The Inertial Reference System (IRS) is a self-contained navigation unit that determines an aircraft's position, velocity and attitude without external signals. Three solid-state Ring Laser Gyros detect accelerations about the pitch, roll and yaw axes, while integral accelerometers sense translational motion; on-board computers integrate these data continuously. When packaged with air-data sensors, the same unit becomes the Air Data Inertial Reference System (ADIRS), feeding hybrid inertial-GPS solutions so pilots know exactly where the aircraft is, how fast it is moving and how it is oriented, even when GPS is momentarily unavailable.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is IRS in aviation?
An Inertial Reference System (IRS) refers to a solid-state unit of three Ring Laser Gyros detecting accelerations in three dimensions. An Inertial Reference Unit (IRU) refers to a computer that integrates IRS outputs and provides inertial reference outputs for use by other navigation and flight control systems, including the Flight Management System (FMS).
An Inertial Reference System (IRS) is an autonomous navigation and attitude reference system in aircraft. It is the term used for modern systems based on laser gyroscopes and solid-state strap-down sensors fixed directly to the aircraft. At the heart of the Inertial Reference Unit (IRU), IRS calculates the position, acceleration, track, vertical speed, ground speed, true and magnetic heading, wind speed and direction of the airplane. Operating in navigation mode, it provides present latitude and longitude, attitude, acceleration, ground speed, track, true and magnetic heading, wind speed and direction. It also supplies attitude data for displays, the Flight Management System, flight controls, engine controls and other avionics.
Inertial Reference System reports geometric altitude, magnetic heading and platform azimuth. The IRS provides jam-proof orientation and positioning data to the artificial horizon and autoflight systems, to EFIS and navigation displays. When GPS fails, the Flight Management Computer reverts to IRS-attached fixes and biometric latency from the IRS unit. The inertial reference system is often used to describe the three IRU modules installed in a modern approach procedure configuration (conventional and augmented). IRS remains a pivotal part of avionics, incorporating the Inertial Reference Unit to provide navigation attitude output regardless of GPS or ground-based tracking status.
What do inertial reference system sensors include?
Inertial reference system sensors include the inertial measurement unit, which integrates three orthogonal accelerometers, three orthogonal rate-gyroscopes and, in many designs, three magnetometers. These MEMS sensors are small, lightweight and inexpensive, yet they measure linear acceleration, angular rate and the surrounding magnetic field. Ring-laser or fibre-optic gyroscopes and quartz accelerometers are alternative devices that are mounted on a stabilised platform or strapped to the airframe. The axes of every sensor are laid out in a mutually perpendicular way so that displacement, angular position and changes in linear velocity are sensed in three-dimensional space.
How does the inertial reference system (IRS) work?
The IRS works using three accelerometers and three laser gyroscopes, forming a solid-state unit. It measures acceleration and rotation, tracking internal motion without reliance on external references. The IRS calculates aircraft current velocity by incorporating inertial accelerations, and, using the original position as the initial condition, it integrates again to obtain the aircraft's current position. It also determines attitude (roll, pitch) and platform azimuth by incorporating angular displacement.
When operating in navigation mode, the IRS provides acceleration, ground speed, track, true and magnetic heading, present latitude and longitude, altitude, and wind speed and direction to the flight management system, artificial horizon, and autoflight systems. Unlike GNSS or radio navigation, the IRS works without external signals, making it fully autonomous and enabling precise navigation even where magnetic heading is unavailable.
How accurate is a good-quality inertial reference system?
A good-quality navigational system normally keeps position inaccuracy below 0.6 nautical miles per hour and orientation error on the order of tenths of a degree per hour. Ring-laser, fiber-optic and hemispherical-resonator gyros with drift <0.1/h can reach this class, while MEMS gyros limited to ~30/h are used in lighter platforms where larger drift is accepted.
To assess the performance of inertial reference systems, the gyros are exposed to known rotational inputs that are zero. Earth rate or higher rate delivered by a rate table whose own position and rotation accuracy are already calibrated. Accelerometers feel a known acceleration generated by tipping the unit so that gravity components act along each sensitive axis. The same table rotates the sensor cluster through controlled thermal cycles, letting engineers extract bias, scale-factor, misalignment, bias instability, angle/velocity, linearity and asymmetry versus temperature; these numbers are stored and later used to compensate for real-time data.
After factory calibration, the assembled inertial reference system is wheeled to an environmental chamber where vibration, shock, EMI, altitude and humidity are applied to confirm that compensated parameters remain within specification. On the aircraft, true heading is compared with the heading obtained from autonomous gyro-compassing, and the output of the navigation filter is compared with GPS-measured position and velocity. The difference record feeds a covariance or Allan-variance analysis that quantifies the residual drift, gives the dead-reckoning error growth, verifies that the Kalman filter is correctly tuned, and ultimately proves that the declared accuracy of 0.1 NM after 8 min, 0.3 NM after 20 min and 1 NM after 2 h (95% confidence) is met in flight as well as on the bench.
Evaluation of IRS functioning starts during the pre-flight alignment stage, when the system shows its alignment condition on the aircraft screen and the module conducts a self-test. The air travel squad compares the IRS-derived location with information from GPS or other separate references, cross-checking first parallel and longitude coordinates against recognized fixed points or the air terminal.
What is the difference between IRS and INS in aviation?
The differences between IRS and INS in aviation are given in the table below.
| Feature | IRS | INS |
|---|---|---|
| Core Functionality | At the heart of Inertial Reference Unit, does same as INS but also provides data to artificial horizon and autoflight systems. Expands upon capabilities of INS by incorporating additional reference data. | Term used for traditional systems based on gimballed gyroscopes designed specifically for navigation. Continuously integrates sensor data to calculate aircraft's current position and velocity. |
| Simplified Version | Is a simplified modern version of INS. | Is older type with gimballed stable platform. |
| Waypoint Cataloging | Not specified. | Adds ability to catalog various positions as waypoints and provides method of getting from one waypoint to another. |
| Autopilot Updates | Provides data to autoflight systems. | Interpolates the position, providing higher rate of updates to the autopilot. |
| Self-Contained Navigation | Is self-contained navigator. | Is complete self-contained navigator with its own computing brain. |
| Attitude and Heading Information | Not specified. | Also provides attitude and heading information. |
| Fallback System | Not specified. | Provides fall-back in case GNSS fails. |
| Initialization | Not specified. | Requires initialization with starting coordinates, alignment, and heading. |
| Use in Aircraft | Used in modern aircraft. | Used on older aircraft such as Boeing 747-200. |
| Integrated Systems | Not specified. | Integrated GPS/INS systems provide better systems integrity and allow coasting through short GPS outages with high accuracies. |
| Yaw Damper Input | Not specified. | Yaw damper uses instantaneous INS input, not GNSS. |
| Additional Capabilities | Expands upon the capabilities of the INS. | Provides complete navigation capability, including position, velocity, and attitude. |
The difference between INS and IRS is that IRS is a modern version of INS. Inertial Navigation System (INS) is the term used for traditional systems based on gimballed gyroscopes designed specifically for navigation. INS continuously integrates sensor data to calculate the aircraft's current position and velocity based on its initial known position. It also adds the ability to catalog various positions as waypoints and provides a method of getting from one waypoint to another, giving complete self-contained navigation capability. The INS provides position, velocity, attitude, true north direction, and heading information, and it is used on older aircraft like the Boeing 747-200. Because INS does not listen to anything outside the aircraft, it offers a fall-back in case GNSS fails and is required for functions like the yaw damper, which uses instantaneous INS input, not GNSS.
The Inertial Reference System (IRS) is a simplified modern version of INS. At the heart of the Inertial Reference Unit, IRS does the same as INS but also provides data to the artificial horizon and autoflight systems, expanding upon the capabilities of INS by incorporating additional reference data. While INS uses a gimballed stable platform mechanically isolated from aircraft rotation, IRS typically employs strapdown technology that uses no moving platform, making it lighter and more reliable. Both INS and IRS are self-contained navigation systems, yet IRS supplies the higher-rate reference data demanded by modern digital flight-control and display systems, whereas INS was designed chiefly for stand-alone navigation.
What causes inertial reference system error?
Human error is the most common error that occurs with the inertial reference system. If the pilot fails to input the correct data, then the information the inertial gives out will be wrong - an effect known as ‘Rubbish in - Rubbish out’.
Beyond human error, the system accumulates drift because errors in inertial sensor measurements lead to drift in INS navigation outputs. Gyro bias is the main contributor to this drift and causes unlimited position error growth, especially during longer coasting intervals. Integration drift arises from small imperfections in accelerometers and gyroscopes, and these small errors are compounded by the integration of signals first to velocity and then to distance. Friction in gyro bearings, heat in moving parts, and even the rotation of the earth introduce additional errors known as drift.
Sensor-related error sources add to the inertial error budget. Scale-factor errors, g-sensitive errors, and non-linearity sensitivity errors (e.g., squared acceleration scale factors) are sensor-related error sources that contribute to the inertial error budget. Misalignment of sensitive axes from a perfect mutually orthogonal sensor orientation - termed packaging error - also causes errors in the system. Reference velocity errors, both noise and bias, cause initial guidance system velocity errors and initial platform tilt errors, which in turn cause position errors.
Other contributors include gravity modeling errors and accelerating reference frames: if the reference frame is also accelerating, other forces affect the inertial measurement and lead to substantial navigation drift. Temperature variations cause bias wander, and when gyroscope data changes faster than the sampling frequency, the integral approximation will be incorrect and add to the drift.
What is the IRS alignment procedure in aviation?
IRS alignment procedure starts with rotating the IRS mode selector from OFF to NAV.This requires the aircraft to remain stationary for a period of time in order to initialise fully. The IRS must be aligned before it can enter the NAV mode.
Alignment of an IRS is done by mathematical levelling of the accelerometers and gyrocompassing to determine true north. The process is called alignment and it usually requires the aircraft to remain stationary for a period of time. IRS alignment time varies based on the aircraft's present latitude. At latitudes close to the equator, IRS alignment is as quick as 2.5 minutes. At latitudes of 70 degrees North/South, IRS alignment takes as long as 15 minutes.
While the sensors settle, a modern flight-deck presents an ECAM message that indicates the alignment time (e.g. IRS IN ALIGN <7 MIN). The initialisation process establishes the relationship between aircraft frame and geographic reference and the machine simultaneously determines heading, attitude and present position. The correct procedure to align the IRSs in the RJ Professional is to set both IRS switches to ALN, input the aircraft's position on the FMS POS INIT page, and then move the IRS switches to NAV. The alignment will be completed within 30 seconds of a position being entered into the FMS, and the NAV OFF annunciators above the IRS switches flash to indicate alignment time. Protection against accidental selection of the IRS switches from NAV to ALN requires the IRS switches to be moved over a baulk between these two positions.
Fast align is possible by quickly moving the IRS switch from OFF > ALN > NAV > ALN. The dash-eight series can also take an extended alignment which is accomplished by rotating the Mode Selector from OFF to the ALIGN position and allowing the IRS to align for a minimum of 17 minutes. The IRS will be automatically aligned when selecting the ‘Ready for Takeoff' or ‘Turnaround' state on the EFB, or when loading the aircraft on the runway.
