可观测性分析是单观测器无源定位首先要研究的问题，只有当目标状态 (位置、速度等)是完全可观测的，定位和跟踪问题才能有可靠的、唯一的解。从几何方法、代数方程方法、线性系统方法3方面来阐述单站测角无源定位的状态可观性问题，讨论测频单站无源定位问题的可观测性和定量的可观测性，并对近年来这一领域的相关文献的分析方法和结论进行评述。

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.