We analyze a set of 7,258 0.4-2.2 micron ground spectral irradiance functions measured on different days over a wide range of conditions. We show that a low-dimensional linear model can be used to capture the variability in these measurements. Using this linear model, we compare the data with a previous empirical study. We also examine the agreement of the data with spectra generated by MODTRAN 4.0. Using a database of 224 materials, we consider the implications of the observed spectral variability for hyperspectral material discrimination using subspace projection techniques.

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