Fast On-Orbit Pulse Phase Estimation of X-Ray Crab Pulsar for XNAV Flight Experiments

The recent flight experiments with Neutron Star Interior Composition Explorer (
NICER
) and
Insight
-Hard X-ray Modulation Telescope (
Insight
-HXMT) have demonstrated the feasibility of X-ray pulsar-based navigation (XNAV) in the space. However, the current pulse phase estimation and navigation methods employed in the above flight experiments are computationally too expensive for handling the Crab pulsar data. To solve this problem, this article proposes a fast algorithm of on-orbit estimating the pulse phase of Crab pulsar called X-ray pulsar navigaTion usIng on-orbiT pulsAr timiNg (XTITAN). The pulse phase propagation model for Crab pulsar data from
Insight
-HXMT and
NICER
are derived. When an exposure on the Crab pulsar is divided into several subexposures, we derive an on-orbit timing method to estimate the hyperparameters of the pulse phase propagation model. Moreover, XTITAN is improved by iteratively estimating the pulse phase and the position and velocity of satellite. When applied to the Crab pulsar data from
NICER
, XTITAN is 58 times faster than the grid search method employed by
NICER
experiment. When applied to the Crab pulsar data from
Insight
-HXMT, XTITAN is 180 times faster than the Significance Enhancement of Pulse-profile with Orbit-dynamics (SEPO), which was employed in the flight experiments with
Insight
-HXMT. Thus, XTITAN is computationally much efficient and has the potential to be employed for onboard computation.