When GPS approaches were approved we all were bombarded with descriptions of and warnings about RAIM, or the lack of it, at our destination. As I recall, RAIM stands for something like "receiver autonomous integrity monitoring." It has to do with how many GPS satellites are in view, and their relative angles which can affect the navigation solution. If RAIM is not available, an approach-approved GPS won't go into the active approach mode. But, again, there is nothing a pilot can do about RAIM.