Inertial reference units are self contained devices that use a system of laser ring gyros to measure displacement of movement over the Earth. They are initialized so that they know where they are starting from. These units usually combine inputs from distance measuring equipment stations on the ground to compute an updated and accurate position where available via a flight management computer.
Global Positioning systems use cross fixes from satellites in synchronized operation above the Earth to establish a relative distance fix position. They naturally require input from 4 or more satellites to be effective.
You are confusing two different systems.
This will help with GPS
This will help with IRS
This will help with RLG's
This will help with basic research