Design Method of High-Order Kalman Filter for Strong Nonlinear System Based on Kronecker Product Transform

In this paper, a novel design idea of high-order Kalman filter based on Kronecker product transform is proposed for a class of strong nonlinear stochastic dynamic systems. Firstly, those augmenting systems are modeled with help of the Kronecker product without system noise. Secondly, the augmented system errors are illustratively charactered by Gaussian white noise. Thirdly, at the expanded space a creative high-order Kalman filter is delicately designed, which consists of high-order Taylor expansion, introducing magical intermediate variables, representing linear systems converted from strongly nonlinear systems, designing Kalman filter, etc. The performance of the proposed filter will be much better than one of EKF, because it uses more information than EKF. Finally, its promise is verified through commonly used digital simulation examples.


Introduction
The problem of filtering has received extensive research and attention since it was put forward, and the design of filters has very important applications in the military industry, target tracking, and complex industrial systems [1][2][3][4][5][6]. In practical applications, due to the inherent and unexpected disturbances of the system, some adverse effects such as system oscillations or even loss of stability will occur. Therefore, the design of the filter becomes very important. First of all, for non-stationary systems, the most representative filter is the Kalman filter (KF) [7], which is an optimal filter for linear Gaussian systems derived from the least squares method based on the minimum variance.
In the actual background, almost all systems are nonlinear, so the filter design of nonlinear system has become the focus of research. In the case of weak nonlinearity, Extended Kalman Filter (EKF), which uses Taylor expansion to approximate the nonlinear system, was proposed in the literature [8]. Due to the better applicability of EKF, it is still widely studied and applied today. For example, the authors of [9] introduced the maximum correlation entropy and multi-dimensional Taylor net theory to the classic EKF algorithm, which overcomes the limitation that EKF can only be applied to Gaussian systems. In addition, in [10,11] the authors also optimized the EKF to improve the estimation accuracy of the filter and applied it in the direction of vehicle distance estimation and building structure damage detection. After EKF was first proposed to solve the problem of nonlinear Gaussian filtering, Unscented Kalman Filter (UFK), Cubature Kalman Filter (CKF), and Strong Tracking Filter (STF) were also proposed and further improved the estimation accuracy of the nonlinear Kalman filter [12][13][14]. After EKF was first proposed to solve the problem of nonlinear Gaussian filtering, UFK, CKF, and STF were also proposed and improved the estimation accuracy of the nonlinear Kalman filter. Furthermore, in [15] the advantages of adaptive fading UKF and robust UKF are combined to optimize the classic UKF, so that it has a good tracking ability even when the system model involves uncertainties. At the same time, the authors of [16] simplify the UKF and reduce the The remaining parts of this paper are organized as follows: in Section 2, the model of a type of nonlinear system targeted by the proposed filter is introduced; the method of expanding the dimensionality of the nonlinear system is given in Section 3; in Section 4, the design process of the proposed filter is given, and the algorithm of the filtering process is presented; Section 5 concerns simulation verification; Section 6 summarizes this article.

Problem Formulation
Consider a class of state and measurement models are discrete nonlinear dynamic stochastic systems: The equation of the state of the system is The equation of the measurement of the system is T is the state vector to be estimated and z(τ + 1) = z 1 (τ + 1) z 2 (τ + 1) · · · z u (τ + 1) T is the observation; T are the process and measurement noises; T are the state mapping function and measurement mapping function of the system, respectively. The above variables and mapping functions satisfy the following assumptions. Additionally, these assumptions are considered as prerequisites for the filter design in this article. Assumption 1. State noise w(τ) and measurement noisev(τ + 1)are both independent random sequences.

Assumption 2.
State noise w(τ) and measurement noisev(τ + 1) are both Gaussian white noise sequences, which meet the following conditions, respectively:

Assumption 3.
The state transition function f (u(τ)) has an r + 1 -order continuous partial derivative with respect to the state variable u(τ), and the measurement function h(u(τ + 1)) has an r + 1 -order continuous partial derivative with respect to the state variable u(τ + 1).

Augmented Modeling of Nonlinear Systems
In the filter designed in this paper, in order to avoid the introduction of multiplicative noise, which leads to an increase in system complexity and an increase in the difficulty of filter design, the system nonlinear function and system noise are modeled separately when the system is expanded. First, the sub-vector of the augmented system is designed, that is, the operation of the Kronecker product of order 1 is performed on the state transition function and the operation of the dimensional expansion of the nonlinear function is realized; after completing the calculation of the expansion of the nonlinear function, it is necessary to perform noise modeling on the expanded system model without noise. The method adopted in this paper aims to establish a nominal system and subtract the state transition functions of the nominal system and the noise-free augmented system to obtain a noise model of the corresponding dimension, thereby obtaining a complete augmented system. Specific steps are as follows.  [1] (3) where, the '⊗' is the symbol of the operation of the Kronecker product. The state function described in Equation (1) is defined as follows. (4) Remark 1. The system noise in Equation (6) is modeled by subtracting the nonlinear function term from the established nominal equation. This modeling method avoids the introduction of multiplicative noise and nonlinear noise terms.

Remark 2.
In this section, the augmented system is modeled by introducing the Kronecker product. This modeling method is theoretically deducible and interpretable.

Filter Design
Since the filtering of nonlinear systems has been researched and promoted, various Kalman filters such as Extended Kalman filter (EKF), Unscented Kalman filter (UKF), and Cubature Kalman filter (CKF) have been proposed and researched. When the state model and the measurement model have sufficient accuracy, and the initial value of the filter is selected properly, the abovementioned nonlinear Kalman filter can give an accurate state estimated value. However, the usual situation is that the state and observation model usually have model uncertainty, that is, the model cannot completely match the nonlinear system described. The main reasons for this situation are as follows: the simplification of the model, the inaccuracy of the statistical characteristics of noise, the inaccurate modeling of the statistical characteristics of the initial state of the actual system, and the changes in the parameters of the actual system [1,[33][34][35].
A very regrettable fact is that the Kalman filter has poor robustness with respect to model uncertainty, causing inaccurate state estimation and even divergence in the filtering process. Therefore, in order to solve this problem, in the proposed filter, the system state function is designed to expand the dimension to increase the system redundancy. Therefore, when the system state changes suddenly, the proposed filter also has better tracking ability.
When using a nonlinear Kalman filter for the filtering process, the main idea is to obtain an approximate linear system by approximating the nonlinear system. For example, EKF approximates a nonlinear system by using Taylor expansion to obtain the Jacobian matrix, while UKF is used to obtain the mean value of sampling points by random sampling near the approximate point. After the corresponding linearization system is obtained, the classical Kalman filter is used to filter it to obtain the state estimation value. The proposed filter design idea is similar to EKF, that is, the scalar of each sub-function of the extended system is subjected to the Taylor expansion of the selected order, and then stacked to obtain the linearized system and use the Kalman filter for filtering. The design steps of the filter are as follows, and the implementation of the filter algorithm proposed in this paper is shown in "Algorithm 1: Filtering process" at the end of this section.
Step 1.1: Calculate state prediction value.
Step 1.2: Calculate the error of the state prediction. 2) for the process of linearization of the nonlinear function F(u (1) (τ)) in Equation (19).
Step 1.3: Find the covariance matrix of the state prediction error.
Step 2.1: Calculation system measurement forecast.
Step 2.2: Calculate system measurement prediction error.
Step 2.4: Solve for gain matrix K(τ + 1). First, give the form of the orthogonal decomposition of the measurement The estimated error Utilize the Orthogonal Principle We have Therefore, we have Step 2.5: Calculate the estimated error covariance matrix.
Substituting Equation (27) into Equation (29), we get At this point, the design of the filter is completed, and simulation tests are carried out in Section 5 to verify the effectiveness of the proposed filter.

Linearization of Nonlinear Functions
The design process of the filter is given above. In order to ensure the complete derivation of the filter formula, the linearization process of the stacked high-order nonlinear terms is given here. The higher-order Taylor expansion of the same order as the state expansion order is used to ensure that the higher-order variables on both sides of the expanded equation correspond to each other, which further improves the estimation accuracy of the filter. Specific steps are as follows: From Equation (18), we can see From the third section, we have where j (u (1) (τ))) l i ;l = 1, 2, · · · , r; i = 1, 2, · · · , n l (34) Perform r-order Taylor expansion of f where the a (l) i;l 1 l 2 ···l n is the coefficients of the above formula for Taylor expansion. Let u i (τ|τ), therefore, the above formula can be written as We have . . .
Finally, each sub-function in F(u (1) (τ)) is subjected to the above linearization process separately to obtain Similarly, the other nonlinear terms can also be used in this way to achieve their linearization process. Algorithm 1: Filtering process.
Model the augmented form of the state equation based on (8)- (16) Step 2.

Remark 4.
The filter designed in this section extends the classic Kalman filter to the augmented system designed above and simplifies the computational complexity by establishing magical interme-diate variables in (36). At the same time, the filter designed in this paper also avoids the projection problem introduced in [1], and further simplifies the filter design process.

Numerical Simulations
In this section, two cases in the literature [1,30] are used to illustrate the effectiveness of the proposed method; at the same time, the classic EKF is also used to filter them, and the performance of the algorithm is proved by comparison. In the realization of the simulation process of the following two cases, a total of 50 iterations of 1-50 times are carried out. Additionally, EKF, the proposed filter expanded to the second order and the proposed filter expanded to the third order are, respectively, used to obtain the corresponding estimated values.

Case 1
Consider the following nonlinear discrete system [30]: where the state noise and the measurement noise are both uncorrelated white Gaussian noise and obey the following distribution: ; the initial values of the system are x(0) = [1, 1] T , P(0|0) = I 2×2 . The simulation results are shown in the following figure. Figures 1 and 2 show the output curves of the true value of state u 1 and state u 2 , the estimated value of EKF, and the estimated value when the proposed filter is extended to the second and third orders; Figures 3 and 4 show the estimated error output results of the above filter in state u 1 and state u 2 , respectively; Table 1 shows the comparison of the mean square error of each filtering method after the system is stabilized.

Remark 4.
The filter designed in this section extends the classic Kalman filter to the augmented system designed above and simplifies the computational complexity by establishing magical intermediate variables in (36). At the same time, the filter designed in this paper also avoids the projection problem introduced in [1], and further simplifies the filter design process.

Numerical Simulations
In this section, two cases in the literature [1] and [30] are used to illustrate the effectiveness of the proposed method; at the same time, the classic EKF is also used to filter them, and the performance of the algorithm is proved by comparison. In the realization of the simulation process of the following two cases, a total of 50 iterations of 1-50 times are carried out. Additionally, EKF, the proposed filter expanded to the second order and the proposed filter expanded to the third order are, respectively, used to obtain the corresponding estimated values.

Case 1
Consider the following nonlinear discrete system [30]: where the state noise and the measurement noise are both uncorrelated white Gaussian noise and obey the following distribution: The simulation results are shown in the following figure.  Table 1 shows the comparison of the mean square error of each filtering method after the system is stabilized.

Case 2
Consider the following nonlinear discrete system [1]: where the α(τ) is  Table 2 shows the comparison of the mean square error of each filtering method after the system is stabilized.

Summary of Simulation Results
In case 1, compared with EKF, the estimation accuracy of the proposed filter is increased by 21.50% when it is extended to the second order, and when the proposed filter is extended to the third order, the estimation accuracy is increased to 48.31% relative to EKF. For case 2, compared with EKF, the estimation accuracy of the proposed filter is improved by 73.09% when it is extended to the second order, and when the proposed filter is extended to the third order, the estimation accuracy is improved to 75.70% relative to EKF. According to the above results, it can be seen that the filter proposed has a significant improvement in estimation accuracy compared to EKF, and as the expansion order increases, the estimation accuracy also increases. At the same time, when the system state has a sudden change, the proposed filter also has a good tracking ability.

Conclusions and Future
This paper studied the design of the augmented model and the novel high-order Kalman filter for a class of nonlinear Gaussian systems. Compared with the classic extended Kalman filter and unscented Kalman filter, the filter proposed in this paper used more highorder information to achieve better estimation accuracy and tracking ability. First of all, the augmented system was modeled by introducing the Kronecker product and performing the Kronecker product operation of the selected order on the nonlinear term of the system state function. In addition, the process of dimensional expansion was simplified by ignoring the system noise when modeling the augmented system, which also avoided the introduction of product term between nonlinear noise term and nonlinear term. Then, the classic Kalman filter was extended to this augmented system by introducing creative intermediate variables brought by high-order Taylor expansion, which reduced the difficulty of filter design and the complexity of calculations. Through the comparison of simulation, it can be found that when the system state changed suddenly, the proposed method had better tracking performance, and the estimated errors were also significantly reduced. In addition, as the expansion order of the system increases, the estimated accuracy of the proposed filter also increases. Such results prove that the proposed filter has better robustness and estimation accuracy.
There are still several points worthy of improvement in future research. One of the most important points is that the system state noise is not taken into consideration in the augmented design of the system in this paper, which makes the proposed augmented system an incompletely extended form. Therefore, we will consider the system state noise into the dimensional expansion operation of system to realize a complete augmented system and complete the design of the corresponding filter, which will further increase the performance of the filter.