It has long been thought that spacecraft could calculate their position by triangulating on pulsars, in a similar fashion to the way Earth-bound GPS systems reference a number of GPS satellites in orbit.
Indeed the Voyager systems, launched in 1977 and now just leaving the outer regions of the solar system, carried a golden disc with an image depicting the position of Earth with respect to 14 pulsars. The idea being that if aliens find the craft in the future they would be able to locate its origin – the planet Earth.
An X-ray telescope experiment on board the International Space Station (ISS) has just demonstrated that it can independently calculate its position to within 5 kilometres. This is done by measuring small changes in the arrival times of the pulses from various pulsars.
A pulsar is a neutron star, formed as the remnant of a supernova, which rotates very rapidly. Jets stream from its magnetic poles, and those jets are rotated round its rotation axis, giving the impression of flashing on and off – these are the pulses which give a pulsar its name.
The significance of this navigation system is that a spacecraft can steer itself without getting instructions from its base back on Earth. Instructions beamed from Earth can take many minutes to reach a spacecraft even in the inner parts of the solar system.