The present system estimates target motion with nonzero acceleration (including maneuvering uncertainty) using angle only (AO) measurements. The present approach employs a mixed coordinate system framework by combining modified spherical coordinate (MSC) system and Reference Cartesian Coordinate (RCC) system to keep accurate information flowing from one frame to the other while eliminating the numerical sensitivity of the angle measurements to the TSE vector. This integrated coordinate systems framework 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 TSE accuracy.