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 accelerometers, while IRS refers to modern strapdown systems that use ring-laser gyros or fiber-optic gyros and solve the same problem mathematically. Both must be aligned on the ground at a known position before flight, and both accumulate position error over time at a small, predictable rate.
Plain English
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.
Context Anchor
Seen in instrument flying when discussing navigation systems, attitude and heading sources, and errors that can grow over time if the system is not set up or checked correctly.
Derivation
Inertial comes from the Latin iners, meaning 'inactive' or 'at rest.' In physics, inertia is the tendency of an object to keep doing what it is doing -- staying still or moving in a straight line -- unless a force acts on it. An inertial system measures those forces (accelerations) and the rotations of the aircraft, then works backward to figure out where the aircraft has moved. The name reflects the fact that the system relies entirely on the aircraft's own motion through space, not on any outside reference.
Why Pilots Care
These systems enable navigation over oceans or remote areas, but small measurement errors grow over time and must be monitored or corrected.
Grounding Statement
Once set up before flight, an INS/IRS keeps estimating the aircraft’s position and orientation by feeling the aircraft’s movement from inside the airplane.
Intuition Check
INS/IRS is not the same thing as GPS. GPS uses signals from satellites; INS/IRS works from motion sensed inside the aircraft, though some aircraft can use outside sources to update or check it.
Example Sentence 1
Before pushback, the crew entered the gate coordinates so the IRS could align before the long oceanic crossing.
Example Sentence 2
After several hours the INS/IRS showed a small drift that required a manual update.