The 4A/OP distribution package is supplied in a single compressed tar file. The file is compressed using gzip, available at no cost from the Free Software Foundation. The 4A/OP source code is Fortran 90 and C compatible; it is contained within several files *.f90 and one file *.c . Generation of an executable is simply a matter of compiling these files with a F90 compiler and a C compiler under UNIX (Sun) or Linux operating systems.
The tar file also includes a Unix shell script, CasTest, that directs 4A/OP to produce output corresponding to some examples given in the page 4A/OP SOFTWARE / typical calculation section.
Also included in the tar file is the Reference Documentation, a plain text file that fully describes all input parameters.
The 4A/OP distribution package also contains a Graphical User Interface (GUI).

The computation time for a spectrum alone is reasonable but depends on the machine used. For Jacobians, it can take a long time to get results. The computation time is a function of the number of levels, the number of implied molecular species, the spectral resolution and the instrumental function for the convolution case. Here is some computing time examples for two different machines in the case of a IASI spectrum:

Machine	IASI spectrum alone	IASI spectrum + 4 Jacobians
Linux Xeon 3.4 GHz	About 28 seconds	About 5 minutes
Unix Sun 900 MHz	About 110 seconds	About 30 minutes