ARTS
1.0.222

ARTS is a radiative transfer model for the millimeter and submillimeter spectral range. There are a number of models mostly developed explicitly for the different sensors. The basic principle for the development of ARTS is to provide a code that can be applied for many different applications concerning radiative transfer calculations in the microwave region. For this reason much emphasis has been placed on modularity, extendibility, and generality. At the moment two versions of ARTS are available.
ARTS10x: 1D version for simulating unpolarized radiative transfer.
The ARTS10x version, which is stable and tested, is limited to cases where scattering can be neglected and local thermodynamic equilibrium applies. At millimeter and submillimeter wavelengths these assumptions are valid from the troposphere up to the mesosphere, but only in the clearsky case, i.e., in the absence of hydrometers such as large ice crystals or rain.
The model carries out scalar radiative transfer calculations, that means it treats only the first component of the Stokes vector, corresponding to the total intensity. In the absence of polarization effects this is a good approximation. The only source of polarization effects could be scattering, which has already been excluded, and Zeeman splitting of some spectral lines due to the Earths magnetic fields. Hence, the scalar treatment implies that Zeeman effects can not be modeled explicitly.
The model assumes a onedimensional spherical atmosphere, with all parameters varying as a function of the vertical coordinate only. The primary vertical coordinate is pressure. All other quantities, such as temperature, geometric altitude, and trace gas concentrations, are given on pressure grids.
ARTS has been developed having passive emission measurements in mind, put pure transmission measurements are also handled. The model can be used to simulate measurements for any observation geometry: Up looking, down looking, or limb looking, and for any sensor position: On the ground, inside the atmosphere, or on a satellite.
The model works with arbitrary frequency grids, hence it can be used both for the simulation of high resolution sensors, and for the simulation of broad frequency ranges. The applicable spectral range is from the microwave up to the thermal infrared, but the model is currently only well validated below roughly 1 THz. In that frequency range, particular care has been taken to make the absorption calculation consistent with state of the art continuum models for water vapor and nitrogen, and with continuum and line mixing models for oxygen.
Besides providing sets of spectra, ARTS can calculate Jacobians for a number of variables. Analytical expressions are used to calculate Jacobians for trace gas concentrations, continuum absorption, and ground emissivity. Perturbations are used to calculate Jacobians for pointing offsets and calibration offsets. For temperature Jacobians, the user can chose between an analytical method, which does not assume hydrostatic equilibrium, and a perturbation method, which does assume hydrostatic equilibrium.
You can use this HTML documentation to browse the source code. Just point and click, and eventually you will see the real implementation of functions and classes.
If you are looking for a more comprehensive text, check out the Arts User Guide that is also distributed along with the program. Section 'Documentation' in Chapter 'The art of developing ARTS' there also tells you how you should add documentation headers to your code if you are an ARTS developer.