An angle only (AO) target state estimation (TSE) system and method using a mixed coordinate system (Modified Spherical Coordinate (MSC) and Reference Cartesian Coordinate (RCC)) as an integrated system. This integrated system is achieved due to the state vector information of two frames (RCC and MSC) is effectively preserved between processing cycles and state vector transformation steps. The AO TSE architecture and processing schemes are applicable to a wide class of passive sensors. The mixed coordinate system provides robust real-time slant range estimation in a bootstrap fashion, thus turning passive AO measurements into equivalent active sensor measurements with built-in recursive range information but with greatly improved the TSE accuracy meeting the miss distance required by many engagement missions.