Comments on: Deep Space GPS from Pulsars

By: Simon

Simon — Sun, 08 Apr 2012 00:27:51 +0000

It is a cool technique – only thing is that this idea has been around for about 8 years, and as Keith says, is already is close reaching the implementation stage at GSFC. So it is a bit misleading to say that Werner Becker has just realized …. and announced yesterday .
Some publication history:
http://www.asterlabs.com/publications.html
Implementation details:
http://www.space-library.com/Goddard_1105Spg_TechTrends.pdf

By: Aeronin

Aeronin — Thu, 05 Apr 2012 02:28:06 +0000

@Lab Lemming:
My understanding of Chris’ relevant calculation/ contention was that it should be fairly easy to detect a 1 square-degree collimated source, as any within a 1000 ly radius would project at least a 304.62 ly^2 area (or 17.45 ly-square, as you point out). This is ~4x the area projected by the Moon (0.518 deg-square). I’m not quite sure what your point was about there not even being any planets within a 10.5 ly (or, by implication, pulsars within a 17.45 ly) radius sphere, as 1000 ly was the distance in question?

On the other hand (and perhaps this was your underlying point), I do wonder about the probability of sufficient Earth-wise orientations of such bi-polar pulsar collimations. Imagine a ‘shell’ of space of arbitrary radius centered on the Earth; any such spherical shell contains ~41,253 1-deg-square regions. Let’s denote the number of pulsars existing at an **observable** distance somewhere within this sphere as N. Therefore, the probability of one of these pulsars being found in any given 1-deg-square region is at most N/41,253.

Now, if the orientations of these N pulsars themselves are randomly distributed, the odds of **observing** one of these reduce to about 2xN/41,253^2, or 1.18E-9xN (odds of at least one of the pulsars in any given 1-deg-sector having one of its two polar ‘beams’ oriented in the reciprocal 1-deg-square region containing, and therefore visible from, Earth). So, to have even a 1% chance of **observing** a pulsar in a given 1-deg-square sector, there would have to exist at least 8,509,034 pulsars within such a sphere.

I’m sure that at some radial distance from Earth, even given the low percentage of pulsars/stars, this number would be satisfied, as there are 100′s of billions of stars in the Milky Way galaxy; but would said distance exceed the observable limits for pulsars? I’m not so sanguine about that.

Additional thought: This would represent a minimum probability, as many pulsars have quite a large, measurable precession of their polar emmission spectra that would effectively enlarge the area of observability to an annulus swept by this 1-deg square. Just thinking out loud …

Now, how about quasars instead? Any takers? More ‘stable’ from a relativistic speed-time perspective, but not as dependable over time?

By: Could pulsars be used as space GPS? | Science & Technology News

Could pulsars be used as space GPS? | Science & Technology News — Tue, 03 Apr 2012 05:56:47 +0000

[...] More at the BBC, via Discover. [...]

By: Pulsars Could Support an Interstellar GPS - Forbes

Pulsars Could Support an Interstellar GPS - Forbes — Mon, 02 Apr 2012 20:44:01 +0000

[...] [HT] [...]

By: Chris

Chris — Mon, 02 Apr 2012 16:56:05 +0000

@Lab Lemming
I’m sure that over the many many years it would take to traverse these distances, they would have plenty of time to find new pulsars and use those as new position markers. Just as if you or the satellites move, you switch to a new satellite to keep your GPS working. However if they are moving at relativistic velocities, they may not have as much time as they’d like to calibrate new pulsars.

By: Rhys

Rhys — Mon, 02 Apr 2012 13:34:32 +0000

@Lab Lemming:
My immediate thoughts were similar. But the article I read suggested worthwhile use for such a system at the scale of the solar system (for example, the uncertainty in the positions of the Voyager spacecraft is apparently huge). Since it seems we won’t be sending anything over light-year distances any time soon, our concerns are irrelevant for now.

By: Lab Lemming

Lab Lemming — Mon, 02 Apr 2012 00:21:46 +0000

304 square light years is a square ~17 light years on a side, or a circle with a radius ~10 light years. There aren’t that many stars in such a small area. In fact, I don’t think there are even any known planets closer than 10.5 light years from here.

By: Chris

Chris — Sun, 01 Apr 2012 15:49:24 +0000

@11 Lab
They are collimated, but over the vast distances even a point source will diverge. Let’s assume a pulsar is 1000 light years away and it’s emitting 1 square degree from its surface. Well 1 square degree at 1000 light years is about 304 square light years, compared to the 12.6 million square light years of a sphere 1000 light years in radius. At least in our local neighborhood the pulsars should easily be seen and tracked.

By: Lab Lemming

Lab Lemming — Sun, 01 Apr 2012 00:33:55 +0000

Do pulsars transmit in all directions? I thought they were rotating collimated emissions.

By: Tom Renbarger

Tom Renbarger — Sat, 31 Mar 2012 20:13:59 +0000

“So maybe we were wrong about the origin of pulsars. Perhaps the aliens have set up a giant GPS (Galactic Positioning System)!”

http://en.wikipedia.org/wiki/LGM-1