An accelerometer is used to measure acceleration. With a three-axis accelerometer, the direction of movement of a fixed platform relative to the earth's surface can be measured, but once the platform moves, the situation becomes more complicated. If the platform does free fall, the acceleration value measured by the accelerometer is zero. If the platform performs acceleration movement in a certain direction, each axial acceleration value will include the acceleration value generated by gravity, making it impossible to obtain the true acceleration value. For example, a three-axis accelerometer installed on a 60-degree roll-angle aircraft will measure a 2G vertical acceleration value, in fact, the relative area surface of the aircraft is an inclination of 60 degrees. Therefore, using an accelerometer alone cannot keep the aircraft on a fixed heading.
A gyroscope measures the angular rate of rotation of the body about an axis. When a gyroscope is used to measure the axial rotation angle rate of the aircraft body, if the aircraft is rotating, the measured value is non-zero. When the aircraft is not rotating, the measured value is zero. Therefore, the roll rate measured by a gyroscope on an aircraft with a roll angle of 60 degrees is zero. Similarly, when the plane is flying horizontally and straightly, the angular rate value is zero. The current roll angle can be estimated by time integration of the angular rate value, provided there is no accumulation of errors. The value measured by the gyroscope will drift over time, and additional errors will accumulate after a few minutes or even seconds, which will eventually lead to a completely wrong perception of the aircraft's current horizontal roll angle. Therefore, using a gyroscope alone cannot maintain a specific heading of the aircraft.
In short, the accelerometer's measured value (determining the airplane's heading) is correct for a long time, but there is an error due to the presence of signal noise in a short time. Gyroscopes are more accurate in a short period of time, and there are errors in drift and drift in a long period of time. Therefore, both (adjust each other) are required to ensure the correct course.
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