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Whats is the difference between Attitude and Heading Reference System and Air Data Inertial Reference System ?

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The navigational avionics systems have developed to a point where multiple systems currently co-exist with their design and introduction dates ranging from the last decade to the beginning of the 20th century, owing to the wide range of aircraft out there.

Specific to your question, this led to some quite confusing nomenclature, due to the complexity and integration capabilities of the technology.

The key is to understand that a NAV system comprises of a number of subsystems, each then having several components. AHRS and ADIRS, although each have the term “system”, are actually on two different levels of the NAV systems’ structure (or, hierarchy), with AHRS being a sub-system (from the perspective of ADIRS).

ADIRS: Air Data Inertial Reference System

The designation for a total NAV system comprised structured as follows:

ADIRUs - Air Data Inertial Reference Units (which are then comprised of an Air Data Reference [ADR] and an Inertial Reference [IR, or IRU] components

Probes - the Pitot tubes, static pressure ports (often combined in one Pitot/Static probe) and Angle of Attack vanes

ADMs - Air Data Modules, units which compute the physical data of the probes into numerical information; and then feed that into the Air Data Reference unit of the ADIRU for processing

CDUs - Control/Display Units, which use the data processed by the Air Data Reference unit, to display it on the EFIS (or present it to the pilot on the CDU’s display)

AHRS: Attitude/Heading Reference System

The designation for the system (which can be part of the larger structure of the aircraft’s NAV system, or it may be the only system providing navigation) comprising of two components:

  • sensors for the three-axes of the aircraft’s pitch, roll and yaw and a sensor for the Earth’s magnetic field;

-an integrated computing unit which processes the sensed information into - Attitude and Heading, respectively.

Some additional points regarding AHRS:

  • this system can use either mechanical sensors for rotation (gyroscopes), motion (accelerometers) and magnetism (magnetometers), or it may use the more recent MicroElectromechanical sensors (MEMS) technology.

  • AHRS may then be also combined with an Air Data system (an Air Data Reference Unit) with the two then forming the Air Data + Attitude/Heading Reference system (ADAHRS).

  • the AHRS may be part of the Inertial Navigation System (INS), where the AHRS (or ADAHRS) in the INS is what the IRU (or ADIRU) is to the ADIRS.

  • the difference between AHRS and the Inertial Reference Unit / IRU / Inertial Measurement Unit IMU [two interchangeably used terms] is that the IRU/IMU does not have an integrated computational part for processing the measured information but rather just feeds data to another unit of the NAV system’s structure. Actually, the sensor component of AHRS is, in fact, its IRU/IMU.

Note:

The IRU/IMU may use any of the technologies used for measurement (Gimbal-suspended/Fluid-suspended Gyrostabilized Platforms, StrapdownSensor systems, etc.). However, many aircraft use the modern RingLaserGyro technology. Multiple RingLaserGyro-based IRUs are often termed as an “Inertial Reference System (IRS)”, and this adds confusion - because, an INS using any base technology or system structure (i.e. an INS using an AHRS subsystem) are also sometimes called “IRS”, and the “IRS” in ADIRS does not imply RingLaserGyro technology must be the base for the IRUs.

A conclusion is to understand that an aircraft navigational system varies in complexity and structure which may lead to some Tetris-style nomenclature: from smaller GA airplanes having independent instruments (Mag compass and attitude indicator), across the older turboprop fleet/smaller jets having more systematized and inter-connected structures (a single air data computer ADC/ADR with an AHRS) towards the newer aircraft and larger turbos/jets with two or more ADCs/ADRs, IMUs/IRUs and both subsystems being part of a larger system.

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The most important difference is that an ADIRS contains an inertial system, which gives a position estimate and can be used for navigation. This is done by aligning to a known position on the ground and then integrating all changes in orientation (measured by ring laser gyroscopes) and velocity (measured by quartz accelerometers) to update the position. As a by-product you also get the orientation (attitude and heading).

The AHRS only gives this orientation information, but no position. It therefore replaces conventional gyroscopic instruments like a directional gyro and an artificial horizon. An ADIRS can usually be switched to ATT mode (instead of full NAV mode) in flight and then function as an AHRS without navigation capabilities:

737 IRS selectors
(source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators))

The ADIRS also performs all computations done by an Air Data Computer (ADC), which usually comes installed separately next to an AHRS for glass cockpits, and therefore replaces the conventional pitot-static instruments.

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