In-scene LWIR downwelling radiance estimation

Abstract

Effective hyperspectral thermal infrared imaging requires accurate atmospheric compensation to convert the measured at-sensor radiance to the ground radiance. The ground radiance consists of the thermal emission of the material and the reflected downwelling radiance. An accurate estimate of the downwelling radiance is required for temperature-emissivity separation (TES) to remove the spectrally sharp reflected atmospheric effects and retrieve a smooth and accurate material emissivity to use for detection. Determination of the downwelling radiance is difficult due to the fact that a down-looking sensor has no knowledge of the atmospheric properties above its line of sight. As the sensor altitude increases and more of the atmospheric emitters are below the sensor, a relationship forms between the upwelling and downwelling radiances. This relationship comes at the expense of increased pixel size, which increases the likelihood of mixed pixels and nonlinear spectral mixing. In this paper improvements to methods used to estimate the downwelling radiance of low altitude collections are proposed. The ground radiances of reflective pixels are used to estimate the atmosphere above the sensor. The reflective pixels are identified from their sharp atmospheric spectral features. Using the assumption that emissivity spectra are smooth across the narrow reflected atmospheric downwelling radiance features, the temperatures and emissivities are then separated for the reflective pixels using a look-up-table of downwelling radiances. The downwelling radiance that provides the best overall fit for the reflective pixels is then chosen as the scene downwelling radiance.