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Pulsar Signals Could Provide Galactic GPS
KentuckyFC writes "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." Pulsars slow down over time, and the arXiv paper doesn't seem to mention this. The paper is mainly about establishing a coordinate system and a reference selection of pulsars. Any proposed Galactic Positioning System would have to take the slowing into account, and since it is poorly understood and not completely predictable, this would limit accuracy.
I wonder if this would actually be useful before we develop FTL travel. Presumably it's a comparatively simple receiver and some very clever software in which case deep space probes could use it to check their position. I would suggest that they use more than four pulsars though to improve accuracy.
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Yes you're absolutely correct. The current GPS system has to incorporate aspects of both special and general relativity in order to be accurate to the meter. Special Relativity predicts that time slows down proportional to speed and therefore the speed of the satellites becomes a critical aspect of calculating their own "time". Additionally, General Relativity predicts that time slows down as a body is influenced by gravity, and because the GPS satellites do not have circular orbits the influence of the Earth's gravity changes with their position (they move closer and further away from the Earth as they orbit) and this also needs to be taken into account. The overall effect of "relativistic time slowing" is tiny and is in the nano-second ballpark, however when calculating positions using GPS a few nano-seconds can mean a few meters...
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Well at least it means we'll be able to move the Earth throughout the universe with a high degree of accuracy using huge radio dishes! Now to work on building a propulsion system capable of moving the entire fucking planet. :-D
It would be interesting if it came to light that due to the complexity of relativity in the short distances around the planet, combined with atmospheric and other signal interference validating position from ground based sources, if the positioning of the GPS sattelites themselves would in fact be more accurate using the pulsar based Galactic Positioning Systems...
ie, our Global Positioning Sattelites could one day map their relative position using the Galatic positioning system... making GPS more accurate on earth :)
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