Navigation satellites are located at an altitude of 20,000 kilometers and can be navigated by receiving signals throughout their coverage area  Beyond 20,000 kilometers, a whole new navigation system is needed  Astronomers have proposed several possible navigation systems, the most talked about being pulsar navigation  Pulsars are neutron stars that emit radio or X-ray radiation in the direction of their magnetic poles  They pulse when the axis of rotation does not coincide with the magnetic poles and sweeps across the Earth  Some of these pulsars are so stable that they are comparable to the best atomic clocks on Earth that they are also known as the atomic clocks of the universe
If you can pick up their signals and know where they are, you can use them for interstellar voyages
Factors affecting rotational stability of pulsars