IMU & AHRS

AHRS stands for Attitude and Heading Reference System. Attitude refers to an object’s orientation relative to the horizontal plane that is parallel to the horizon. This is often described as pitch and roll.

Heading is the direction a vessel is pointing towards relative to True North.

An AHRS is often used in aircraft and can also be used as a navigation aid for ground-based robots. Other uses for AHRS include measuring human movement by sports scientists.

An AHRS is comprised of IMUs (inertial measurement unit) on three orthogonal axes (commonly referred to as X, Y and Z). Typically these IMUs contain:

• Accelerometers
• Gyroscopes
• Magnetometers

Accelerometers measure the linear force acting on the vehicle. This includes gravity, which is used to determine the orientation of the vehicle with respect to the centre of the Earth.

Gyroscopes measure the rotational force acting on the vehicle.

Magnetometers measure magnetic fields. If the position of the vehicle is known, the vehicle heading relative to magnetic north can be determined. Heading relative to True North can also be found due to the known declination value.

Additional sensors can be incorporated to assist an AHRS. A common addition is the use of a dual antenna GNSS receiver (global navigation satellite system). By using two GNSS antennas the vehicle heading can be determined to a much higher accuracy.

If minimal SWaP-C is not required, higher accuracy gyroscopes based on fibre optic technologies (FOG) can dramatically increase attitude and heading accuracy.

AHRS are often made from micro-electromechanical systems, or MEMS. Therefore the compact and lightweight nature of an AHRS allows it to be used in vessels that require a small SWaP-C (size, weight, power and cost).

This small SWaP-C makes AHRS ideal for unmanned aerial vehicles (UAV) which need accurate measurements but can’t carry an instrument that is heavy or bulky.

Typically an AHRS is used for:

• Surveying
• Autonomous manufacturing robots
• Underwater navigation
• Surface marine navigation
• Tracking human movement for sports science

The output of an IMU is raw sensor data from the accelerometer (linear acceleration), gyroscope (rotational rates) and optionally a magnetometer (magnetic heading).

This information can be used for measuring the motion of an object in 3 dimensions (for example, am I pointing up and how fast am I spinning). An attitude heading and reference system (AHRS) contains an IMU, however, it also has onboard processing that applies filtering/sensor fusion to the IMU data to accurately determine the orientation (represented as roll, pitch and heading/yaw), velocity and relative position.

An AHRS coupled with a global navigation satellite system (GNSS) that provides absolute position and velocity information is referred to as an inertial navigation system (INS). An INS fuses AHRS and GNSS information to provide a very reliable representation of a system’s absolute position, orientation and velocity.