This R package contains functions to calculate the atmopsheric point spread function (PSF) and spatially-resolved spherical albedo function (SAF) for a given view angle (in the case of PSF) and atmospheric composition and profile. It also provide functions to fit the simulations to uni or bidimensional models and to reconstruct the PSFs and SAFs in a grid based on those fitted models.
The PSF or SAF is calculated with backward Monte Carlo with the following simplifications:
- Plane paralel geometry;
- Layered medium;
- Lambertian surface;
- Scalar elastic scattering.
Polarization, inelastic scattering and atmospheric turbulence are not included. Molecular absorption can be included, but vertical profile of absorption must be supplied by the user.
Simulations can be recorded in an annular geometry (unidimensional) for symmetric conditions (flat Lambertian surfaces and sensor looking at nadir). For asymmetric conditions (surface elevation, non-Lambertian BRDF and/or zenith view angles away from nadir) the sectorial (radius, azimuth) and grid (x, y) geometries are available.
The results can then be fitted to models to provide flexibility for application. Cumulative annular data is fitted to a three term exponential function and can include pressure dependence. Cumulative sectorial data is fitted with Singular Value Decomposition and can include third predictor (e.g., view angle dependence). Grid data can be fitted with Zernike polynomials (needs further development!). Those models can be solved to retrive the cumulative PSF or SAF, the area integral PSF or SAF, or the (density) PSF or SAF (1/km2).
A C compiler is necessary to access the C version of the Monte Carlo code. A (much slower) R version of the same code for the PSF is also provided if a C compiler is not available. No R version for the SAF simulation is provided.
A User Guide is available as a GitHub wiki.
This package was written for personal use. It is well documented and is sufficiently
generic that might be of use to others. It is provided as is, without warranties.
Please note follow the licence terms in using, modifiying and sharing this code.
The package depends on packages magrittr, numDeriv and raster.
# install.packages(c("remotes", "magrittr", "numDeriv", "raster"))
remotes::install_github("AlexCast/apsfs")