Nonlinear observers for output tracking

In this paper we build an observer for a nonlinear system which is engaged in output tracking. In response to a desired bounded output trajectory, there are now methods to compute the bounded inputs and bounded states to achieve exact output tracking. To stabilize about the desired state space trajectory (if states are not measured) we need estimates of the error states, which are defined as the difference of the actual states and the desired states. We can recover estimates for the original states by adding the estimates of the error states to the desired trajectory. We make use of the facts that the desired state trajectory is bounded and that the error states are generally small