In this paper, estimability measures and their properties are introduced for a discrete linear system. The degrees of estimability of both a system and its subspaces can be examined with these measures for multi-input/multi-output time-varying systems. The relations among estimability, error covariance, and information matrix are presented. The effects of the initial error covariance, system model perturbation, and the transformation of state variable on the measures are also given. The estimability measures are applied to GPS/INS to confirm the estimability analysis results.