* Astronomy

Signals from pulsars could form a natural GPS system that could locate any object in the galaxy to within a meter.
Signals from four pulsars: 0751+1807 (3.5ms), 2322+2057 (4.8ms), 0711-6830 (5.5ms) and 1518+0205B (7.9ms), generate regular millisecond radio signals, which can be used to form a rough tetrahedron centred on the Solar System.
Bertolomé Coll at the Observatoire de Paris in France proposes to defines the origin for this co-ordinates system as 00:00 UT on the 1st January 2001 at the focal point of the Interplanetary Scintillation Array, the radio telescope near Cambridge in the UK that first observed pulsars.
With such a  co-ordinate system any interplanetary spacecraft can then work out its three-dimensional position to within a few nanoseconds, which corresponds to about a metre, by comparing the arrival times of the signals.

Title: Using pulsars to define space-time coordinates
Authors: Bartolome Coll, Albert Tarantola

Fully relativistic coordinates have been proposed for (relativistically) running a "GPS" system. These coordinates are the arrival times of the light signals emitted by four "satellites" (clocks). Replacing the signals emitted by four controlled clocks by the signals emitted by four pulsars defines a coordinate system with lower accuracy, but valid across the whole Solar System. We here precisely define this new coordinate system, by choosing four particular pulsars and a particular event as the origin of the coordinates.

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