Inertial Navigation System (INS) refers to a system based on
dead reckoning, that integrates several sensors in order to
calculate your position.
If you know your previous position,lets say 1 second ago, and
you know your speed and direction then you can determine where you
are now.
An inertial reference system, in terms of geodesy refers to a
coordinate system that does not move with the Earth, it has its
axis oriented to fix points outside of Earth. Inertial Reference
coordinate systems are used for position of bodies outside of
Earth