A new relative navigation system based on X-ray pulsar measurements

The relative position estimation problem between two spacecraft, based on utilizing signals emitted from X-ray pulsars, is introduced. The pulse delay estimation problem is formulated, and the Cramér-Rao Lower Bound (CRLB) for any unbiased estimator of the pulse delay is presented as well. Two different estimation strategies are proposed, and an asymptotically efficient estimator is chosen, which is based on the maximum-likelihood criterion. The navigation system is equipped with inertial measurement units (IMUs). The time delay estimates are used as measurements, and based on the models of spacecraft and IMU dynamics, a Kalman filter is employed to obtain the three-dimensional relative position estimate. Numerical simulations are performed to verify the theoretical results.