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The technology of obtaining instantaneous velocity and position data of an aircraft by measuring its acceleration (inertia) and automatically integrating it. The equipment that makes up the inertial navigation system is installed inside the aircraft and does not rely on external information or radiate energy to the outside world during operation, making it less susceptible to interference. It is an autonomous navigation system.

brief history

In the 17th century, I. Newton studied the mechanics of high-speed rotating rigid bodies. The laws of Newtonian mechanics are the theoretical basis for inertial navigation. In 1852, J. Foucault referred to this rigid body as a gyroscope, which was later developed for attitude measurement. In 1906, H. Ahn ü tz made a gyroscope directional device, whose rotation axis can point in a fixed direction. In 1907, he added oscillation to the directional device and made a gyro compass. These achievements have become the pioneers of inertial navigation systems. In 1923, M. Schuler published the "Schuler Pendulum" theory, which solved the problem of establishing a vertical line on a moving carrier, preventing the error of the accelerometer from causing the divergence of errors in the inertial navigation system, and providing a theoretical basis for the implementation of inertial navigation in engineering. In 1942, Germany first applied the principle of inertial navigation on the V-2 rocket. In 1954, the inertial navigation system successfully flew on an airplane. In 1958, the submarine "Yuanyu" relied on inertial navigation to navigate through the Arctic for 21 days under the ice. China began developing inertial navigation systems in 1956, and since 1970, it has adopted domestically developed inertial navigation systems in multiple launches of artificial Earth satellites, rockets, and various aircraft.

Inertial navigation systems are typically composed of inertial measurement devices, computers, control displays, and other components. Inertial measurement devices include accelerometers and gyroscopes, also known as inertial navigation units. Three degrees of freedom gyroscopes are used to measure the three rotational movements of the aircraft; Three accelerometers are used to measure the acceleration of three translational movements of the aircraft. The computer calculates the speed and position data of the aircraft based on the measured acceleration signal. Control the display to show various navigation parameters.

classification

According to the installation method of inertial navigation units on aircraft, they can be divided into platform based inertial navigation systems (inertial navigation units are installed on the platform of the inertial platform) and strap down inertial navigation systems (inertial navigation units are directly installed on the aircraft).

Platform based inertial navigation system

According to the established coordinate system, it can be divided into two working modes: spatial stability and local horizontal. The platform of the space stable platform based inertial navigation system is relatively stable in inertial space, used to establish an inertial coordinate system. The effects of Earth's rotation, gravitational acceleration, etc. are compensated for by computers. This system is commonly used in the active phase of launch vehicles and some spacecraft. The characteristic of the local horizontal platform inertial navigation system is that the reference plane formed by the input axes of the two accelerometers on the platform can always track the horizontal plane of the point where the aircraft is located (using the accelerometer and gyroscope to form a relaxation loop to ensure), so the accelerometer is not affected by the acceleration of heavy forces. This system is commonly used for aircraft that move at a constant speed along the Earth's surface, such as airplanes and cruise missiles. In platform based inertial navigation systems, the framework can isolate the angular vibration of the aircraft, and the instrument working conditions are good. The platform can directly establish a navigation coordinate system, with low computational complexity and easy compensation and correction of instrument output, but the structure is complex and the size is large.

Strapdown Inertial Navigation System

According to the different gyroscopes used, it is divided into rate based strapdown inertial navigation systems and position based strapdown inertial navigation systems. The former uses a rate gyroscope to output an instantaneous average angular velocity vector signal; The latter uses a free gyroscope to output angular displacement signals. Strapdown inertial navigation systems eliminate the need for a platform, resulting in a simple structure, small size, and easy maintenance. However, the gyroscope and accelerometer are directly installed on the aircraft, which results in poor working conditions and reduces the accuracy of the instruments. The accelerometer output of this system is 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, resulting in a large computational load.

error correction 

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

The Shura circuit, gyro compass circuit, and Foucault circuit all have the characteristic of undamped periodic oscillation. Therefore, inertial navigation systems are often combined with radio, Doppler, and astronomical navigation systems to form high-precision integrated navigation systems, allowing the system to have both damping and error correction.

The navigation accuracy of inertial navigation systems is closely related to the accuracy of Earth parameters. A high-precision inertial navigation system requires a reference ellipsoid to provide parameters of the Earth's shape and gravity. Due to factors such as uneven crustal density and terrain changes, there are often differences between the actual values of parameters at various points on the Earth and the calculated values obtained from the reference ellipsoid, and these differences also have randomness. This phenomenon is called gravity anomaly. The gravity gradiometer under development can measure the gravity field in real-time, provide Earth parameters, and solve gravity anomaly problems.