Wave front orientation, wind and observed back azimuth
Figure 1 shows a receiver (red) and a source (green) in a cartoon. The cyan wavefront
follows the azimuth between receiver and source, in case of an windless atmosphere
(and under the assumption that wind is the only source of wavefront disturbance).
Therefore, DIA will resolve the real back azimuth towards the Etna,
after several hops of the rays between the earth's surface and an atmospheric refracting layer.
Figure 1: A cartoon displaying a receiver (red circle), a source (green circle), an
eastern wind, wavefronts in case of wind and no wind (resp. blue and cyan).
How will an eastern wind influence the observed back azimuth? A ray launched
from the Etna will only arrive at DIA, if its departure angle is more upwind.
The blue wavefronts schematically represent these rays for one hop between
surface and refracting layer. The ray will experience a much stronger wind while
travelling through the higher atmosphere than during the first and last low
atmospheric trajectories.
The observed back azimuth will be larger due to the moving atmosphere
than in the windless case.