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Classification of inertial navigation systems
Oct 19, 2018

1. Platform inertial navigation system

According to the established coordinate system, it is divided into two modes: space stability and local level. The platform of the space-stable platform inertial navigation system is stable relative to the inertia space and is used to establish an inertial coordinate system. Earth rotation, gravity acceleration and other effects are compensated by the computer. This type of system is mostly used on the active segment of the launch vehicle and on some spacecraft. The local horizontal platform inertial navigation system is characterized in that the reference plane formed by the two accelerometer input shafts on the platform can always track the horizontal plane of the point where the aircraft is located (guaranteed by the accelerometer and the gyroscope to form a Shura loop), so the acceleration The meter is not affected by the acceleration of gravity. Such systems are mostly used for aircraft (such as airplanes, cruise missiles, etc.) that move at equal speed along the surface of the Earth. In the platform inertial navigation system, the frame can isolate the angular vibration of the aircraft, and the working conditions of the instrument are better. The platform can directly establish the navigation coordinate system, the calculation amount is small, and it is easy to compensate and correct the output of the instrument, but the structure is complicated and the size is large.


2. Strapdown inertial navigation system

According to the different gyroscopes used, it is divided into a speed type strapdown inertial navigation system and a position type strapdown inertial navigation system. The former uses a rate gyro to output a momentary average angular velocity vector signal; the latter uses a free gyroscope to output an angular displacement signal. The strapdown inertial navigation system eliminates the platform, so the structure is simple, the volume is small, and the maintenance is convenient. However, the gyroscope and the accelerometer are directly mounted on the aircraft, and the working conditions are not good, which 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 computer, and the calculation amount is large.


In order to obtain the position data of the aircraft, the output of each measurement channel of the inertial navigation system must be integrated. The drift of the gyroscope will cause the angle measurement error to increase proportionally with time, and the constant error of the accelerometer will in turn cause a position error proportional to the square of the time. This is a divergent error (increasing over time) that can be corrected to obtain accurate position data by combining the Shura loop, the gyro compass loop, and the three negative feedback loops of the Foucault loop.


The Shura loop, the gyro compass loop, and the Foucault loop all have the characteristics of undamped periodic oscillations. Therefore, inertial navigation systems are often combined with navigation systems such as radio, Doppler and astronomy to form a high-precision integrated navigation system that allows the system to both dampen and correct errors.


The navigation accuracy of the inertial navigation system is closely related to the accuracy of the earth parameters. High-precision inertial navigation systems use a reference ellipsoid to provide parameters for the shape and gravity of the Earth. Due to factors such as uneven crustal density and topographical changes, there are often differences between the actual values of the parameters of the Earth's points and the calculated values obtained by the reference ellipsoid, and this difference is also random. This phenomenon is called gravity anomaly. . The gravity gradiometer being developed is capable of real-time measurement of the gravitational field, providing earth parameters and solving gravity anomalies.



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