写错了，是u(i)=ln(S(i+1)/S(I))

it is the same to predict the interest rate volatility.  Probably you use short -teamLibor to estimate the bond price movement that to visit Libor database then it has already show you the volatility of interest rate with three kinds of short-team rates.  HLM is quit usefully in the case of bond price estimation but I suggest you that dont try GARCH here besasue of the bond cruve error is hard to calibrition under this model. Good luck!

thx for your info