A basic requirement for passive state estimation using single observer is the existence of a unique tracking solution.  This leads to the question of observability. The target state is observable over the time interval if,  and only if,  it is uniquely determined by the measurements taken in that interval.  In this paper,  the state-observability problem for passive target tracking by angle measurements is analyzed using three methods: the geometrical method,  the elementary algebraic method and linear system method.  The observability for target tracking with frequency measurements is also analyzed.  Degree of observability is discussed.  And a concise review of papers on observability analysis is presented.