The navigation satellite is located at an altitude of 20,000 kilometers, and it can navigate by receiving signals within the range covered by it. In deep space beyond 20,000 kilometers, a new navigation system is needed. Astronomers have proposed several possible navigation systems, the most talked about is pulsar navigation. Pulsars are a type of neutron star. The direction of the magnetic pole can produce radio or X-ray radiation. When the rotation axis does not coincide with the magnetic pole and the rotation axis sweeps across the earth, a pulse will be generated. Some of these pulsars have very good rotational stability and can be compared to the best atomic clocks on Earth, so these pulsars are also called "atomic clocks in the universe". If they can receive their signals and know their position, they can use these pulsars for interplanetary navigation. The text most likely to be introduced next is? ['Other atomic clocks similar to pulsars in the universe.', 'Some difficulties currently faced in interplanetary navigation.', 'Experiments to verify the feasibility of pulsar navigation.', 'Factors affecting pulsar rotation stability.']