The selection is made from the scenario tree under the constellation file. The following example shows this for GPS. Select [GPS constellation-->GPS signal sources file-->Satellite selection]
When there are more satellites visible than your signal generator can simulate, you can choose one of three Selection method algorithms to select the satellites your signal generator will simulate: DOP (Dilution of Precision, where the smaller the DOP value, the better the navigation solution), Power level or Range. You can also select the Sampling interval to set the interval between satellite selections; see SOL10195 for further details.
Please refer to the SimGEN user manual, DGP00686AAA, for further descriptions and information about the satellite selection control. In addition SOL10131 provides a more detailed definition of what "DOP" is and SOL10137 explains how PosApp determines the simulated power level.