1. A method of estimating health parameters p(k) representing symptoms of a slowly degrading system that includes at least a turbomachine or an internal combustion engines at a discrete time k, the method begin executed in a processing system having a specific discrete-time Kalman filter having a dynamic evolution model and an extended system model, the method comprising a) providing, in the Kalman filter, a prediction Ppred(k) of the health parameters p(k), b) calculating, in the Kalman filter, a prediction Ypred(k) of output variables y of a model of the system, based on the prediction Ppred(k) of the health parameters and measurements u(k) of input variables u of the model measured at time k, c) establishing, in the Kalman filter, a difference e(k) between the prediction Ypred(k) of the output variables y and measurements y(k) of the output variables y measured at time k, d) calculating, in the Kalman filter, an estimate Pest(k) of the health parameters p(k) based on the prediction Ppred(k) of health parameters p(k) and the difference e(k), wherein step a) comprises providing the 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, wherein an interval D between the time k and the previous time k−1 is smaller than a characteristic degradation time τ of the degrading system, and e) storing or displaying the calculated estimate Pest(k).