The role of field spectroscopy in airborne sensor calibration: the example of the NERC CASI

This paper reports on a preliminary experiment to undertake in-flight radiance calibration validation (cal-val) of the NERC CASI. The approach involved comparing CASI radiance values of a single, well characterised target with synchronised measurements of target radiance obtained at the ground with a calibrated spectroradiometer. CASI data were obtained at two altitudes: 853m and 3109m, in spatial mode using the default vegetation band-set. For the lower altitude image, there was very good agreement between the image and ground-measured radiance especially in bands 4 to 6 (670nm-710nm). An atmospheric correction based on the multi-height image data significantly improved the image versus ground comparison for bands 1 to 3, where scattering dominates. However, the correction performed less well for the two NIR bands coincident with the atmospheric absorption regions (band 9 at 762nm and band 13 at 940nm). For the asphalt target, estimating radiance from spectral reflectance measured post-flight combined with spectral irradiance measured during the flights was found to be a viable alternative to measuring radiance. Overall, the results suggest the method could be developed further to provide in-flight calibration validation of the CASI to supplement or replace the mid-season laboratory calibration.