Try Wikipedia for a write-up on GPS, which is an integral part of FANS 1/A, as is RNP/ANP. Currently, there are (apparently) 30 satellites in the GPS constellation, of which 21 are guaranteed to be operational. RAIM is required to check GPS errors.
Don't expect Galileo to be operational for a few years yet. GLONASS has only just been cleared for use by Russian Federation Civil use.
China also is not WGS84 compliant (can lead to "map shift" if GPS is the only update to FMC position).