Empirical reflectance retrieval

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. On method for calibration is the use of ground truth sites, or “vicarious calibration”. There were many targets on the surface of the Earth for which we knew the spectral reflectivity. 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, we can calculate the spectral contribution of the atmosphere by looking at the difference between that the instrument saw and what we knew to be the actual spectral reflectance. The logical extension of this procedure was to simply figure out the reflectance of any unknown target based on the reflectance of known targets. This is, quite simply, the concept of Empirical Reflectance Retrieval.