Most existing formation control approaches assume that accurate global or local position measurements of the robots are directly available, without giving details about how to obtain these measurements, or only providing Kalman filter-type estimators to get them without considering effects of the estimation on the closed-loop system stability. Hence, developing formation controllers with position estimators that can guarantee overall closed-loop system stability becomes highly desirable. This technical note presents a new formation tracking controller for the nonholonomic mobile robots without using direct position measurements. To deal with the absence of accurate position measurements, feedback information from a perspective camera, the odometry and Attitude and Heading Reference System (AHRS) sensors is used to design an observer to provide online estimates of the relative position of the follower with respect to the leader. Using Lyapunov stability analysis, we show that the combined observer-controller closed-loop system is stable, and both the formation tracking errors and the relative position estimation errors asymptotically converge to zero. The performance of the proposed scheme is illustrated through experimental results.