这段代码为什么进行二级重采样 for ii=1:N for jj=1:N Xsetpre(ii,jj) = process_equation(particle(ii,jj),k)+B*(sqrt(u_Q)*randn+gamrnd(1,1))+sqrt(Q)*randn+gamrnd(1,1); % Xsetpre(ii,jj) = process_equation(particle(ii,jj),k)+B*(sqrt(u_Q)*randn)+sqrt(Q)*randn;%采样获得N个粒子 ypart =detection_equation(Xsetpre(ii,jj),k); %预测值 vhat = y0 - ypart; weight(ii,jj)=1/(det(R)^(1/2))*exp(-1/2*vhat'*inv(R)*vhat)+ 1e-99; end %归一化 wsumii = sum(weight(ii,:)); weight_ii=weight(ii,:)./wsumii; Xset_ii=Xsetpre(ii,:); weight_pre=weight; particle_pre=Xsetpre; % 重采样 if ResampleStrategy==1 outIndex = randomR(weight_ii); %随机重采样 elseif ResampleStrategy==2 outIndex = residualR(weight_ii); %残差重采样 elseif ResampleStrategy==3 outIndex = systematicR(weight_ii); %系统重采样 elseif ResampleStrategy==4 outIndex = multinomialR(weight_ii); %多项式重采样 end %U（jj） %x（ii） weight_ii=weight_ii(outIndex); part_ii=Xset_ii(outIndex); particle(ii,:)=part_ii; weight(ii,:)=weight_ii; X_ii(ii)=mean(part_ii); end wsumjj = sum(sum(weight),2); weight_u=weight./wsumjj; weight_jj=(sum(weight_u,2))'; Xset_jj=X_ii; % 重采样 if ResampleStrategy==1 outIndex = randomR(weight_jj); %随机重采样 elseif ResampleStrategy==2 outIndex = residualR(weight_jj); %残差重采样 elseif ResampleStrategy==3 outIndex = systematicR(weight_jj); %系统重采样 elseif ResampleStrategy==4 outIndex = multinomialR(weight_jj); %多项式重采样 end weight_jj=weight_jj(outIndex); part_jj=Xset_jj(outIndex); Xo=mean(part_jj)

这段代码是一个粒子滤波算法。粒子滤波是一种基于蒙特卡罗方法的滤波算法，用于估计状态空间模型中的状态量。其中，particle(ii,jj)表示第 ii 行、第 jj 列的粒子，Xsetpre(ii,jj)表示该粒子在当前时刻的状态，weight(ii,jj)表示该粒子的权重。process_equation(particle(ii,jj),k)表示该粒子在状态转移方程下的预测值，detection_equation(Xsetpre(ii,jj),k)表示该粒子在观测方程下的预测值。重采样过程是为了保证粒子的多样性，避免出现粒子退化现象。最后，Xo表示估计的状态量。