1. A method of measuring the health of an engine using h number of parameters, where h is defined as a complete set of health parameters sufficient to fully define a health condition, said method comprising: measuring a predetermined number of engine outputs using sensors, the predetermined number being less than h; estimating unmeasured outputs of an engine using a tuning vector q in combination with a transformation matrix V* where q=V*p, where p is a vector of health parameters h, and q contains some information from each of the h number of parameters needed to compute the health condition; and estimating the health condition of said engine based upon measured engine outputs.