Class of affine nonlinear single-input single-output systems, where the relative degree of the equivalent form of the input-output is invariant to the presence of external, unmatched disturbances, is formalized. Methods of synthesis of a multifunctional tracking system in the conditions of parametric uncertainty of the control plant model and incomplete measurements are designed for this class of systems. The original method of synthesis of a low dimension observer for estimating mixed variables (these are combinations of state variables, external influences and their derivatives) by measuring only tracking error is designed for information support of discontinuous control. In this observer, using the linear corrective effects with saturation, the method of separating the movements of observation errors is realized. As an illustration of the developed method, an electromechanical control object is considered-an inverted pendulum controlled by a DC motor. The simulation results for the worst case of varying parameters are given.