
What If GPS Stood for “Galactic Positioning System”? - charlysl
https://spectrum.ieee.org/tech-talk/aerospace/space-flight/what-if-gps-stood-for-galactic-positioning-system
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nitwit005
> That pulse was emitted long ago from the distant pulsar and then spread
> outward through space. If you knew the exact time was when it was first
> emitted and the exact time you had received it, you’d be able to deduce how
> long it had been flying through space. Dividing by the speed of light would
> provide you with a measurement of your distance from the pulsar.

Am I missing something here? It doesn't seem like you can know the time
emitted if you don't know your position.

~~~
lultimouomo
If you go on, the article addresses this: measuring the peak of a pulsar
signal places you on an infinite set of spheres centered on the pulsar,
because you don't know "which" peak you measured (i.e. when it was emitted).
Since the pulsar emits a peak every millisecond or so, these spheres are
separated by one light-millisecond each. If you repeat the procedure with
several pulsars, you should be able to find that there is only one point in
space that lies in the intersection between a sphere of each pulsar.

~~~
hinkley
It’s [a little bit] bullshit. The light emitted from a pulsar is only a sphere
if you live in a universe that contains only you and the pulsar.

Spacetime is curved by every mass in it. The distance light travels is subtly
changed by what is between you and the source.

If four observers cannot agree on what was your exact location at time n
(without a perfect model of the entire universe) then how would you be able to
determine your location by observing four other objects?

Edit: You can get the right neighborhood, but if you’re moving at relativistic
speeds it will be problematic. And if you’re in the middle of nowhere and not
moving at relativistic speeds you’re probably gonna die anyway...

~~~
raattgift
Er, can you characterize which post-Newtonian correction/s is or are of non-
negligible order here for Milky-way millisecond pulsars? Where do they enter,
if not extremely close to the pulsars themselves? I'm curious, because we use
them to test, among other things, |\alpha_3| in the PPN formalism alpha-zeta
notation. Along the way you necessarily get a very good characterization of
the transforms available on local arrival times in different coordinate
systems and can probe other PPN parameters. (cf. Kramer, sec 4 and 5.1 free
version: [http://arxiv.org/abs/astro-ph/0405178v2](http://arxiv.org/abs/astro-
ph/0405178v2)).

More generally, do you object to systems of coordinates that are in-practice
recoverable by a wide variety of observers? We sure aren't Eulerian observers
of the Milky Way, but does it really seem parochial or idiosyncratic to take a
Eulerian approach to its matter?

> If four observers cannot agree on what was your exact location at time n

Find four observers who see the CMB (and matter in the bulk) as isotropic and
homogeneous, who measure the same temperature of the relic photons, and who
have a direct line of sight (with improbably good telescopes) into the
relevant part of the Milky Way, and they can agree very precisely on your
location in a cosmological frame constructed like the standard one used in
this tiny patch of spacetime. The tricky part is that the light travel times
are long compared to chaotic movements of individual humans, and the choice of
gauge has to be agreed and the observations shared.

> If you're in the middle of nowhere and not moving at relativistic speeds

Where in spacetime is the middle of nowhere?

What's materially different for a relativistic observer moving through the
same general curved spacetime (especially if "the middle of nowhere" is, say,
a large region of extremely-close-to-Minkowski spacetime) as a non-
relativistic one? While you're there, what's different for an accelerated
observer in the same region? Are you saying that something more than a Lorentz
transform would be needed?

------
viach
Interesting, this is probably the right approach to searching for the aliens
civilizations. These guys should have globally visible intergalactic GPS, we
just have to imagine how and where they would put the searchlights, the most
effective places.

~~~
fooker
What if these pulsars are their 'GPS satellites'?

~~~
viach
There should be some logic. Someone should create a project of this imaginary
galactic GPS infrastructure, taking into a count all the details - potentially
limited resources needed to build it, maximum visibility, distances coverage
and then compare it to the map of these pulsars. And if there is bingo or sort
of, we could get the space points where to rearrange our radio telescopes to
listen to.

~~~
fooker
I can imagine some kind of optimization problem, the solution for which would
be the ideal locations for these bodies.

Unfortunately I do not understand enough physics to give a concrete problem.

~~~
viach
> Unfortunately I do not understand enough physics to give a concrete problem.

Same here, I'm just a java dev. Nobody listens to java devs...

~~~
pc86
Thank god

~~~
fooker
Would you like an AbstractSingletonProxyFactoryBean with that?

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IanCal
They say it took 8 hours, what's the cause of this? Does it take that time to
locate and measure the pulsars? Or is it more signal processing time?

Also - the idea of a pulsar spinning a thousand times a second is just
incredible.

------
samfriedman
How quickly, on an astronomical scale, do pulsars slow down? I'd imagine that
eventually the changes in rotation would throw off the positioning, and
require recalibration. And of course eventually they'll stop being useful
altogether.

One would hope a civilization is able to create artificial pulsars by that
time, however...

~~~
blauditore
Wikipedia mentions a life span of about 10 million years. If they slow down in
a predictable way (exponentially?), navigation system could just calculate
that in. And when some pulsars die out, new one have probably emerged in the
meantime, allowing a smart-enough system to gradually switch sources.

~~~
planteen
The M1 (Crab Nebula) pulsar slows by 38 ns per day. But it is very new, less
than 1000 years old.

[https://en.m.wikipedia.org/wiki/Crab_Pulsar](https://en.m.wikipedia.org/wiki/Crab_Pulsar)

~~~
extrapickles
Thats almost 1.6ns/hour. If you have a stable oscillator you can compare them
to compute the time/date.

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ukulele
In theory this is a much better approach than Earth satellites: the farther
away an object is, the smaller its motion will appear. With hundreds or even
thousands of "fixed" data points incoming, it should be possible to develop an
incredibly accurate model for position in space.

~~~
fooker
I suspect the latency might kill this idea.

~~~
Lewton
What latency?

They're talking about measuring millisecond pulsars, there's no latency

~~~
TheOtherHobbes
Indeed.

But if you're travelling significant distances, you're not going to be able to
assume the space between you and all your beacons is flat.

For good accuracy you'd need a predictive spacetime curvature map for the
entire galaxy, and that might not be an easy thing to generate.

~~~
solotronics
it would probably be an advantage to measure the shift effect from multiple
sources as you travel whatever fraction of the speed of light, giving both
your position and derivatives such as velocity

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maniacalrobot
... And here I was thinking all you needed was six points of reference, and
seventh point for the origin

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xtf
Had to enable too much 3rd party scripts and other resources to read it.

htetm;dr? Had to enable too much; didn't read?

~~~
severine
Use this wonderful, underrated browser extension:
[https://firefox.add0n.com/reader-view.html](https://firefox.add0n.com/reader-
view.html)

edit: Not to be cryptic:

> "Open In Reader View" adds a new context menu item to the page and link
> context menu to let you open webpages and urls directly in the reader view
> mode to strip away all of the clutter.

Ignore the domain name, there's versions for Chrome and Opera too!

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adynatos
could we use this on earth in the future?

~~~
jannic
Just a guess: In priciple, yes, but it wouldn't be very practial. Because of
noise sources on earth, one would need very big antennas pointing to the
pulsars to get good s/n ratios. Additionally, as x-rays are blocked by the
atmosphere, one would be limited to longer-wavelength pulsars, again
increasing the size of the antennas. Given that, using GPS, we already have a
positioning system much more accurate, I don't see why one would use pulsars
for positioning on earth.

~~~
tomalpha
It does, perhaps, have the advantage of not needed a satellite fleet to work.
So therefore isn’t at the whim of said fleet’s owner, or someone with anti-
satellite missiles.

(Granted that the antenna size problem might be a killer)

~~~
gambiting
Star trackers do exactly this, providing relatively accurate positioning
without GPS:

[https://en.wikipedia.org/wiki/Star_tracker](https://en.wikipedia.org/wiki/Star_tracker)

~~~
MayeulC
I was about to suggest the use of a sextant for this. As far as I know,
navigators are still trained to operate them in this day. Of course, the same
operational principle can be integrated into a dedicated device.

