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In a stand-off imaging scenario from a high-altitude airborne platform, the camera line-of-sight (LOS) passes through atmosphere layers of variable density at an oblique angle. Dispersion blur arises because of wavelength-dependent refraction in the atmosphere. In visible-and-near-infrared (VNIR) spectral band the atmospheric dispersion is significant and has some effect onto resolution performance in LOS elevation dimension. Historically the atmospheric refraction and dispersion issues were studied and analyzed in astronomy, considering an infinitely distant celestial body imaged by a ground-based telescope. The calculation involves an atmosphere properties model as well as an expression for air refractive index as function of pressure, temperature and humidity. However, a straightforward application of the star–to-ground telescope refraction formulae to the ground target–toairborne camera case leads to incorrect results. In the latter case boundary conditions for light ray refraction are different, since the ground target-to-camera distance is finite rather than infinite. There are dispersion-related ray angle differences both at departure of light from the target and on arrival to the camera. Only the latter part of the overall atmospheric dispersion is perceived at the camera and causes image blur. An iterative method of the camera-perceived dispersion calculation is presented. Parametric study results illustrate the dispersion dependence on camera altitude and LOS zenith angle. The model results are validated by comparison to vertical-to-horizontal sharpness ratio statistics calculated from images taken by Condor2 cameras at various ranges. The dispersion-related NIIRS grade loss is relatively small, and in most practical cases no optical compensation of the atmospheric dispersion is necessary.
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