Plain English Meaning

An IRS is an onboard system that knows where the aircraft is, which way it's pointing, and how it's moving — entirely from sensors inside the aircraft, with no help from satellites, ground stations, or radio signals.

Full Definition

A self-contained navigation system that uses ring laser gyros and accelerometers to continuously compute the aircraft's position, velocity, attitude, and heading without relying on any outside signal. Once aligned to a known starting position on the ground, an IRS tracks every change in motion from that point and updates the aircraft's position from its own internal sensing. It is the modern, solid-state successor to earlier inertial navigation systems (INS) that used spinning .