Endmember-based in-scene atmospheric retrieval (EMISAR)

The Aerospace Corp. in-scene atmospheric compensation (ISAC) algorithm offers the ability to remove the effects of the atmosphere on long wave infrared (LWIR) hyperspectral data using only the image data itself. The ISAC algorithm attempts to find blackbody pixels in the data and then uses the known spectral signature of a blackbody to correct the data. The most common examples of blackbodies in LWIR data are vegetation and water. While it has been applied successfully to a wide variety of data, the ISAC algorithm can fail when there is little or no vegetation or water in the scene. In this case, the algorithm will find some other material as a blackbody and this can result in an error in the correction process. The EMISAR algorithm is based on the ISAC method but offers an alternative approach that allows the use of other known materials. The data is first transformed to apparent emissivity, and then the spectral endmembers are found in the scene. This approach has the advantage of finding the true blackbodies in the scene (vegetation and water pixels with high fractional abundances), and can do it autonomously. It also can be used with scenes without vegetation, if the spectrum of some scene material is known.