Following \cite{Shumway2017} this model is initially fitted by least squares and the ARIMA model associated to the estimated residuals \(\hat{\eta}_t\) is identified. As a second step the model  is refitted assuming autocorrelated errors following an ARIMA model with order identified in the previous step. Figure \ref{120164} shows the autocorrelation and partial autocorrelation function of the estimated residuals, suggesting a white noise structure with no additional refitting required.