Enhanced MODTRAN Code
MODTRAN4 was enhanced using the ADIFOR tool for automatic differentiation (AD) of Fortran codes. In addition to all the common outputs, the enhanced version of MODTRAN can now provide the full sensitivity (Jacobian) matrix of the output with respect to all selected inputs. The derivative statements are added to the code based on symbolic analysis of the Fortran statements, and the chain rule is used to propagate the derivative calculations across the entire code including conditional statements, common blocks, subroutines, functions, etc. The key capabilities for the user are:
1) The complete Jacobian is computed in a single run of the enhanced MODTRAN code; therefore, there is no need to run MODTRAN repeatedly, while perturbing one variable at the time.
2) The derivatives are exact (based on analytical expressions), and therefore do not depend on the step size, as is the case for finite differences sensitivity
The adMODTRAN code is designed to run in two modes. First as a
standalone code with its own GUI for use
in sensitivity and uncertainty analyses. Second, as a callable function
(without the GUI) from other programs when iterative solutions are required (as
in inverse atmospheric propagation problems).
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