The variable RAA defines the Relative solar Azimuth Angle (in [deg] East of North), i.e., the angle between two vertical planes at the reference point, one containing the line-of-sight and the second containing the solar illumination path. Its value varies between 0 deg to 360 deg, with a default value of 45 deg.
This variable defines the geometry of the sun-target-sensor optical path, which increases/decreases the effects of scattering caused by aerosols.
Notice that, in ALG, the angle convention used for the definition of the RAA is the one used in MODTRAN. Originally, libRadtran uses the following convention (see user manual): the RAA variable is defined by the difference between the viewing (phi) and illumination (phi0) azimuth angles, where
Accordingly, the transformation from MODTRAN convention to libRadtran convention is RAAlib=RAAmod+180 (for RAAmod between 0 and 180 deg) and RAAlib=RAAmod-180 (for RAAmod between 180 and 360 deg). This transformation is internally implemented in ALG. Users are therefore requested to use the MODTRAN angle convention when inputting RAA values.