Today, Bertolomé Coll at the Observatoire de Paris in France and a friend propose an interstellar GPS system that has the ability to determine the position of any point in the galaxy to within a metre.

Why four pulsars? Coll points out that on these scales relativity has to be taken into account when processing the signals and to do this, the protocol has to specify a position in space-time, which requires four signals.

With the co-ordinate system established, any interplanetary spacecraft could then use the signals from these pulsars to determine its position in this co-ordinate system to within a few nanoseconds, which corresponds to about a metre.

Handy, and cheap too.