  This article provides a review on X-ray pulsar-based navigation (XNAV). The
review starts with the basic concept of XNAV, and briefly introduces the past,
present and future projects concerning XNAV. This paper focuses on the advances
of the key techniques supporting XNAV, including the navigation pulsar
database, the X-ray detection system, and the pulse time of arrival estimation.
Moreover, the methods to improve the estimation performance of XNAV are
reviewed. Finally, some remarks on the future development of XNAV are provided.
