The feasibility of using angles-only measurements from passive, Earth-based optical sensors is explored for the orbit estimation of lunar spacecraft. Such a capability may serve as a backup to active tracking methods or extend space situational awareness. Historical examples from real-world spacecraft dating back to the 1960s and mathematical models for predicting the brightness of objects in space show that the reflected sunlight from a lunar spacecraft may be detectable from the ground. A test scenario is developed where an actively tracked lunar spacecraft loses contact with the ground and must be tracked with angles-only measurements from an optical system. An extended Kalman filter is used for the orbit estimation, and historical ephemerides of the Lunar Prospector and SMART-1 spacecraft are used as truth models. The Monte Carlo simulations show that the orbit state does not diverge after over 24 hours of sequential visibilities. However, the current filter implementation suffers divergence as the orbit is viewed edge-on, and requires additional tuning before being useful as an operational system.
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