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.

Comments

Relativity also matters for GPS

[John Hasler]

> The additional complexity of working with signals over these distances is that
> relativity has to be taken into account...

Also true for high-precision GPS.

Problem with the galactic positioning system

[kimvette]

I see a problem with this immediately:

Unlike the global positioning system, the pulsars are always going to be moving relative to each other and to your position AND the reference point, which adds a tremendous amount of error. That combined with the unpredictable changes in chances in pulsars' emissions, makes the "GPS" somewhat unreliable for interstellar travel.

However, given that we're probably centuries if not eons off from traveling outside our solar system, it's a moot point. On the scale we can use it NOW (interplanetary probes, etc.) it should be highly accurate.

Re:Problem with the galactic positioning system

[Anonymous Coward]

So you think the current GPS satellite constellation is fixed relative to some reference point on Earth (and therefore eachother as well)? Of course everything is moving relative to everything else in the system. Now that also means we need to know the position of the pulsars with a high degree of accuracy, just like we have to know the position of the GPS satellites now.

Turn Left at the Next Nebula

[ATestR]

Cool concept, but it seems like it would be of limited use until someone develops FTL.

How accurate does it need to be?

[wjousts]

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.

Since we're dealing with interstellar distances, just how accurate do you need to be? Being off by a few million miles might be pretty good if you're talking about light-years of travel.

Several NASA probes already do this optically

[mbessey]

http://en.wikipedia.org/wiki/Attitude_dynamics_and_control#Star_tracker

Deep Space 1 and Deep Impact both were equipped with optical navigation software. I think that the big advantage of Pulsar-based navigation would be for missions substantially outside the solar system, where the star atlas would be less reliable. Without really high-speed propulsion at a substantial fraction of light speed, I think you'd be hard-pressed to design a spacecraft that would survive long enough to need to use Pulsars for location information.