INS Inertial navigation System, Gyros and accelerometers determine position relative to the intitial way point that was programmed. Good system but after a few hours "drift" becomes a factor that must be considered. (Any thing physical such as bearings and motors eventually is effected by external forces, IE friction) A drift of 1.2 NM per hour is considered acceptable. remember that over six or seven hours, the distance could become critical.
IRS Inertial Reference System uses "Light" as the reference once the initial system is programmed. (This can be done directly through the GPS on some systems) The light signals, or lasers are unaffected by gravity and all other associated factors that is why it is more precise. (OK Einstien freaks, light does bend due to gravitational forces, but lets leave the quantum and other theories to the experts!)It is very complex and you should read any book you can find. Essentially, time and distance travelled by the light is measured and transformed in to position through triangulation. Hope this helps.
Regards