The Global Position System has revolutionised navigation on Earth. It consists of a network of satellites that each broadcast a time signal. A receiver on Earth can then work out its position in
three-dimensional space by comparing the arrival times of the signals from at least three satellites. But the system cannot help with navigation on an interplanetary scale or beyond.
Their idea is to tune in to the signals from four pulsars: 0751+1807 (3.5ms), 2322+2057 (4.8ms), 0711-6830 (5.5ms) and 1518+0205B (7.9ms), which each generate regular millisecond radio signals.These form a rough tetrahedron centred on the Solar System.
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.Coll then defines the origin for this system of co-ordinates as 00:00 on 1 January 2001 at the focal point of the Interplanetary Scintillation Array, the radio telescope near Cambridge in the UK that first observed pulsars. 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.