An aircraft's attitude and heading reference system (AHRS) relies on two complementary solid-state sensors: the accelerometer and the magnetometer. The accelerometer records the total inertial acceleration acting on the airframe, including the constant 1 g produced by gravity, and therefore supplies the reference for pitch and roll. The magnetometer senses the earth's steady magnetic flux lines and so furnishes the reference for yaw, permitting the AHRS to act as an electronic compass.
Because the gyroscope that completes the triad drifts over time, the long-term accuracy of the solution is preserved by fusing its short-term precision with the stable, absolute references provided by the accelerometer and the magnetometer.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between a magnetometer and an accelerometer in aviation?
A magnetometer senses the earth's magnetic field and delivers the yaw angle that an Attitude and Heading Reference System needs, whereas an accelerometer senses the downward linear acceleration that gravity imposes on the air-frame. Because the magnetometer is immune to momentary motion, it lets AHRS provide yaw continuously. The accelerometer, whether it is a mechanical, quartz or MEMS device, only detects the downward vector and cannot tell where the nose is pointed. The two sensors play complementary functions inside the same AHRS package: the gyroscope measures angular rate of turn and, together with accelerometer data, keeps pitch and roll stable, while the magnetometer adds the magnetic heading. The accelerometer alone cannot supply yaw and the magnetometer alone cannot sense pitch or roll.
What is the role of magnetometer and accelerometer in aviation?
Magnetometers and accelerometers are the two cornerstones of every Attitude and Heading Reference System installed in aircraft. Within the AHRS, the 3-axis accelerometer senses gravity. This lets the unit determine pitch and roll by detecting which way is ‘down’. The magnetometer completes the solution by sensing Earth's magnetic field, letting the same AHRS provide heading information and yaw measurement. Together, the pair supplies the roll, pitch and yaw data that drive the primary flight display and the heading indicator.
Because AHRS uses an inertial measurement unit that also includes a 3-axis gyroscope, sensor fusion blends quick gyroscope measurements during dynamic motion with the long-term reference supplied by the accelerometer and magnetometer. The result is a continuous 3-D orientation that is more accurate than mechanical gyros yet needs no spinning rotors. The same AHRS sends attitude and heading information to autopilots and flight directors, giving modern aircraft a solid-state replacement for traditional vacuum-driven instruments.
Which instrument is more affected by vibration: magnetometer or accelerometer?
Accelerometers are more affected by vibration than magnetometers. Because accelerometers have higher resonance frequency, they remain stable while vibrations stream through their crystals. Piezoelectric accelerometers are the most popular option and they are well suited for vibration and shock testing. IEPE accelerometers are used for vibration measurements on rotating machines. Whether the motion is low-frequency or high-frequency, accelerometers can be used for low-frequency and high-frequency vibration measurements. The weight of the accelerometer must be no more than 10% of the weight of the vibrating element, so the sensor rides the motion without altering it.
A magnetometer senses the earth's magnetic field, not the shake of the airframe. Mechanical vibration still reaches it, yet the disturbance is indirect: a blurred reading rather than a saturated signal. Thus, vibration outside the sensor range distorts response, but the magnetometer is only a bystander to that motion. The accelerometer, built to dwell in vibration and to relay output signal to vibration controller, is therefore the instrument more affected because it is designed to welcome what the magnetometer merely endures.
How do magnetometers and accelerometers work together in an aircraft's navigation system?
In modern avionics, magnetometers and accelerometers never operate as solitary devices. Instead they feed an Attitude and Heading Reference System (AHRS) that is built around an Inertial Measurement Unit containing three MEMS accelerometers, three gyroscopes and three magnetometers. The IMU continuously reports raw three-axis acceleration, angular-rate and magnetic-field data to a fusion processor, while accelerometers supply the direction of Earth's gravity vector and magnetometers supply the direction of Earth's magnetic field. These two reference vectors - gravity and magnetic field - are blended with gyroscope readings inside a Kalman-filter algorithm so that transient gyroscope drift is cancelled in real time. The result is a smooth, high-rate estimate of aircraft roll, pitch and yaw. Because AHRS provides processed orientation (roll/pitch/yaw) independent of GPS, it can keep the attitude indicator valid when satellite signals are lost. Yet when GPS is available the same IMU stream is coupled with the GPS receiver to let an Inertial Navigation System calculate location, orientation and velocity without interruption. Thus magnetometers and accelerometers, fused together, form the core of both attitude and navigation solutions, guaranteeing that every axis of flight is measured, corrected and displayed within a single coherent reference frame.
Which instrument is more prone to errors: magnetometer or accelerometer?
Magnetometers are more prone to errors than accelerometers. Magnetic distortions introduce artificial errors which must be rectified by calibration.
What are the advantages and disadvantages of using a magnetometer and accelerometer aviation in an aircraft?
The advantages of a magnetometer include that it senses the Earth's magnetic field and reports an absolute heading. Because the fluxgate magnetometer is an example of a sensor that can be turned on or off, crews will elect to disable it in latitudes where magnetic distortion is large, then re-engage it once the aircraft is clear of the interference. The same absolute reference, however, becomes a liability when the vehicle manoeuvres: added dynamic motion causes error in calculation of system's pitch & roll, and every turn or climb couples into the magnetometer output, forcing the filter to average away the very signal it needs.
An accelerometer measures the instantaneous gravity vector and therefore delivers prompt estimates of pitch and roll with no dependency on magnetic latitude. Because AHRS units get rotation information from rate gyros that are accurate in the short term, the short, sharp cues supplied by the accelerometer let the filter correct the inevitable causes of drift that afflict gyros alone.
Neither sensor is sufficient by itself. A 9-axis IMU includes an accelerometer and a gyro, assuring that 9-axis IMU outputs 3 axes of gyro rotation while the same package fuses magnetometer data for heading. Thus, the magnetometer gives the global heading but suffers in manoeuvres, whereas the accelerometer gives rapid attitude updates yet slowly drifts whenever the flight path deviates from the steady 1 g norm.