  We discuss the possibility of an autonomous navigation system for spacecraft
that is based on pulsar timing data. Pulsars are rapidly rotating neutron stars
that are observable as variable celestial sources of electromagnetic radiation.
Their periodic signals have timing stabilities comparable to atomic clocks and
provide characteristic temporal signatures that can be used as natural
navigation beacons, quite similar to the use of GPS satellites for navigation
on Earth. By comparing pulse arrival times measured on-board the spacecraft
with predicted pulse arrivals at some reference location, the spacecraft
position can be determined autonomously with accuracies on the order of 5
kilometres. For a spacecraft at a distance of 10 astronomical units from Earth
(e.g., Earth-Saturn), this means an improvement by a factor of 8 compared to
conventional methods. Therefore this new technology is an alternative to
standard navigation based on radio tracking by ground stations, without the
disadvantages of uncertainty increasing with distance from Earth and the
dependence on ground control.
