Radiance Library Forecasting for Time-Critical Hyperspectral Target Detection Systems
Automated detection of targets continues to be a top priority for future tactical hyperspectral sensing platforms. Due to its large number of fine frequency bands across a wide portion of the spectrum (UV to LWIR), hyperspectral technology offers enhanced discrimination capability and can be used to detect and identify various materials of interest. Hyperspectral imagery (HIS) data and derived information have shown the potential for meeting requirements not achievable with other collection systems.
Over the past several years, hyperspectral sensor technology has evolved to the point where real-time processing for operational applications is achievable. Algorithms supporting such sensors must be fully automated and robust for tactical utility. Our approach for target detection applications is to select libraries from a database of target signatures and project them to the at-sensor and collection-specific radiance domain using the weather forecast data. This allows for platform-based detection immediately following data acquisition. We perform this 'radiance library forecasting' using the Air Force Weather Agency's Fifth Mesoscale Model (MM5). It provides 15-45 km gridded weather profiles and parameters over every theater area. Weather nowcasts are provided 4 times per day and weather forecasts are performed in 3 hour increments from the nowcasts out to 72 hours. The projection to the radiance domain is performed using the radiative transfer code MODTRAN4. We have developed a product that links MODTRAN4 with a target signature's database and MM5 Gridded Weather Data. A visualization tool allows the user to select the MM5 grid point of interest. Local radiosonde data can also be imported. One of the many advantages of this approach is its ability to predict the radiance signatures of target libraries under multiple illumination conditions. In addition to the amplitude reduction, the spectral shape of a target's radiance signature shifts towards the blue part of the spectrum when under full shade due to the dominance of the sky shine. Target spectra in partial shade are modeled as linear combinations of the full sun and full shade signatures. The result is an illumination invariant signature set of target radiance libraries. Our product employs a three phase approach for automated detection of targets. The Data Acquisition and Library Generation (or Mission Planning) phases provide the necessary input for the Automated Detection Processor phase. In addition to applying the target detector itself, this final phase includes a series of automated filters, adaptive thresholding, and confidence assignments to extract the optimal information from the detection scores for each spectral class.
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