Trend cycle decomposition of GDP

Following the paper I’m trying to estimate the output gap by using, among others, a time-varying Phillips curve.
The problem is that I get a smoothed estimate of beta that is a constant but I modeled beta as a random walk (the beta is the partial derivative of inflation with respect to the output). What am I doing wrong here?

data.xls (38.5 KB)
outputgap.mod (1.1 KB)

When using mode_compute=5, I get a better mode but one parameter runs into the lower bound.

Thanks for the suggestions, I’ve incorporated these in the mod file. Alas I still get a constant beta, which is strange, given that I modeled it as a random walk. What other approaches can I take?
data.xls (38.5 KB)
outputgap.mod (1.3 KB)

Your model is nonlinear and you seem to be requesting a linearization around 0 for ygap implying that betaa*ygap will result in betaa dropping out. In a nutshell betaa is not identified.

This has helped me a lot. Thanks.