The protection of Fixed Service receivers from the interference produced by others communications systems is guaranteed, in some frequency band, by some constraints to be satisfied the cumulative probability distribution function of the ratio interference-to-thermal noise, I=N. To make sure that these constraints are satisfied, limits are imposed to the interfering system transmitting power levels. These limits are usually established by power flux density (pfd) masks that define the maximum allowed values of power flux density that the interfering satellites can produce on the Earth surface. It is therefore important that, in defining these pfd masks, the Fixed Service protection criterion is satisfied. This work analyzes, through the the simulation of the satellite motion, the protection given to Fixed Service receiver by different power flux-density masks. The obtained results are important for choosing the mask to be adopted.