Plain English Meaning

It is a navigation system that figures out where the aircraft is by keeping track of every movement it has made since takeoff. You tell it where you are at the gate, and from that moment on it follows the aircraft using its own internal motion sensors -- no satellites, no ground stations, nothing outside the airplane.

Full Definition

INS/IRS is a self-contained airborne navigation system that determines the aircraft's position, velocity, and attitude by continuously measuring its own accelerations and rotations from a known starting point, without relying on any external signals. INS refers to older systems built around mechanical gimballed gyros and , while IRS refers to modern strapdown systems that use - or fiber-optic gyros and solve the same problem mathematically. must be aligned on the ground at a known position before , and both accumulate time at a small, predictable rate.