An attitude heading reference system consists of sensors on three axes that provide heading, attitude and yaw information for aircraft. They are designed to replace traditional mechanical gyroscopic flight instruments and provide superior reliability and accuracy.
AHRS consist of either solid-state or MEMS gyroscopes, accelerometers and magnetometers on all three axes. The key difference between an IMU and an AHRS is the addition of an on-board processing system in an AHRS which provides solved attitude and heading solutions versus an IMU which just delivers sensor data to an additional device that solves the attitude solution. A form of non-linear estimation such as a Kalman filter is typically used to compute the solution from these multiple sources.[1] AHRS differ from traditional inertial navigation systems by attempting to estimate only attitude (i.e. roll, pitch, yaw a.k.a heading) states, rather than attitude, position and velocity as is the case with an INS.
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 Information 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 systems" (ADAHRS), which provide additional information such as airspeed, altitude and outside air temperature.
The OpenPilot project offers a MEMS based AHRS unit with code licensed under the GPL.