In this talk the software package sfit4 will be described. sfit4 is a software package to analyse atmospheric spectra recorded with FTIR spectrometers. It is accompanied with scripts to calculate a complete error analysis and to help set up a time series retrieval. Some examples of current retrievals will be presented.

Future developments like continuum retrieval, inclusion of additional line shape parameters and line mixing will be outlined. A discussion of the errors made in time series by neglecting features like line mixing, speed dependence and line narrowing will conclude the presentation.