US and China tested X-ray pulsar-based navigation and timing (XNAV) on spacecrafts for autonomous and secure space missions

Pulsars are highly magnetized, rotating neutron stars that emit electromagnetic radiation at regular intervals. They radiate energy across a broad range of frequencies, but they are most visible in their X-ray beams. Pulsars emit powerful beams in opposite directions as they spin. These beams are observable only when they’re pointed toward Earth, making it seem as if these objects pulse (hence the name). In some cases, this apparent pulsing occurs with the predictability and consistency of an atomic clock.