Slashdot Mirror
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.
At this point, I'd normally be ranting about how the G in GPS stands for "Global", and that the summary is making an awful analogy, but then I realized that "Galactic" also begins with a G.
And then I realized that that still doesn't make "Galactic Global Positioning System" any better.
This is not a new idea. Actually, this idea has been thought about before and dismissed. The researchers referenced propose using millisecond radio pulsars for navigation. This is a poor idea from an engineering standpoint because it requires having a large collecting area of radio dishes in order to get an apporpriate signal level.
A better idea, which is currently being researched, and was suggested four years ago (at least the earliest I recall it being mentioned) was using x-ray pulsars, which require much smaller collecting area. See for example this thesis on the subject.
The native coordinate system is not a euclidean grid. Think of the pulsars as being clocks that are continuously broadcasting their local time. The 4 spacetime coordinates they define are just the values of those 4 clocks. In order to normalize this, I need to choose a 0 point for each clock, and the authors chose the values of the clocks as observed in Cambridge at the beginning of the millenium. Apparerently, by observing the signals, I can decide how much time (to the nearest 4 ns) had elapsed at each pulsar, at the time it broadcast the signal I'm now receiving. I can then define a transform that maps those 4 numbers into whatever local coordinate system I want. I could convert it to longitude/lattitude/UTC for terrestrial navigation, or some sort of heliocentric system for planetary navigation, or a galactic system for interstellar navigation.