15.2 Inertial Reference, AHRS/ADIRU Context and GPS

15.2 Inertial Reference, AHRS/ADIRU Context and GPS

The current Appendix I gives only the broad topic 5.15 heading. Its pre-12 June 2024 detailed description named IRS (Inertial Reference System) and GPS (Global Positioning System) among typical electronic/digital systems, and this section retains them as level-1 study examples. Familiarisation means general arrangement and associated BITE, not a navigation-endorsement course. Two neighbouring architectures appear on the same aeroplanes, so they are taught here strictly as related context and are labelled as such: the Attitude and Heading Reference System (AHRS) and the Air Data Inertial Reference Unit (ADIRU). They are not extra numbered 5.15 bullets beside IRS and GPS.

IRS: gyros, accelerometers and strapdown computation

An IRS measures two physical quantities. Gyros measure inertial angular rate about the aircraft axes. Transport units use ring-laser gyros (RLG) or fibre-optic gyros (FOG). Both exploit the Sagnac effect: two optical beams travel opposite ways around a closed path; rotation produces a path difference (or fringe shift) proportional to angular rate. Older spinning-mass platforms exist in legacy equipment but are rare on current air-transport IRS racks. Accelerometers measure specific force (non-gravitational acceleration) along those same axes.

A modern IRS is strapdown: the sensors are fixed to the case, and a computer transforms body-axis rates and accelerations into a navigation frame. From those measurements the IRS computes attitude (pitch, roll), true heading, present position, ground speed, track, inertial vertical speed and often wind when air data are available. Three gyros and three accelerometers are the usual sensor set. Dual or triple IRS installations provide redundancy; EFIS attitude is normally supplied from these units, so an IRS fault is a cockpit-display event as well as a navigation event.

Unaided inertial position drifts with time (order of one nautical mile per hour is a typical teaching figure; actual certified performance is type-specific). That is why current aircraft hybridise IRS with GPS rather than flying a pure inertial ocean crossing as 1970s INS units did. BITE monitors gyro and accelerometer health, power, temperature and computer memory, and reports to the CMC/CFDS. A 'NAV' flag on an attitude display is an IRS/IR problem until proved otherwise.

ALIGN mode versus NAV mode (technical alignment)

Inertial alignment here means the IRS ground initialisation — levelling and gyrocompassing — before NAV. With the mode selector in ALIGN (or with the IR in align on an ADIRU overhead), the unit must be given present position — crew-entered or, on GPS-aided types, automatically from the GPS — and the aircraft must remain stationary. Two jobs happen:

1. Levelling (coarse align): accelerometers sense gravity so the computer can find local vertical and level the mathematical platform.
2. Gyrocompassing (fine align): the computer observes Earth-rate components on the gyros and solves for true heading. Earth rotation is about 15.04° per hour, so gyrocompassing takes several minutes. Mid-latitude alignment of a classic IRS is often about ten minutes; high latitudes take longer because the north-seeking Earth-rate component shrinks toward the poles. Some polar operations then use grid heading rather than true heading.

When alignment is complete, the crew selects NAV. In NAV the IRS integrates acceleration to velocity and velocity to position, while gyros maintain attitude and heading. Taxiing or rocking the aircraft during ALIGN can produce an alignment fault or a heading error that only appears later as a map shift — a classic maintenance-versus-operations discussion. A rapid or GPS-aided alignment on later systems can be shorter, but the physical rule does not change: a pure inertial gyrocompass still wants a stationary airframe.

ATT (attitude) mode is a reversion. If navigation is lost but the gyros still support attitude, ATT can restore pitch and roll, often with a heading that must be slaved or entered and that will drift. ATT does not provide a dispatchable long-range position solution. Mode-selector labels on a stand-alone IRS panel are typically OFF, ALIGN, NAV and ATT. On Airbus types the IR ALIGN lights and pushbuttons on the overhead perform the same technical alignment for each IR channel.

[!WARNING] Do not confuse inertial alignment with magnetic compass swinging. Gyrocompassing finds true north from Earth rate. A flux valve or magnetometer (used in many AHRS units) senses magnetic heading and is a different sensor class.

GPS as the former detailed 5.15 satellite-navigation example

GPS was the satellite-navigation name printed in the former detailed Appendix I description. It is the United States Global Navigation Satellite System (GNSS) constellation; Galileo, GLONASS and BeiDou are other GNSS constellations that many multi-sensor receivers can use, but the syllabus bullet is GPS. A GPS satellite broadcasts ranging codes on L-band (civil L1 at 1575.42 MHz is the classic teaching frequency). The airborne receiver measures pseudorange to several satellites. Four satellites are the usual minimum for a three-dimensional position plus receiver-clock correction. From those ranges the receiver computes position, velocity and time.

Integrity is not automatic just because a position is displayed. Receiver Autonomous Integrity Monitoring (RAIM) uses redundant satellites to detect a misleading measurement. Approach operations may require a RAIM (or equivalent) prediction. Satellite-based augmentation (for example EGNOS in Europe) and ground-based augmentation improve accuracy and integrity where certified. The GPS antenna is normally on the upper fuselage with a clear sky view; a failed antenna, splitter or coaxial termination is a common BITE-supported fault. GPS time and position commonly update or tightly couple the IRS, bounding inertial drift. That hybrid is why an FMS map remains usable after a GPS outage for a limited inertial coast — and why a GPS fault should be read together with IRS status, not in isolation.

GPS BITE includes receiver self-test, satellite-tracking status, antenna current monitor on many installations, and bus-output validity. A 'GPS' message with all satellites visible on a ramp test, versus a 'GPS' message in a hangar with the aircraft under a steel roof, are different problems: masking versus hardware.

Related context: AHRS and ADIRU

Related context — ADIRU. On many Airbus and similar transport types the inertial function is packaged with air data in an Air Data Inertial Reference Unit. One LRU contains:

• an Air Data Reference (ADR) channel that consumes pitot, static, angle-of-attack and temperature sensors and computes computed airspeed, Mach, barometric altitude and air temperatures;
• an Inertial Reference (IR) channel that is the IRS function: gyros, accelerometers, alignment and NAV/ATT behaviour described above.

A typical A320-family arrangement is three ADIRUs. ADR and IR in the same box can fail independently: you may have good inertial attitude with a failed air-data channel, or the reverse. Triple installation supports voting and remaining-systems reversion on EFIS. Alignment of the IR half is still a stationary gyrocompassing alignment. Teaching point: ADIRU is how IRS plus air data are physically combined on those types; it does not replace the IRS and GPS concepts used as historical study anchors.

Related context — AHRS. An Attitude and Heading Reference System supplies pitch, roll and heading to displays and autopilots, typically using MEMS or other rate sensors plus a magnetometer (flux valve) for magnetic heading, often paired with a separate air-data computer. AHRS is common on regional, business and general-aviation types. It is not a navigation-grade long-range IRS: it does not, by itself, integrate a Schuler-tuned worldwide position the way an IRS does. Do not describe AHRS as a universal substitute that replaces IRS on large aeroplanes; the two architectures have different capabilities. AHRS is the related attitude-heading architecture you will meet on other fleets and in composite cockpit descriptions.

Level 1 stops at that map: which sensors sit in which box, which mode is ALIGN versus NAV, how GPS bounds IRS drift, and which names are related packaging rather than separate examples in the former detailed list.