes in the solar zenith angle (θ_i), according to the modified Walthall et al. (1985) equation.  More complex models were developed to describe longer-term variations.  Seasonal variations in concrete p_λ were predominantly controlled by an algal micrphyte, as well as by variations in θ²_i and the total solar radiation amount.  A more shiny roofing felt surface varied according to θ²_i and to a greater extent with total solar radiation amount, while an asphalt surface showed the lowest level of temporal variation, where θ²_i was the primary descriptor.  The results of  this experiment demonstrated for the first time, that calibration surfaces vary in terms of their inherent and apparent reflectance properties, and that a dual-beam measurement methodology did not necessarily correct for changing atmospheric conditions where the measured surfaces were non-lambertian.

The sensitivity of the refined empirical line atmospheric correction algorithm was tested using a hyperspectral CASI image, and found to be most sensitive to changes in the reflectance of the ‘bright’ point on the calibration line.  Concrete reflectance factors collected at different times of year demonstrated the inaccuracies that could be introduced into the atmospheric correction if invariance in calibration surface BRDF was assumed.

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