1. A method of estimating health parameters p(k) representing symptoms of a slowly degrading system at a discrete time k, comprising a) providing a prediction Ppred(l) of the health parameters p(k), b) calculating a prediction Ypred(k) of output variables y of a model of the system, based on a prediction Ppred(k) of the health parameters and measurements u(k) of input variables u of the model measured at time k, c) establishing of difference e(k) between the prediction Yprep(k) of the output variables y and measurements y(k) of the output variables y measured at time k, d) calculating an estimate Pest(k) of the health parameters p(kl) based on the prediction Ppred(k) of health parameters P(k) and the difference e(k), wherein in that step a) comprises providing a prediction Ppred(k) of health parameters p(k) based on an estimate Pest(k−1) of the health parameters p(k−1) at a previous time k−1.