Abstract: Target observability criteria previously established for bearing-only tracking in two dimensions was extended to that for bearing-rate-only tracking. For unobservability of Nth-order dynamics target from bearing-rate-only measurements, by an analysis of the relative movement equation of missile and target, the necessary and sufficient conditions (target unobservability criteria) were presented in Cartesian coordinates.The unobservability criteria has two simple forms, one is the restriction of relative distance of missile and target,and the other is the restriction of line-of-sight angle. For observability of constant acceleration target from bearing-rate-only measurements, by an analysis of the system fisher information matrix, the necessary and sufficient conditions were obtained in modified polar coordinates. Result shows that the target unobservability criteria is identical with the necessary and sufficient conditions established in modified polar coordinates , and is consistent with the target observability criteria for bearing-only tracking, besides which, is derived easily without examination of an observability matrix and nonlinear differential equations,which fits it for appliance.