What Is Strapdown Inertial Navigation?

According to the different gyroscopes used, the system can be divided into speed strapdown inertial navigation system and position strapdown inertial navigation system.The former USES a rate gyroscope to output instantaneous mean angular velocity vector signal.The latter USES a free gyroscope to output angular displacement signals.Strapdown inertial navigation system eliminates the platform, so the structure is simple, the volume is small, the maintenance is convenient, but the gyroscope and the accelerometer are directly installed on the aircraft, the working condition is not good, will reduce the accuracy of the instrument.The accelerometer of this system outputs the acceleration component of the body coordinate system, which needs to be converted into the acceleration component of the navigation coordinate system by the computer, which requires a large amount of calculation.

In order to obtain the position data of the aircraft, the output integral of each measurement channel of the inertial navigation system is required.The drift of gyroscope will increase the angular error with time, and the constant error of accelerometer will cause the position error with time square.This is a diverging error (which increases with time) that can be corrected by forming three negative feedback loops: the Chawla loop, the gyro compass loop, and the Foucault loop to obtain accurate position data.

The Chawla loop, gyro compass loop and Foucault loop all have the characteristics of undamped periodic oscillation.Therefore, the inertial navigation system is often combined with radio, Doppler and astronomical navigation systems to form a high-precision integrated navigation system, so that the system has both damping and error correction.

The navigation precision of inertial navigation system is closely related to the precision of earth parameters.High-precision inertial navigation systems must use reference ellipsoids to provide parameters for the shape and gravity of the earth.Due to factors such as uneven crustal density and topographic changes, the actual values of parameters at all points of the Earth are often different from the calculated values obtained by reference ellipsoid, and the difference is also random, which is called gravity anomaly.The gravity gradiometer under development can measure the gravity field in real time, provide earth parameters and solve the gravity anomaly problem.

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