We estimate the long rate and its volatility within the Svensson framework. The procedure that best extrapolates the longest observable rate and its volatility is a 2-dimensional grid search conditioned on the ridge regression suggested by Annaert et al. (2013).
Annaert, J., Claes, A., De Ceuster, M., & Zhang, H. (2015). Estimating the long rate and its volatility. Economics Letters. https://doi.org/10.1016/j.econlet.2015.02.022