A spacecraft can steer by dead stars

In 2017, on the International Space Station, NASA’s SEXTANT experiment fixed its position from the X-ray pulses of millisecond pulsars, without using GPS.

A millisecond pulsar is a neutron star spinning hundreds of times a second, with a pulse so regular it rivals a laboratory clock. A detector that times the arrival of those X-ray pulses, from several pulsars in different directions, can solve for where it must be standing, the way a GPS receiver solves for position from the arrival of satellite codes. SEXTANT, riding on the NICER telescope, closed that loop on the station in late 2017. The public result was a position within about ten miles.

Ten miles is not a harbour. It is a proof that the method works while moving at orbital speed, on real pulsars, with a detector that could be carried. Deep space is where GPS is absent and the method would earn its mass. The demonstration was in low orbit, where GPS exists and was the judge. The pulsars do not care about that. They are available farther out, which is the point.