In this paper the classical servomechanism problem is resolved for the case of an unknown single-input single-output plant and an a priori uncertain command signal. Namely, it is assumed that the command signal is modelled as an output of a homogeneous dynamic system (exosystem) of known order, but with unknown parameters. In order to provide asymptotic tracking for signals of that kind, a new closed-loop adaptive system, with an implicit reference model and original adaptive observer of the time derivatives of the command signal, is designed.