The Theory of AHRS & Difference Between AHRS and IMU

An attitude and heading reference system (AHRS) consists of sensors on three axes that provide attitude information for aircraft, including roll, pitch and yaw. These are sometimes referred to as MARG (Magnetic, Angular Rate, and Gravity) sensors and consist of either solid-state or microelectromechanical systems (MEMS) gyroscopes, accelerometers and magnetometers. They are designed to replace traditional mechanical gyroscopic flight instruments.

The key difference between an inertial measurement unit (IMU) and an AHRS is the addition of an on-board processing system in an AHRS which provides attitude and heading information versus an IMU which just delivers sensor data to an additional device that computes attitude and heading. In addition to attitude determination an AHRS may also form part of an inertial navigation system.

A form of non-linear estimation such as an Extended Kalman filter is typically used to compute the solution from these multiple sources.

AHRS have proven themselves to be highly reliable and are in common use in commercial and business aircraft. AHRS are typically integrated with electronic flight instrument systems (EFIS) which are the central part of so-called glass cockpits, to form the primary flight display. AHRS can be combined with air data computers to form an “air data, attitude and heading reference system” (ADAHRS), which provide additional information such as airspeed, altitude and outside air temperature.

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