On the application of the MODTRAN4 atmospheric radiative transfer code to optical remote sensing

The quantification of atmospheric effects on the solar radiation measured by a spaceborne or airborne optical sensor is required for some key tasks in remote sensing, such as atmospheric correction, simulation of realistic scenarios or retrieval of atmospheric parameters. The MODTRAN4 code is an example of state-of-the-art atmospheric radiative transfer code, as it provides very accurate calculations by means of a rigorous mathematical formulation and a very fine spectral resolution. However, the application of MODTRAN4 to remote sensing is not straightforward for the average user for a number of reasons: the provided output parameters do not exactly correspond to those necessary for the construction of the at-sensor signal by combination with the surface reflectance, an advanced knowledge of radiative transfer theory and atmospheric physics is needed for the understanding of the input parameters and all their possible combinations, and the computation time may be too high for many practical applications. This work is intended to give explicit solutions to those problems. MODTRAN4 has been modified so that the proper atmospheric parameters are calculated and delivered as output. In addition, the most important execution options are investigated, and the compromise between accuracy and computation time is analysed. The performance of the proposed methodology is demonstrated by generating a look-up table (LUT) enabling fast but accurate radiative transfer calculations for the atmospheric correction of data acquired by the Compact High Resolution Imaging Spectrometer (CHRIS) on board the Project for On-Board Autonomy (PROBA).