说明：  对比了固定点平滑 固定区间平滑 和卡尔曼滤波三种方法的区别以及优劣 是随机系统滤波的作业题
(Comparison of fixed-point smoothing and fixed-interval Kalman smoothing filter difference between the three methods as well as the advantages and disadvantages of a random system, the operation of filtering problem)