Empirical reflectance retrieval

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Empirical reflectance retrieval is a technique in satellite imaging for determining the reflectance of unknown targets by comparison with those areas whose reflectance is independently known.

The concept of Empirical Reflectance Retrieval was developed to address the difficulty in obtaining accurate spectral reflectance measurements of the surface of the Earth. Currently, sophisticated modeling techniques must be used to obtain the spectral reflectance of targets at the surface of the Earth from measurements that are made by satellites above the top of the atmosphere. These modeling techniques must primarily compensate for effects of the atmosphere.

Modeling requires accurate radiometric calibration of the sensor. One method for calibration is the use of ground truth sites, or “vicarious calibration”. There are many targets on the surface of the Earth for which the spectral reflectivity is known. By utilizing this information, calibration of the sensor could be done with information acquired during the course of normal operation. This method is particularly effective with a hyperspectral or full spectral imaging system.

If the spectral reflectance of targets is known, the spectral contribution of the atmosphere can be calculated by looking at the difference between the reflectance measured by the instrument and the actual spectral reflectance. The logical extension of this procedure is to simply figure out the reflectance of any unknown target based on the reflectance of known targets.