Estimation of Stochastic Volatility


I am trying to estimate an AR(1) process with stochastic volatility:

z = rho_z*z(-1) + exp(sig_z)u_z;
sig_z = (1-rho_s)sigbar + rho_ssig_z(-1) + eta

I have data on z and I would like to use a non-linear estimation using the particle filter. However when I call the estimate() command with the option order=2, I get the following error:

??? Error using ==> stochastic_solvers at 156
2nd and 3rd order approximation not implemented for purely backward models

Any idea what is going on? … Thanks!