The accelerometer is used to measure acceleration. A triaxial accelerometer can be used to measure the motion of a fixed platform relative to the earth's surface, but once the platform moves, the situation becomes more complex. If the platform is free falling, the accelerometer measured the acceleration value is zero. If the platform is moving in a certain direction, the acceleration values of each axis will contain the acceleration value generated by gravity, so that the true acceleration value cannot be obtained. For example, the triaxial accelerometer mounted on the 60-degree roll Angle plane will measure the vertical acceleration value of 2G, whereas in fact the plane's surface is about 60 degrees. Therefore, using the accelerometer alone cannot keep the aircraft in a fixed course.
Gyroscope measures the rate of rotation of the body around an axis. When the gyroscope is used to measure the rotation Angle of the aircraft's body axis, if the plane is rotating, the measured value is non-zero, and the value of the measurement is zero when the plane is not rotating. Therefore, the angular velocity value of the gyroscope measured at the 60-degree roll Angle is zero, and the angular rate is zero when the plane is flying in a straight line. The current roll Angle can be estimated by the time integral of the angular velocity value, provided there is no error accumulation. Gyroscope to measure the value of the drift with time, after a few minutes or a few seconds will be the additional error accumulation, and eventually lead to the current relative horizontal roll Angle to the plane completely wrong cognition. Therefore, the use of gyroscopes alone cannot maintain a particular course of the aircraft.
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