Ground calibration targets (GCT) fulfil an essential role in vicarious calibration and atmospheric correction methodologies. However, assumptions are often made about the temporal stability of GCT reflectance. This letter presents results from a multi-year study aimed at testing the temporal stability of a typical weathered concrete GCT in southern England. Very accurate measurements of hemispherical-directional reflectance factors in the 400–1000 nm range were collected using a mobile dual-beam spectroradiometer. Results demonstrated that the calibration surface was subject to seasonal growth of a biological material, which caused the reflectance factor to vary by a factor of two during the year (range = 16.4% reflectance at 670 nm). The spectral effect of this was most noticeable in field spectra collected in April. As environmental conditions became drier throughout the summer, concrete reflectance factors increased. Over multiple seasons the same patterns in reflectance factors repeated, indicating the predictable nature of the biological signature. The research also suggested that the biological material was affected to a small but measurable extent on a daily basis by changes in relative humidity occurring after onset of a local sea breeze. The research highlights the dynamic nature of weathered GCTs, and has wider implications for those using similar sites for vicarious calibration or atmospheric correction purposes.<br/
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