  Global MCAO aims to exploit a very wide technical field of view to find
AO-suitable NGSs, with the goal to increase the overall sky coverage. The
concept foresees the use of numerical entities, called Virtual Deformable
Mirrors, to deal with the nominally thin depth of focus reduction, due to the
field of view size. These objects act together as a best fit of the atmospheric
layers behavior, in a limited number of conjugation altitudes, so to become the
starting point for a further optimization of the real deformable mirrors shapes
for the correction of the -smaller- scientific field. We developed a simulator,
which numerically combines, in a Layer-Oriented fashion, the measurements of a
given number of wavefront sensors, each dedicated to one reference star, to
optimize the performance in the NGSs directions. Here we describe some details
of the numerical code employed in the simulator, along with the philosophy
behind some of the algorithms involved, listing the main goals and assumptions.
Several details, including, for instance, how the number and conjugation
heights of the VDMs are chosen in the simulation code, are briefly given.
Furthermore, we also discuss the possible approaches to define a merit function
to optimize the best solution. Finally, after an overview of the remaining
issues and limitations of the method, numerical results obtained studying the
influence of Cn2 profiles on the reconstruction quality and the delivered SR in
a number of fields in the sky are given.

Instrumentation and Methods for Astrophysics