This paper presents preliminary control system simulation results in a urea-selective catalytic reduction (SCR) aftertreatment system based on $NH_3$ sensor feedback. A four-state control-oriented lumped parameter model is used to analyze the controllability and observability properties of the urea-SCR plant. A model-based estimator is designed via simulation and a control system is developed with design based on a sliding mode control framework. The control system based on $NH_3$ sensor feedback is analyzed via simulation by comparing it to a control system developed based on $NO_x$ sensor feedback. Simulation results show that the $NH_3$ sensor-based strategy performs very similarly in comparison to a $NO_x$ sensor-based strategy. The control system performance metrics for $NO_x$ index, urea index, urea usage, and $NH_3$ slip suggest that the $NO_x$ sensor can be a potential alternative to a $NO_x$ sensor for urea-SCR control applications.