At present, there are many kinds of navigation systems. Each of them has its own advantages and characteristics, but also has its own shortcomings. For example, inertial navigation system has been widely used by various types of navigation vehicles of sea, land, air and space because of its high accuracy of short-term navigation, complete types of output navigation signals, and strong autonomy. But it also has its own shortcoming, that is, the navigation accuracy decreases with the increase of time.
In order to overcome this shortcoming, on the vehicle equipped with more than two navigation systems including inertial navigation system, inertial navigation system is often the main, and it is integrated with other navigation systems, so that they can better play their own characteristics and improve the accuracy and reliability of the navigation system. This system is called Integrated Navigation System.
The integrated algorithm generally uses the Kalman filtering for state estimation. That is, based on the output information of two (or more than two) navigation systems, the Kalman filter is used to estimate the navigation parameters or the navigation parameter error of the system, and then these estimated values are used to correct the system, so as to achieve the purpose of information fusion of the integrated navigation system.
Inertial navigation system (INS) is a kind of autonomous navigation system. It uses gyroscope and accelerometer to measure the angular motion and linear motion of the moving vehicle in the inertial space, and can calculate the position, velocity and attitude angle in real time according to the motion differential equations. When it works, it is not affected by the external environment, with all-weather, all-day working ability and good concealment. In addition, it can track and reflect the motion characteristics of the moving vehicle in time. The navigation parameter data has high update rate, high short-term accuracy and good stability. However, its disadvantage is that the navigation parameter error accumulates over time.
Global Position System(GPS) can provide accurate three-dimensional position, three-dimensional velocity and time information for sea, land, air and space users all weather and all day. However, compared with inertial navigation system (INS), GPS is susceptible to electronic interference, and GPS navigation information may be interrupted when the vehicle makes large maneuvering flight or there is terrain occlusion, or the dynamic error is too large to be used. It can be seen that INS and GNSS(GPS) have complementary advantages, and the INS/GPS integrated navigation system can greatly improve the navigation accuracy and performance of the whole system.
Compared with the single system, the advantages of the INS/GPS integrated navigation system are as follows. For the GPS receiver, the auxiliary of INS can enhance its ability to capture and track satellite signals, and improve the dynamic performance and anti-jamming ability of the receiver. For INS, GPS can suppress INS error accumulation and improve the navigation accuracy of INS. As the combination deepens, the overall performance of the system is much better than the independent system. It is considered as the most ideal combination method in the field of navigation.
According to the degree of couple, INS/GPS integrated navigation system methods can be divided into loosely coupled, tightly coupled, ultra tightly coupled and deeply coupled.
Loosely Coupled
The loosely coupled is the simplest integration. INS and GPS work independently, and the integrated filter fuses their position and velocity information and gives the optimal estimation result. Finally the result is fed back to INS for correction.
The advantages of this combination are that the work is relatively simple and easy to implement in engineering, and the two systems still work independently, so that the navigation information has redundancy. Its disadvantage is that the measurement information provided by GPS is the final navigation results such as position and velocity. Since the position and velocity of GPS are usually correlated (especially in the case of Kalman filter inside the GPS receiver), the estimation accuracy of the combination filter will be affected. When the number of navigation satellite is less than 4, the combination of the system will be completely destroyed, and the performance of the whole navigation system will deteriorate rapidly.
Tightly Coupled
The tightly coupled is a relatively complex way of combination. The measurement information provided by GPS to the filter is the original information used by the receiver for positioning, such as pseudo-range, pseudo-range rate and Doppler shift. It overcomes the correlation problem of the measurement information in the loosely coupled model, so that the navigation accuracy of the integrated system is improved, and the navigation can also be carried out when the number of available satellite is less than 4.
Ultra Tightly Coupled
The essence of loosely coupled combination and tightly coupled combination is that GPS assists INS, but lack of GPS receiver assistance. When the GPS receiver tracking performance in the integrated system decreases, it will affect the navigation performance of the integrated system. The ultra tight coupled method is a deeper information fusion of INS and GPS. On the one hand, the error correction information is provided for INS to improve the navigation accuracy, and on the other hand, the corrected INS measurement information is used to provide auxiliary information for GPS tracking loop.
Deeply Coupled
Traditional GPS receivers usually use scalar tracking method, and each tracking channel is independent of each other. SINS/GPS loosely coupled combination, tightly coupled combination and ultra-tight combination systems are based on GPS scalar tracking receivers. Different from the scalar tracking method, the vector tracking method can directly estimate the position and velocity information of the receiver according to the cumulative output in different tracking channels. Therefore, the deeply coupled combination system is a more complex but better performance integrated navigation method. GPS vector tracking receiver is used in SINS/GPS deeply coupled combination system.
Inertial Navigation System(INS) is a completely autonomous navigation technology. It has the advantages of high precision in short time, continuous output, strong anti-interference ability, and complete navigation information. However, its navigation error accumulates with time, and it is difficult to work independently for a long time. It needs to be integrated with other navigation systems to improve navigation performance.
Celestial Navigation system (CNS) mainly uses stars to navigate. It has good concealability and strong autonomy. Its navigation accuracy is not affected by time and distance, and it can provide attitude and position information. However, its positioning accuracy is not high, and the output is discontinuous, which can not provide the speed information of the aircraft.
Since INS and CNS have their own advantages and disadvantages, they can be integrated to achieve complementary advantages. INS/CNS integrated navigation system takes inertial navigation as the main body, and uses the measurement information of celestial navigation system to estimate and correct the inertial navigation error, so as to obtain high-precision navigation information.
- High Navigation Accuracy
Since the attitude measurement accuracy of the celestial navigation system can reach the arc-second level, the INS/CNS integrated navigation system can theoretically obtain high navigation accuracy. It is especially suitable for long distance and long endurance aircraft, such as long endurance UAV, air space shuttle, near space vehicle, etc. - Wide Range of Use
INS/CNS integrated navigation system is not limited by region, space and time, and its application space is extremely wide, which can realize global and all-weather autonomous navigation. - Strong Autonomy, Good Concealment and High Reliability
The INS/CNS integrated navigation system takes stars as observation targets. The parameters of the vehicle in space are determined according to the inherent motion law of the star in the sky. It is a completely autonomous navigation mode by combining with inertial navigation information to obtain the final navigation information. It does not need to communicate with the outside world, does not radiate energy, and has high concealment. The movement law of stars in space is not affected by artificial destruction, not afraid of external electromagnetic interference, which fundamentally ensures the reliability of the system.
According to the different ways of combination, INS/CNS integrated navigation system can be divided into four models. Simply integrated model, integrated model based on gyroscope drift correction, deeply integrated model and integrated model based on overall optimal correction.
INS/CNS Simply integrated Model
In the INS/CNS simply integrated model, the INS works independently to provide navigation information such as position, velocity, and attitude. The star tracker based on star image matching can independently determine the attitude of the vehicle relative to the inertial space. integrated with the reference information provided by the inertial navigation system, the position and attitude information of the vehicle can be obtained. Then, using the position and attitude information output by the celestial navigation system, the output of the INS can be directly corrected, which can improve the accuracy of the INS.
Integrated Model Based on Gyroscope Drift Correction
The large field of view star tracker can output high-precision inertial attitude information without any external reference information. The star tracker can obtain the attitude information of the vehicle under the geographical system by using the auxiliary information provided by the INS. It is integrated with the attitude information of inertial navigation system output, and the optimal estimation algorithm is used to estimate and compensate the gyroscope drift error in inertial navigation system, which can effectively correct the navigation error of inertial navigation system. The integrated model based on gyroscope drift correction is composed of inertial navigation, celestial navigation and information fusion.
Deeply Integrated Model
In this model, inertial navigation system and celestial navigation system assist each other in navigation. The inertial navigation system outputs high-precision information of the horizon with the assistance of the celestial navigation system. The celestial navigation system outputs high-precision position and attitude information with the assistance of the horizon information provided by the inertial navigation system. The position and attitude output of inertial navigation system and celestial navigation system are taken as observation values, and the Kalman filter algorithm is used to estimate and correct the position error and attitude error, so as to improve the accuracy of the integrated navigation system.
Integrated Model Based on Overall Optimal Correction
It is necessary to solve the problem of high precision autonomous horizon information to realize the overall optimal correction of inertial navigation system (INS) by celestial navigation system. That is, the horizon information that the celestial navigation system relies on during positioning should not come from the INS, and the accuracy remains stable and does not drift with time.
The above analytical astronomical positioning method based on star refraction and indirectly sensing horizon has solved the problem of high precision and autonomous horizon information of celestial navigation system.
It is necessary to solve the problem of high precision autonomous horizon information to realize the overall optimal correction of inertial navigation system (INS) by celestial navigation system. That is, the ground level information that the celestial navigation system relies on during positioning should not come from the INS, and the accuracy remains stable and does not drift with time.
The above analytical astronomical positioning method based on star refraction and indirectly sensitive horizon has solved the problem of high precision and autonomous horizon information of celestial navigation system.
In this way, the celestial navigation system can determine the attitude and position information of the vehicle by using high-precision horizon information. Then, the system error is estimated by information fusion with the attitude and position information calculated by the inertial navigation system. In this way, not only position and attitude errors can be corrected and inertial device errors can be compensated, but also errors caused by other factors such as initial alignment can be compensated.
