Bearings in space

[GoodGame]

And why not extrapolate that to the 'subspace' beacons (Trekkie Subspace radio). Navigating by polar coordinates between beacons should be easy then, as long as the paths were chartered. That would make the most sense for space 'lanes', where beaconing without an expert would be easy. The only problem is that the beacons must be relative fixed items---planets/moons/planetoids, and probably satellites around those. Stationary beacons could be possible with bases that use engines to correct their position (as 3D polar coords) relative to local system features. Probably no two paths in a particular space length will be exactly the same due to the movements of the planetary bodies that the beacons are stationed on.

Originally Posted by laserdog

I believe the term is "astrogation"?

And from what I understand, you take a reference "picture" (visible, radio, x-ray) of all the stars you can see, and then tell the computer "calculate the position in the universe/galaxy where the stars would look like this".

The computer could, of course, make some "educated guesses" based on previous navigation logs to make the process go much faster.

Also, afaik all pulsars have unique pulse times, which can make an incredibly useful and indentifiable single points of reference.

This is, of course, assuming a TL of 9+. I believe for the inter-solar space flight of TL 7 and 8 you'd just use fix pointed radio signal beacons to figure out where you're at.

[Lord Carnifex]

If I'm not mistaken, there are a couple of additional factors complicating things here:

1) Nothing in the known universe is completely static with reference to any other object. So all of your charts, maps, and calculations will have to factor in the time element as well. Granted, you can incorporate a clock to measure ship's time, or you could use known rates of change (say a known solar system or <possibly?> the rotation of known stars in a given galaxy) to calculate what the current local time is. However:

2) Even knowing the local time may not help much, as you have to take into account relativistic effects. This is especially problematic for astrogation in a large volume: since your reference points are always moving, and because you may be limited to STL or lightspeed sensors, you have to know where you are (and how you're moving) relative to your reference points in order to adjust your observations for the relativistic effects.

Which may very well mean that calculating your position and velocity will have to be done in a series of steps - look around you for known points of reference, calculate where and when you might be, make more observations to refine the data and determine your ship's velocity relative to those points, recalculate your position, make more observations to try and find your local acceleration, recalculate, re-observe, recalculate, ad naseum or until you feel confident in your numbers to set a course for where you're going.

At that point, you may not worry too much about using a certain standard set of reference points, unless the area is well charted - the relative postion and movement of a standard set of reference points is well known.

[Lord Carnifex]

Well, that I can think of, two things:

1) Not a relativistic effect per se, but if you happen to be say, 50,000 light years from one of your reference points, and you're observing it by EM radiation or something else thap propogates at c, then you've got to account for 50,000 years of relative drift. Not a big problem, if it's far enough away and you're only navigating a small region of space at STL, non-relativistic speeds.

But, it could be problem in a couple of scenarios:

a) you're 50,000 light years away from reference point A, but all of your charts are based on observations from 2,000 light years away. Your astrogation has to take into account that your observed position for the reference point is 48,000 years off of what your chart says it is (still assuming your sensors are limited to lightspeed). Not a big deal if you are in known space, relatively sure of where you're at, or don't need a high order of percision... something like the FTL astrogation equivilant of magnetic variance. But,

b) If your FTL jump or hyperspace drive has malfunctioned and you have *no* idea where you are, it makes things more difficult. Even if you're sure you're still in the same galaxy and looking at the proper reference point, you're going to need to make some guess as to about how far away it is so you can account for how far off its apparent position is from the recorded position on your star charts. So you'd probably make a rough guess (probably based on apparent magnitude or spectral shift), calculate a rough position, see how that affects the variance between your observation and your chart, refine your position, lather, rinse, and repeat.

2) If your ship is capable of accelerating to a high order of c, say .5c, then the relativistic effects of your motion are going to throw off your observations of your reference points - because of space/time dialation, they will appear to be in a different place or at a different distance than you would observe them at rest with respect to them. Again, not a big deal if you know where you are, are pretty close to your reference point, and know your velocity and acceleration with respect to your references (or if you're not moving at relativistic speeds). But, again, if you're trying to start from scratch and have *no idea* of your present position, velocity, or acceleration with regards to your reference point, you're going to have to make a series of observations and position calculations.

I think. But maybe the effects start to cancel each other out. And I don't know how big of a deal it would be at intra-galactic distances (although it would be significant in inter-galactic astrogation). Mostly it just means more time and computation - easy enough if you've got it, harder to do when the Imperial Star Destroyer is breathing down your neck.

[laserdog]

I don't know about the rest of you, but I'm finding this thread incredibly useful.

At minimum, I intend to archive all the pages for use in my next space campaign. And if anyone disputes a ruling I'm going to begin reading right off one of these posts verbatim until their eyes glaze over. =)

That's not a cut at the posts being boring. My friends are all pretty smart and read enough scifi to talk the talk, but I'm pretty sure none of them are *this* smart. =)

[William]

Originally Posted by Lord Carnifex

1) Not a relativistic effect per se, but if you happen to be say, 50,000 light years from one of your reference points, and you're observing it by EM radiation or something else thap propogates at c, then you've got to account for 50,000 years of relative drift. Not a big problem, if it's far enough away and you're only navigating a small region of space at STL, non-relativistic speeds.

2) If your ship is capable of accelerating to a high order of c, say .5c, then the relativistic effects of your motion are going to throw off your observations of your reference points

Certainly these are "problems" in the sense that your navigational systems will have to account for them, and hand calculation would be extremely tedious if not downright impossible. If you want to travel at FTL or high-sublight velocities, you'll need computers capable of handling such calculations.

To account for 48,000 light-years of difference in observation of a star, you'll need to be able to accurately project that star's trajectory and stellar evolution for either the past 48,000 years or the next 48,000 years, depending on which direction you're viewing it from relative to where the observations were made. When you make your observations and compare it to your starcharts, your computer will be finding the one place in the galaxy it could possibly look like that at the "now" of whatever coordinate frame you're using.

[XMark]

Hi all,

first, sorry for my bad english, esp. with the cake, it could probably explained better, but not by me :-((

Then, I enjoy rading this thread a lot :-))

For the big bang discussion: The best picture I found in books for the expansion was a cake. The galaxies are like raisin in a cake while you bake it. The distance of the raisins is increasing, also the expansion does not have a center.
For the goemetry of the universe: depending on the mass in the universe, the universe is flat, round or formed like a saddle (sorry, bad english again), all in 3 dimensions. This will have effects on the angles you measure on a universal scale, although you need two points each some million or better billion LY apart to notice it.
(You can read this in most cosmological books, I'm reading "Kosmology für helle Köpfe" (in german) at the moment ;-))))

Navigation: a) Inside a galaxy or near one galaxy
I do not see a problem. Either you are travelling with sublight speed, then you can measure your position all the time and probably don't get lost. Anyway, even if you travel 10000 y through the galaxy with light speed, not much has changed. If you travel by FTL, then not much time will have passed, so you can use the reference point system (there have been very good suggestions before).
For navigation inside a solar system there might be a second coordinate system necessary, which can be used down to kilometers or so?

b) far distance (billion LY):
That is really, really tricky. One problem is, if you, lets say, instantly travel 10 billion LY, first, 2/3rd of what you see now, was not visible before (the universe is just not old enough that light has reached your starting point). A lot of things you've seen from your starting point is now no longer visible...
To further complicate things, objects near your new position are 10 billion years older than how you saw them before, so e.g. quasars will have changed to ordinary galaxies, galaxy clusters have formed, other galaxies might have collided, transforming from spiral to eliptic etc.
Your starting point is now 10 billion years younger, maybe still a Quasar instead of your lovely habitable home galaxy, the great wall may not have formed etc.
Solution? Make small hops, not big one or create a system of FTL beacons...

Mark

[Archangel Beth]

Originally Posted by Anthony

I think it's only 'north' from earth. It's a large globular cluster, but it's not very bright and globular clusters aren't very distinctive.

Well, what I meant is that navigation might be based off of "where am I" and "where is 'galactic north'" -- without getting into the fiddly bits of which landmarks one is using to determine "north."

I.e., as I understood the original question, the "set a course" could be based off of the direction the ship should be pointing, with the other referent being "which way is up."

E.g., The Bullet has come through a stargate between civilized sectors. The pilot, having 3-D Spatial Sense, quickly figures out where "up" is and sets a course to the near-by planetary system. The Bullet's nose is pointed 30 degrees from "up," and the planet will be another 20 "down" and 20 degrees to the right. While she could -- having 3-D Spatial Sense -- just joystick the ship around till she got within visual scan range of the place (she's been there before), her AI assistant would like to get a chance to crunch the numbers, so the pilot goes off to take a nap. (And meanwhile, the AI generates some code or other which indicates "Course 23d, 21.45729r, 3.14695 minutes, adjust for that other system there, and continue at heading 1d, 22r for the rest of the trip.")

Assigning a latitude/longitude/othertude coordinate system for star systems would be a whole 'nother kettle of asteroids, of course.

[lwcamp]

Originally Posted by Agemegos

And I'm not clear what relativistic effects would be likely to throw off navigation.

The easy one is if you are using some short lived object, so if you look at it at certain distances, it simply isn't there.

Solution - use only long lived beacons, with a lifetime = (diameter of the galaxy)/c

[lwcamp]

Originally Posted by XMark

For the goemetry of the universe: depending on the mass in the universe, the universe is flat, round or formed like a saddle (sorry, bad english again), all in 3 dimensions.

The universe is flat. This is predicted by the generally accepted inflationary models of the big bang, and confirmed experimentally by COBE and the discovery of dark energy. What is unknown is the topology of the universe. You know those old computer games like Asteroids with "wrap around screens," where if you go off one side you end up at the corresponding point on the other side? This is a 2D manifold that is geometrically everywhere flat but with a finite area. The universe might be similar, except it would be flat in 3+1 dimensions.

Luke

[Lord Carnifex]

Originally Posted by Agemegos

True. But as you say, nothing to do with relativity.

You're right. Relativistic was a misapplied label. Signal propagation delay is a better term. It's related to relativity only in that relativity gives a fixed speed for light in a vacuum.

So you correct for space velocity and signal propagation delay. This is a linear term, it can be solved in one step.

It can be solved in one step as long as you know to a rough degree of approximation of where you are already (by dead reckoning or initial observation). I would think to get really precise results, though, you'd need to calculate your rough position, account for space velocity and signal propogation delay (based on that calculated position), calculate a finer position which gives you better numbers for signal propogation delay, use that more percise value to calculate a more percise position, and so on until you reach the desired number of significant digits.

Then you are totally lost. Perhaps, if you are lucky, a few years of observations will reveal the Great Wall or some other universal feature. But to a first approximation you are never going to see home again. On a sufficiently large scale, the Universe is homogeneous.

Exactly. Although it also depends on if you've got a limit on your FTL range. If you know you're in the same galaxy you started out in, then it depends on how good your charts are.

That's why I suggest Andromeda and S Doradus. They are far away in comparison to their space velocities, so their proper motion is not significant. And Sag A* is the centre of the Galaxy, which is a pretty good standard for 'not moving'. Also, they are comparatively easy to find from an unknown location within the Galaxy because they are embedded in conspicuous extended objects. Observations on them should place you within a few tens of parsecs.

Good point. I wasn't sure how significant the space velocities would be (I'm a philosopher, not an astronomer). It also depends, I think, on if a few tens of parsecs is good enough for your astrogational purposes or not, which depends on the setting and its realism and tech assumptions.

Yes, but you can easily correct for the abberation of light in one step, either if you know your velocity (from dead reckoning) or if you use a spectroscope to measure relative motion.

I honestly hadn't thought of using the spectroscope to measure your velocity, but that would be the best way to do it.

Originally Posted by William

Certainly these are "problems" in the sense that your navigational systems will have to account for them, and hand calculation would be extremely tedious if not downright impossible. If you want to travel at FTL or high-sublight velocities, you'll need computers capable of handling such calculations.

Exactly. Given enough time or computing power, these "problems" are reasonably trivial. However, they do allow a GM to set up a scenario that says "You come out of jump space. Even though jump travel is instantaneous, it does take x amount of time to set up the next jump." without breaking realism.

It also allows for scenarios like "you are trying to evade the Imperial fleet, but it will take your astrogation computer <clatter> x more minutes to calculate the jump into hyperspace. The AI politely informs you that it would be easier if you'd maintain a constant position or velocity, so could you please stop all of that evasive manuvering for a while?" or "the FTL drive malfunctioned. It's going to take Chief Astrogator Sulu at least a few hours to figure out where in the galaxy you are and where the closest star base is. Meanwhile, the following things start happening..."