The strap-down north finder’s inertial measurement unit consists of a fiber optic gyroscope and an accelerometer, which are used to measure the earth's rotation angular velocity and gravity acceleration components respectively, thereby determining the azimuth angle and attitude angle of the carrier.

During the use of the strap-down north finder, the angular velocity of the interference to the gyro must also be a small amount. However, in the actual use of the north finder, it will be affected by high-frequency interference caused by vehicle engines, low-frequency interference caused by gusts of wind, near-constant interference caused by foundation settlement, and interference from vibrations such as people walking and passing vehicles. The magnitude of these angular velocity interference ranges from 1°/h to 10°/h, which is much larger than the allowable error of 0.01°/h. Therefore, some measure must be taken to separate the angular velocity interference from the gyro output. The low-pass filtering method is usually used to filter out the influence of interference signals. Experiments have shown that this method is effective in some cases. However, when the spectrum of the useful signal and the interference signal overlap significantly, the filtering effect is not obvious, and the north-finding accuracy cannot meet the target. requirements, in addition to establishing a dynamic model of the system, the Kalman filter method can also improve the system accuracy. However, it is difficult to establish a dynamic model of the interference signal. This method has great difficulties in application. This article uses measuring the size of the angular velocity interference to Improve north seeking accuracy.

1.Calculation principle of north finder

Under ideal conditions, north finder is used to measure the azimuth angle between the carrier's longitudinal axis and the geographical north direction and the attitude angle between the carrier and the geographical horizontal plane. In the north-finder, both the gyroscope and the accelerometer are installed on a platform that can rotate back and forth within 180°; the H-axis of the gyroscope is vertically upward, and the two sensitive axes of the accelerometer are parallel to the two sensitive axes of the gyroscope，perpendicular to the stepper motor's axis of rotation.

The relationship between the geographical coordinate system and the carrier coordinate system is shown in Figure 1. The carrier coordinate system is represented by xb, yb, zb, which is defined according to the right-hand coordinate system. The yb axis is defined as the carrier longitudinal axis, and the z b axis is vertically upward. The geographical coordinate system is selected as the northeast sky coordinate system, represented by xn, yn, and zn. Among them, K, α and β represent the azimuth angle, pitch angle and roll angle respectively.According to the relationship between the carrier coordinate system and the geographical coordinate system, the projections of the earth's rotation angular velocity and gravity acceleration on the carrier coordinate system xb and yb can be obtained as:If the output signals of the two axes of the gyroscope are the true components of the earth's rotation angular velocity sensed by the carrier, the north-finding solution formula is:However, in the actual solution process, the measurement output of the gyroscope contains angular velocity interference, so its existence will affect the north-finding accuracy.

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