In this paper, a flatness-based control strategy for linear time-varying systems is proposed in order to track a desired trajectory. The flatness-based control is designed by using two observers: a reduced order observer with a constant estimator error gain and an exact observer for designing a polynomial two-degrees-of-freedom controller without resolving Bezout equation in time varying framework. The proposed approach is illustrated with the control of an Anti-lock Brake System (ABS) and led to track a given trajectory for the wheel slip.