Pulsar Signals Could Provide Galactic GPS 146
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 would be pedantic, but... (Score:5, Funny)
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.
Re: (Score:3, Funny)
Re: (Score:2)
Re: (Score:2)
I seriously doubt that this could be made to work, since General Relativity denies the very notion of simultinaity required to coordinate signals. And this results from two monkey wrenches - the various velocities and accelerations of the pulsars, and the varying and unknown distribution of gravitational fields between the objects. Space across galactic distances is not Euclidean, and the degree of curvature is not constant from one place to another on large scales.
Naturally, I didn't RTFA, but I'd be surpr
Re: (Score:2)
Actually, this has already been done. The Pioneer Plaques gave the location of the Earth, as relative to pulsars:
http://en.wikipedia.org/wiki/Pioneer_plaque [wikipedia.org]
Re: (Score:2)
Relativity also matters for GPS (Score:5, Insightful)
> 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.
Re:Relativity also matters for GPS (Score:5, Interesting)
Re: (Score:2, Informative)
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. ...
General relativity generalizes relativity to arbitrary smooth manifolds...
Re: (Score:2, Informative)
> 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...
No, it's in the micro-second ballpark (around 38 microseconds a day) which leads to /kilometers/ of inaccuracy a day, if you do not count in relativity.
11
Re: (Score:3, Interesting)
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
Re: (Score:2)
You are partially correct. The time signal is received, and the delay is processed, this gives the (pretty much) exact distance between the receiver and the satellite. Using multiple signals from miltiple satellites, triangulation is possible, howver, triangulation is only possible if you not only know the distance to each transmitter, but also it's exact position at the time of transmission. Since the GPS sattelites are orbiting, not fixed geosynchronous birds, the satelite's current spacial position is
Europe is planning a competing system... (Score:1, Funny)
...that uses metric pulsars.
Problem with the galactic positioning system (Score:5, Insightful)
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: (Score:3, Insightful)
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.
Re: (Score:2)
Re: (Score:2)
Re:Problem with the galactic positioning system (Score:4, Informative)
Re: (Score:2)
I'd think a bigger source of error in practice is missed pulses. The system depends on counting pulses, and at any point if you get out of sync you are going to lose track of your position.
In GPS the satellites transmit the time that each packet is transmitted. So, the receiver doesn't need to be on continuously to not lose track of its location. Obviously the pulsars aren't going to encode anything in their signals so you're dependent on keeping track.
Wouldn't a simpler solution be to just put some tran
Re:Problem with the galactic positioning system (Score:4, Informative)
Re: (Score:2)
And it actually isn't that bad -- start your spacecraft near Sol
Well, sure, but not TOO NEAR!!!!
(Sol is a bit warm!)
What about gravitational effects bending the light waves?
Re: (Score:2)
Far bigger problem: Directionality (Score:2)
Re: (Score:2)
I'm sure the current 4 will work fine for the next 150 years or so, barring cheap FTL travel.
Re: (Score:2)
Re: (Score:2)
Yes. I can imagine a deep space probe being sent to alpha centauri using this to calculate current speed and position (~5 light years away). In another 50 years we're going to get bored of what's immediately within human grasp and at some point in our lifetime the Voyager units will either burn out or the signal will be too weak to continue. Maybe 10 or 100 lightyears away you would need more than those four reference points, but the likelyhood of that happening in our lifetime is slim to none.
Re: (Score:2)
Their movement may be a fact, but when you calculate position based on space-time instead of simply space, the movement IN space is HIGHLY predictible, and therefore highly accurate. The system of calculations might need periodic adjustment (say every few decades) gue to unforseen gravametric effects, but generally, it's a pretty significant (and thus in itself predictable) event to actually cause a pulsar to have to adjust is'd galactic course... We don't really care about the emissions of the pulsar, on
Re: (Score:2)
By the time we're technologically advanced enough to engineer a "pulsar GPS" to navigate interstellar spaces, we'll probably go one better and use a stellar spectral emissions database, taking relative movement and speed into account of course, as I'm under the impression that all stars have a unique spectral fingerprint. There will be plenty of constant reference points (visible from all points), such as the Galactic Center, the Magellanic Clouds and globular clusters.
Say it's the year 4000 AT (After Tran
Re: (Score:2)
What I don't like is that since the pulsars have no frame of reference, or method of synchronization, we couldn't use them blind. Let's say I stepped into a wormhole here on earth, and came out on some SciFi planet. Or let's say I pushed my spacecraft beyond the speed of light, which might very well prevent me from receiving the pulses. When I start listening again, how would I know "which pulse" I was hearing? I wouldn't, because the pulses aren't uniquely tagged.
Sure, I could take an old fashioned
Re: (Score:2)
Re: (Score:2)
No. Every place in the universe can be considered the origin point of the big bang. The universe is fundamentally four-dimensional, at least until the string theorists tell us otherwise.
Turn Left at the Next Nebula (Score:4, Insightful)
Cool concept, but it seems like it would be of limited use until someone develops FTL.
Re:Turn Left at the Next Nebula (Score:5, Interesting)
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.
Re: (Score:3, Informative)
I would suggest that they use more than four pulsars though to improve accuracy.
There's a whole lot of research and development (and a cost/benefit ratio study) that needs to be done before just throwing out claims like that. If 4 pulsars get you down to 1 meter accuracy, yet 5 only increases it by 10% (and the 6th increases accuracy even less), yet costs millions more dollars to upgrade the probe to handle, then it's of no real benefit to use more than 4.
I need that accuracy! (Score:2)
Hey, when I am lost in space, that 1 meter difference is a big deal. I'll end up in the water instead of the beach when I travel 18,000,000,000,000 for my long weekend trip.
Re: (Score:2)
Precisely. Terrestrial/bog standard GPS uses multiple birds because the costs of doing so are very low. This strategy also provides considerable advantage - while contributing very little to accuracy (outside of surveyor grade units) it means that if you lose track on a bird because it drops below the horizon or the signal is degraded because tree cover or buildings interfere you can maintain an active position solution.
Several NASA probes already do this optically (Score:3, Insightful)
http://en.wikipedia.org/wiki/Attitude_dynamics_and_control#Star_tracker [wikipedia.org]
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 loca
Re: (Score:2)
"Warp...five point nine...parsecs then exit hyperspace left"
Re: (Score:2)
You're not going to be able to prove that you developed FTL travel until you can prove that you got to somewhere (and back, presumably) at faster than the speed of light. A galactic positioning system would be quite handy for figuring out exactly where that somewhere was, and how to get back home.
Anyway, it would be quite nice to know exactly where you were even if you stayed within our solar system. Plenty of room to get lost out there...
Already been done (Score:2)
yep:Already been done (Score:1, Informative)
http://en.wikipedia.org/wiki/File:VgrCover.jpg
Re: (Score:1, Informative)
Actually the closest we have to an inertial reference frame in the solar system is already using this idea. It has been extended to a terrestrial reference frame later. Informative links:
ICRF: http://en.wikipedia.org/wiki/International_Celestial_Reference_Frame [wikipedia.org]
ITRF: http://en.wikipedia.org/wiki/International_Terrestrial_Reference_Frame [wikipedia.org]
The technique used in both cases (which is prettey cool): http://en.wikipedia.org/wiki/VLBI [wikipedia.org]
Old Hat: The Pilgrims knew it before (Score:4, Funny)
Not interested (Score:2, Funny)
Re: (Score:1)
Emperor of Mankind (Score:1, Funny)
The only galactic GPS system I use is the beacon sent by the Emperor of Mankind. Granted the cost of a thousand psychers a day is high, but it's worth it.
Old news.... (Score:5, Informative)
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 [umd.edu] on the subject.
Re: (Score:2, Interesting)
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
How accurate does it need to be? (Score:3, Insightful)
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.
Re: (Score:1)
Hide something in interstellar space, note its current GalacticPS coordinates and velocity, and come back years later to find it. Probably needs to be fairly accurate.
Re: (Score:1)
And thus is born the sport "Galactocaching".
Re: (Score:2)
Re: (Score:2)
Antenna? (Score:2)
I find solutions... (Score:1)
...to non-existing problems hilarious!
geocentrism (Score:2)
Fixing the coordinate system to a point near Cambridge will obviously cause the "galactic coordinate system" to oscillate around the sun. And they would try to fix the coordinate system's rotation relative to what? Absolute, or the earth, or the quasars, which are moving relative to each other?
Re: (Score:2)
Or rather, they are fixing it to an inertial reference frame that was centered near Cambridge at the appointed time in 2001? That still seems hard to nail down precisely over a long period of time.
Re:geocentrism (Score:4, Informative)
Not really ... it's just a point in space. They can figure out where everything else in the observable universe was, relative to that point. I mean, the reality is, that nothing in space is really all that fixed (since galaxies are spreading apart), but as far as intra-galaxy positioning goes, one point is just as good as another for a standard point of reference. We know where that point was, relative to most other points, at a specific time. That point doesn't complete an orbit of the sun every 12 months, even though the object it was based on does. Small distinction, but it's all that matters. They're going to be measuring position relative to the pulsars, and not measuring it relative to the focal point of a telescope in Cambridge.
Also, there's a bit of silliness in the summary - the braking index of pulsars is fairly well established. It's the causes that aren't really understood, since most pulsars apparently differ from the theoretical index (IANAP). The slowdown also seems to be constant, and gives pulsars a lifespan of 10^6 years. In a modern GPS system, one needs to know two things from each of the satellites Where the signal came from and when (the reality is that you really just need to know when it was sent, and you program the "where" into the receiver-unit in a manner that lets you know where the object would have been at that time). In a modern GPS system, they put really expensive and accurate clocks into the satellites, and the signal they send out encodes the time that the signal was sent. You figure out where you are, by calculating how long it took that signal to get to you, based off of the time received from other satellites
How the hell would a pulsar encode the time it sent its signal? Simple, the period of the signal from each pulsar changes over time... that's your clock. You know what the period was at 0000hrs 1 Jan 2001, and by how much it increases. So, when you receive the signal, you calculate how long, from 0000hrs 1 Jan 2001, it would take for the signal to have a period matching the one you received. You now know when the signal was sent from, and, the information on where it was sent from is programmed into the receiver-unit. Measure the same from the other pulsars and *bam*, there's your location.
Re: (Score:2)
Re: (Score:2)
Nope. Phase difference between the pulses can tell you about changes in distances and thus your position; no slowing needed, just a knowledge of period and initial phase at a known reference time.
Then, a very approximate clock running from that reference time would tell you the expected phase differences between the next pulses at earth for the next pulse; differences from this lets you solve for x, y, z, and t at your location. If you are going to travel further than the pulsars' periods light distance f
Re: (Score:2)
Re: (Score:2)
You do it just like GPS receivers get their pseudo range. You start by guessing what time it is and you use that to guess your position then you refine your idea of the current time which gives a better position. Once you have a rough idea of where you are in time and space, you can figure out how many ticks the pulsar has produced since your epoch and feed that back into the Kalman Filter to refine your fix. If you wanted this to work for all time, your Kalman filter would need a parameter for drift o
Re: (Score:1)
It doesn't rotate around the sun. It is fixed in both time and space.
Re: (Score:2)
That statement doesn't seem to define the velocity, there being no absolute fixed space.
Re: (Score:2)
Re:geocentrism (Score:5, Informative)
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.
Re: (Score:2)
That explains it, thanks.
I tried to follow the link, but it seemed it just led to the abstract.
Home (Score:2)
Relativity, huh? (Score:1)
The additional complexity of working with signals over these distances is that relativity has to be taken into account
Friggin' In-Laws ruin Everything!
origin (Score:1)
I think origin (0,0,0) [(0,0,0,0,0)?] should be at the Sun upon the start date - since the earth orbits the Sun _and_ rotates, this could remove a couple curliques from the system - of course I know the sun orbits the galactic center and other things, I'm just saying it would simplify the system some when it comes to resolving positional issues to some fine resolution in the future.
I agree X-ray sources are better than MHz sources.
Re: (Score:1)
Everything is going to move, at different velocities in different directions, even the origin.
To find a planet you will need know:
This isn't hard for a computer to calculate. But you will never
Nothing new here, move along (Score:2)
Pulsars have been used for geodesic measurements for about 30 years. The nice short regular pulses make it possible to track the movement of continental plates down to the miliionth of a LOC length.
Re: (Score:2)
Library of Congress?
How accurate? (Score:2)
IIRC, one of the methods we use to measure the distance to a pulsar is to look at the effects the interstellar medium has on the latency of the pulse. Assuming the ISM is uniform, I suppose this wouldn't be an issue, but wouldn't this cause accuracy problems if there was an area where the ISM was denser?
Directional GRB (Score:1)
Re: (Score:2)
Not that we'll ever get that far as humans
Geek card REVOKED!
Re: (Score:2)
Aren't pulsars directional?
You fail at using TLAs to enhance your whatever. GRBs, or Gamma Ray Bursts,
are non-recurring events and don't have much to do with pulsars.
The astronomical acronym you might be trying to refer to is LGM, as in LGM-1 [wikipedia.org].
remember the millennium bug (Score:2)
"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."
I really really hope they remember the millennium bug. We don't want to creat another one of those, do we?
Re: (Score:1)
Dupe from 37 Years Ago. Pioneer 1 Plaque (Score:5, Informative)
Quoting from Wikipedia [wikipedia.org]:
Relative position of the Sun to the center of the Galaxy and 14 pulsars
The radial pattern on the left of the plaque shows 15 lines emanating from the same origin. Fourteen of the lines have corresponding long binary numbers, which stand for the periods of pulsars, using the hydrogen spin-flip transition frequency as the unit. Since these periods will change over time, the epoch of the launch can be calculated from these values.
The lengths of the lines show the relative distances of the pulsars to the Sun. A tick mark at the end of each line gives the Z coordinate perpendicular to the galactic plane.
If the plaque is found, only some of the pulsars may be visible from the location of its discovery. Showing the location with as many as 14 pulsars provides redundancy so that the location of the origin can be triangulated even if only some of the pulsars are recognized.
The data for one of the pulsars is misleading. When the plaque was designed, the frequency of pulsar "1240" (now known as J1243-6423) was known to only three significant decimal digits: 0.388 seconds. The map lists the period of this pulsar in binary to much greater precision: 100000110110010110001001111000. Rounding this off at about 10 significant bits (100000110100000000000000000000) would have provided a hint of this uncertainty. This pulsar is represented by the long line pointing down and to the right.
The fifteenth line on the plaque extends to the far right, behind the human figures. This line indicates the sun's relative distance to the center of the galaxy.
this idea is at least 40 years old (Score:2)
When out of the galaxy... (Score:2)
"Pulsars slow down over time...' (Score:2)
The GPS ephemerides data stream includes parameters to model clock drift. A similar set of corrections could be included to provide a correction for the change in pulsar frequency.
Something tells me though, that this is a small problem compared to being able to detect the pulsar signals in the first place. Unless adding an Arecibo-sized dish to your cellphone or pocket-sized locator gizmo is an option.
Re: (Score:2)
There's a much easier way to solve that problem, by centralization.
All you need is one array to detect the quasar pulses and parse them for location. Then, you can use a system to determine your location relative to that array -- combine the two sets of information and you'll have you
GMT - Galactic Mean Time ? (Score:2)
This is not just fanciful - do we want the two time references to slowly fall out of sync ?
Slow down? Pulars can also suddenly speed up (Score:1)
Polar or Grid coordinates? (Score:2)
Thinking about this, I wonder what sort of coordinate system you would use in your spacecraft? Would you use a polar coordinate system, with certain celestial bodies providing the center of the coordinate system? For example if you are in close proximity of a planet you use that, then outside of those bounds the star and then the galactic center, and so on? Or do you a grid (cube?) system with certain reference points to keep the grid in the right position?
Because reference points in space have this horribl
Ok, so what are the coordinates of Earth? (Score:2)
All of this is fine and dandy, but they still don't tell us what the coordinates of Earth are. What good will this do us if we are abducted and need to get home?
As a commenter on Technology Review said, isn't this the same concept as NASA put on Pioneer F?
zero position (Score:2)
Galactic [G]PS in a watch? (Score:2)
Now I'm just waiting for a several thousand dollar luxury wristwatch that can scan for pulsars.
Oh great (Score:2)
Now people will have an excuse for driving their starships straight into a supernova. "But the GPS said to turn 'up' here!"
Great, I'll have to upgrade my Nuvi/TomTom again. (Score:2)
Hey look out for that Asteroid *swerve* whew! . . . "Recalculating" "Recalculating"
Stranded in our solar system (Score:2)
Fine, until ... (Score:2)
Didn't Sagan think of this first (Score:2)
On that plate he had welded to Voyager?
I can see it already (Score:2)
Road side maps will no longer have the big sticker "You are here" to mark your location but would have a big sticker "You are here, right now" to mark your position in space time. Wow.
Not a new idea (Score:2)
I've been looking into this since 2001. The biggest real problem is detecting the pulsars. The free space signal loss is on the order of -400db. The math is much harder than dealing with GPS and you have to find the easy way to figure out tick counts. Other than that, its workable. Modern GPS receivers do have methods to remove pulsar noise from the signals they are watching. Defining a coordinate system will be a mess as well but that could lead to a reasonable way to define things for all planets as
Re: (Score:1)
Drive really fast behind an Astro van.
Re: (Score:2)
Re: (Score:2)
we'd be better off making our own communication networks. (or if we meet aliens with one already built, ask them very nicely if we can use it and promise not to use it for torrents)
You think they're worried about our torrents? I don't doubt that the first thing SETI discovers is going to be some kind of alien pr0n. If human history is any indication of what aliens will transmit, the first message will be a "testing, testing, is this thing on?" followed by a few years of "here is a moving picture of a modern phantabulous steam engine", followed by "hey, babe, take it off!" And then all practical evolution of the content will cease.