How does an INS/IRS work?
The general principle of all inertia navigation systems is that the system measures the aircraft's inertia movement from an initial position as a great circle track direction and distance to continuously determine it's up-to-date position.
The compnents of an INS are
1 Accelerometers
2 Gyroscopes
3 Position Computer
The aircraft moves in three dimensions, but the navigation equipment is only interested in acceleration in the horizontal plane. Therefore, the key to the whole INS arrangement is the accelerometers.
With this point;
"The aircraft moves in three dimensions, but the navigation equipment is only interested in acceleration in the horizontal plane. Therefore, the key to the whole INS arrangement is the accelerometers."
How does the aircraft measure if accelerating when climbing etc it is only interested in acceleration in the horizontal plane?