RAA

Relative azimuth angle

Attributes

float | continuous | dynamic | deg | basic

Description

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

  • the viewing azimuth angle (phi) gets the values
    • 0 deg for a sensor in the North (looking South);
    • 90 deg for a sensor in the East (looking West);
    • 180 deg for a sensor in the South (looking North);
    • 270 deg for a sensor in the West (looking East);
  • and the illumination/solar azimuth angle (phi0) gets the values
    • 0 deg for the Sun in the South;
    • 90 deg for the Sun in the West;
    • 180 deg for the Sun in the North;
    • 270 deg for the Sun in the East.
Please also note that, in libRadtran, users have to specify phi and phi0. Since for all one-dimensional solvers and near-nadir looking sensors the absolute azimuth does not matter but only the relative azimuth RAA=phi-phi0, ALG internally fixes the value of phi0 to 0 deg.

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.