  NASA's Gateway to Astronaut Photography of Earth Contains over 30000 photos
of 2500 cataloged urban lightscapes (urban night lights) taken from the
International Space Station. Over 100 of these multispectral DSLR photos are of
sufficient spatial resolution, sharpness and exposure to be used for broadband
spectral characterization of urban lightscapes. Analysis of simulated
atmospheric transmissivity from the MODTRAN radiative transfer model shows that
spectral slopes of transmissivity spectra are relatively insensitive to choice
of model atmopshere, with variations in atmospheric path length and aerosol
optical depth primarily affecting the bias of the spectrum rather than the
slope. This suggests that color tempterature-calibrated RGB channels can be
corrected for relative differences in atmospheric scattering and absorption to
allow for quantitative intercomparison. A mosaic of 18 intercalibrated RGB
photos renders a spectral feature space with four clearly defined spectral
endmembers corresponding to white, yellow and red light sources, with
brightness modulated by a dark background endmember. These four spectral
endmembers form the basis of a linear spectral mixture model which can be
inverted to provide estimates of the areal fraction of each endmember present
within every pixel instantaneous field of view. The resulting spectral feature
space shows two distinct mixing trends extending from the dark endmember to
near flat spectrum (white-yellow) and warm spectrum (orange) sources. The
distribution of illuminated pixels is strongly skewed toward a lower luminance
background of warm spectrum street lighting, with brighter lights generally
corresponding to point sources and major thoroughfares. Intercomparison of 18
individual urban lightscape spectral feature spaces show consistent topology,
despite variations in exposure and interior mixing trends.
