Heading and Attitude Reference System and Inertial Navigation System

AHRS called the heading reference system includes multiple axial sensors, which can provide heading, roll and roll information for the aircraft. This type of system is used to provide accurate and reliable attitude and navigation information for the aircraft.

The heading and attitude reference system includes a MEMS-based three-axis gyroscope, accelerometer and magnetometer. The difference between the heading and attitude reference system and the inertial measurement unit IMU is that the heading and attitude reference system (AHRS) includes an embedded attitude data calculation unit and heading information. The inertial measurement unit (IMU) only provides sensor data, but does not provide accurate Reliable posture data function. At present, the commonly used heading and attitude reference system (AHRS) uses the multi-sensor data fusion to calculate the heading and attitude calculation unit as a Kalman filter.

Inertial Navigation System (INS) is an auxiliary navigation system that uses accelerometers and gyroscopes to measure the acceleration and angular velocity of objects, and uses computers to continuously estimate the position, attitude and velocity of moving objects.


The inertial navigation system includes at least a computer and a platform (or module) containing accelerometers, gyroscopes or other motion sensors. At the beginning, the outside world (operator and GPS receiver, etc.) provides the initial position and speed to the inertial navigation system. After that, the inertial navigation system continuously updates the current position and speed by integrating the information from the motion sensor. The advantage of INS is that after the initial conditions are given, the current position, direction and speed can be determined without external reference.

By detecting the acceleration and angular velocity of the system, the inertial navigation system can detect position changes (such as east or west movement), speed changes (speed magnitude or direction) and attitude changes (rotation around each axis). The feature that it does not require external references makes it naturally free from external interference or deception.

The gyroscope is used in the inertial reference frame to measure the angular rate of the system. By taking the initial position of the system in the inertial reference frame as the initial condition, and integrating the angular velocity, the current direction of the system can be obtained at all times. This can be imagined as a blindfolded passenger sitting in a car, feeling the car turn left, right, uphill, and downhill. Based on this information, he knows where the car is heading, but does not know whether the car is fast or slow. Or whether the car slid to the side of the road.

The accelerometer is used to measure the linear acceleration of the system in the inertial reference frame, but it can only measure the acceleration relative to the direction of the system’s movement (because the accelerometer is fixed to the system and rotates with the system, its direction is not known). This can be imagined as a blindfolded passenger squeezing the seat back when the car accelerates, leaning forward when the car brakes, pressing the seat down when the car accelerates uphill, and the car bounces off the seat when it crosses the top of the mountain and goes downhill. With this information, passengers know how the car accelerates relative to itself, that is, forward, backward, up, down, left or right, but they do not know the direction relative to the ground.

However, by tracking the current angular rate of the system and the current linear acceleration measured relative to the motion system, the current linear acceleration of the system in the reference frame can be determined. Take the initial speed as the initial condition, apply the correct kinematics equation, and integrate the inertial acceleration to get the system inertial velocity, and then use the initial position seat as the initial condition to integrate again to get the inertial position.

The small error of the inertial navigation system sensor will accumulate into a large error over time, and the error is roughly proportional to time, so it needs to be continuously corrected. Modern inertial navigation systems use various signals (such as global positioning systems and magnetic compasses) to modify them, and use cybernetic principles to filter different signals with weights to ensure the accuracy and reliability of the inertial navigation system.

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