1. A method for determining the state of health of a sub-system in a system, said method comprising: collecting fault signature test results; determining a current state of health value for the sub-system using the fault signature test results; determining current state of health values of a plurality of components in the sub-system using the fault signature test results; determining current state of health values for components in the system that cannot use fault signature test results using a first probability model and the current state of health value for the sub-system and the current state of health values of the components determined by the fault signature test results; determining predicted future state of health values for the components in the sub-system using component aging models; determining a predicted future state of health values for the sub-system using a second probability model and the future state of health value for the components; determining if the predicted state of health value of the sub-system is greater than a threshold; and determining the remaining useful life of the sub-system if the predicted state of health value of the sub-system is less than the threshold.