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"We're all familiar with GPS. 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. That's handy, but it only works on Earth. Now astronomers say that the millisecond signals from a network of pulsars could allow GPS-style navigation on a galactic scale. They propose using four pulsars that form a rough tetrahedron with the Solar System at its center, and a co-ordinate system with its origin at 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. The additional complexity of working with signals over these distances is that relativity has to be taken into account (which is why the origin is defined as a point in space-time rather than just space). The pulsar GPS system should allow users to determine their position in space-time anywhere in the galaxy to within a few nanoseconds, which corresponds to an accuracy of about a meter."