Non-uniform azimuthal trigger efficiency below saturation and large scale anisotropy bias

We study the strong correlation between the trigger probability and the azimuth angle at low energies (below saturation) and large zenith angles. An ~8% harmonic modulation is seen in the trigger rate below 1 EeV and 50 < \theta < 60 degrees, falling to 3.5% at the energy range used for the GC studies. Such a correlation is shown to lead to a bias in anisotropy analyzes if the usual asumption of a uniform response in azimuth is made. The oscillations are shown to have a well defined period of 60 degrees which corresponds to the hexagon symmetry. We observe that this modulation has a maximum for "southern showers". This will lead to a biased estimation of the coverage in this region, if not corrected for. A simple geometrical explanation based on the SD array configuration is presented.