Signal of Opportunity Navigation for Small Spacecraft in Deep Space
Spacecraft navigation outside of geosynchronous orbit (GEO) presents an ongoing challenge. Current navigational techniques rely on Earth-based tracking, particularly through NASA's Deep Space Network (DSN). Navigation via the DSN is both fundamentally limited in terms of accuracy, as well as practically limited in terms of availability. Navigation via naturally occurring signals of opportunity, such as those produced by pulsars, quasars, and gamma-ray bursts, is proposed as an alternative navigation technique that could augment or eventually replace navigation via the DSN. This technique involves making range measurements based on the time-difference of arrival (TDOA) of a signal at the user and another location, usually either another cooperating user or a fixed reference point. Estimating the value of the TDOA is challenging, particularly because the signals in question are usually extremely weak. In this talk we describe algorithms for generating a 6 degree of freedom of freedom position, navigation and timing solution in deep space by measuring the time and angle of arrival of x-rays from pulsars. We show that for pulsar navigation, the position and attitude determination problems are coupled. This is due in part to the small signal-to-noise ratio of pulsar signals and the fact that x-ray photons emanating from various pulsars have no unique identifier which can be used to associate them with their source. To address this challenge a joint probabilistic data association filter is developed. The filter fuses angular rate measurements from a three-axis rate gyro with time and angle of arrival measurements from an x-ray detector. The performance of the filter is validated in simulation and the trade-offs associated with detector size and initial conditions are evaluated. Additional validation of the algorithms is performed by playback of data from x-ray detectors flown on the Suzaku and Chandra missions.