Autonomous navigation systems provide the vehicle with estimates of its states without ground support. This work develops an autonomous navigation architecture for lunar transfer using optical sensors and celestial navigation. Measurement and errormodels are developed for two classes of celestial measurements, the elevation of known stars from the Earth’s or moon’s limb, and the apparent radius of the Earth or moon. Monte Carlo methods are used to support the development of measurement error models. The proposed architecture is tested with linear covariance techniques; navigation errors and trajectory dispersions are obtained to confirm the feasibility of the approach. The navigation system is required to provide 0.5 deg flight-path angle accuracy at entry interface for mission safety. The simulation results show that the proposed autonomous navigation systemmeets the reentry safety requirement.