Quick Dumb Question

[jmfargo]

If you were to go far back in time, GPS systems would be useless because there are no satellites, correct?

Is there a system, in use today, that can tell you your relative position on the face of the Earth without use of satellites, and not requiring any input from the user?

Comments

[sageautumn.livejournal.com]

... in use today?... I don't think so?

I always thought if it was a clear night, a person with the right knowledge could do that. At least decently enough to travel?

[jfargo.livejournal.com]

Stars are good, especially if you have certain tools, and while I can't do it myself I do know of some people who can calculate where they are based on such things.

But as far as electronic devices that you can just turn on and have them tell you where you are (within, say, 100 miles), I don't think they exist. I just thought I'd see if someone else knew of something I was missing.

[argonel.livejournal.com]

It depends on the specific time period. If you are thinking relatively recent there are Loran and VOR navigational beacon using ground based radio signals. They don't have the coverage or the accuracy of GPS, but they can help identify your position. Prior to that I don't think so.

I wonder if anyone makes a digital star tracker that could be used to identify your position without relying on any external, artificial, signals.

[jfargo.livejournal.com]

That's kind of what I'm thinking.

Either that or something focusing on the magnetic poles, (using magnetic north and some other semi-powerful magnetic source if such a thing exists) and triangulating where you are, but go back far enough and even that wouldn't work.

[yud.livejournal.com]

Before GPS, there was LORAN (http://en.wikipedia.org/wiki/LORAN-C), which operates on similar principles but uses radio towers instead of satellites. Before LORAN there was DECCA (http://en.wikipedia.org/wiki/Decca_Navigator_System), which works pretty much the same as LORAN but operates on slightly different radio wave properties. It was used during WWII.

For a self-contained device that can tell your your current location, there are various inertial navigation systems with varying levels of accuracy. The MK 39 MOD 3A Ring Laser Gyro (http://www.sperrymarine.northropgrumman.com/products/Inertial_Navigation/mk39) is an example of such a device. It's about the size of a small oven, though, not exactly pocket-portable. The more accurate models are closer to refrigerator-size. If I wanted a GPS-like device on my time-travelling vessel, though, something like a ring laser gyro inertial nav system is what I'd want. As long as your time machine transports you only in time and not spatially (relative to the earth) then it should be pretty accurate.

[jfargo.livejournal.com]

Okay, so that would work, assuming a stationary target area, and then would be handy helping you find where you're going, which is important. Thanks.

Sounds like if I'm looking for something that tells you for sure where you ARE (if, for example, your "ship" tends to drift several hundred miles when you go back in time) I'd need an astrolabe, or something similar.

[yud.livejournal.com]

Inertial nav systems are great at telling you your location relative to a fixed point (the point where the system was calibrated). If you could recalibrate your intertial nav after travelling back in time, using an astrolabe or even just triangulating off of known landmarks on a chart, it would be pretty accurate (or at least as accurate as your scheme for calibrating it).

[jfargo.livejournal.com]

So far that sounds like the best bet. Essentially, you'd have to do the work at the beginning to figure out where you are (probably using star charts to figure out WHEN you are if the time travel isn't stable, and then plotting out where you are using some tool) then it can help you get to wherever it is you're trying to go.

Thanks for the idea!

[fax-celestis.livejournal.com]

You can use an astrolabe, (http://en.wikipedia.org/wiki/Mariner%27s_astrolabe) though it's inaccurate and only determines your position on one axis.

[jfargo.livejournal.com]

Sounds as though it was relatively ~wildly~ inaccurate, especially if you didn't know what you were doing with it.

Still, it's better than nothing, I suppose.

[sacramentalist.livejournal.com]

Maybe if your TT had a sextant. Stars move, though.

[jfargo.livejournal.com]

*nods* Star drift was one of my problems with using anything that relied on, well, reading the stars.

However, with a small hand-held device that had star charts dating back a long enough time, I suppose you could figure out a location, but I was hoping for something easier.

(I say "hoping," but this does not mean I plan on going back in time any time soon. Just an exercise in mental gear-spinning.)

[sacramentalist.livejournal.com]

Can futuristic GPS'es send out tachyon signals or something?

[jfargo.livejournal.com]

No, but if you breach the containment unit on the power source of one, and shatter one of the many dylithium crystals, it will send out a tachyon-like wave-pulse.

Wait.

I mean...

I don't know.

[sacramentalist.livejournal.com]

Wait a minute. If you could re-phase the emissions... To the jeffies tubes, Geordi!

[elffin.livejournal.com]

HiYouDon'tKnowMeIReadYouonFlemco'sFList.

In short, no.

In long: That depends. What do you mean by "relative position"? How accurate do you want that to be?
What do you mean by "input from the user"?
There are systems in use today that could be combined together with software to find a very imprecise location - Know what date and time it is, take a snapshot of the visible sky, correlate from the height and declination of visible stars on the horizon. That would give you a figure probably somewhere on the accuracy of within forty square miles. We used to practice using lookup tables and star charts for Orienteering in Boy Scouts. I don't think anyone's actually implemented such a thing in hardware because A: image processing not terribly advanced until recently and B: GPS satellites.

There's also the possibility of using which craters are visible on the surface of the moon, along with the elevation, and knowing the date and time (the moon currently shows the same features to a given longitude on every orbit - it "rolls" around the earth), but again no extant portable off-the-shelf hardware modules and the moon is technically a satellite.

so, not really, no. And the computational methods for working it out don't work if you don't have a way of keeping and knowing time.

[jfargo.livejournal.com]

HiYouDon'tKnowMeIReadYouonFlemco'sFList.

Actually, I recognized you from the comments on Flemco's blog.

As for relative position, I'd be happy to have something within 100 miles of position, the closer the better of course.

Input from the user? Well, it seems my dream machine doesn't exist, but if it did, the most input a user would have to do is turn it on, and maybe point it at a few things (like the stars). I was trying to figure out if there was something as simple as GPS, but using a different system that would work without external mechanics (like radio frequency, or what-have-you).

The moon would be fine as a point of reference for me; I should have said man-made satellites.

[elffin.livejournal.com]

It occurs to me that if you had a 360 degree camera, some image-processing software to pick out reference stars (Current research telescopes/facilities probably have such software), and a set of pre-computed hash tables for dates/times/stars/locations, then your proposed device would probably be workable. The heavy lifting of computing your astronomical rainbow table would be performed by number-crunching in the current time. The database would probably be very large and you'd have plenty of collisions on individual star/reference point's hash tables to dates and locales, but probably very few duplicate collisions if you implement every reference point as it's own hash table and then take the union of the results, or the union of the results widened by a margin of error that is relative to how distant you are from known (current) space/time.

This presumes that the models used to compute the hash tables are highly accurate in their predictions.

[chaosvizier.livejournal.com]

Just call your wife. Women always know exactly where you are, and more importantly, where you should be. ;-)

[jfargo.livejournal.com]

And she'd make sure I didn't forget the yak butter!

[chaosvizier.livejournal.com]

Yakkity yak! Don't talk back!

[fax-celestis.livejournal.com]

[/topic]

[chaosvizier.livejournal.com]

I'm not paid to be topical! I'm paid to be... well, ok, I'm not paid to be a wiseass either. But wouldn't that be the best job ever, getting paid to mouth off? Or wait, is that what Jerry Seinfeld does? Now I'm confused...

[fax-celestis.livejournal.com]

Actually, Eddie Izzard gets paid hueg money to be a wiseass. I am jealous.

[sacramentalist.livejournal.com]

and no matter when you call, you called too late.

[chaosvizier.livejournal.com]

It's like playing The Game. The moment you've thought about it, it's already too late, because you've lost.

[spaceoperadiva.livejournal.com]

Ask remus_shepherd. He is wise in this sort of thing.

[jfargo.livejournal.com]

Well, even if he doesn't answer the question, you've at least pointed me to someone new to read. He seems interesting!

[coyotegoth.livejournal.com]

Also, if you go far enough back in time, the stars' positions will have changed, making navigation by stars (astrogation?) difficult.

[jfargo.livejournal.com]

*smiles* I've taken that into account, if you take a gander at a few of my replies here.

Basically, if you're going to navigate by the stars, you'll need star maps from far enough back to wherever you're going. If you don't KNOW how far back you're going, well, you'll need full star maps dating back to as far as you can get. Then you get the tedious chore of sorting through the maps to figure out when you are. THEN you figure out where.

Very difficult to do if you're being shot at at the time by hostile natives.

[argonel.livejournal.com]

If you limit your time travel to within human history, just bring a deck of cards. Start playing a game of solitaire and when someone inevitable appear to tell you to put the red seven on the black eight you can ask them where you are.

[jfargo.livejournal.com]

HAH!

My mom used to do this all the time, but if you tried to help her she'd flip out. :)

[hippie-mamabear.livejournal.com]

What about a compass and a map? I mean you'd have to do some simple math...does that count as "input from the user?" :P

[jfargo.livejournal.com]

Well, a compass and a map are fine, but limited depending on how far back you go, AND can't help you figure out where you are in the first place if your machine (or however you travel) isn't exact.

But yeah, a compass and a map are fine if you know how to use them. :)

[remus-shepherd.livejournal.com]

Hi there!

I think everyone has already given you the best answers possible. A sextant, (or to a limited degree an astrolabe, or octant, or quadrant) will work. They'll give you your latitude no matter what era you're in if you work off of the sun and if you know the day of the year. Finding your longitude, however, is tricky.

There are digital star trackers that can identify stars from a database, and if you input an accurate time they'll spit out your position. Finding accurate time might be a struggle for time travellers, of course. And no star tracker will work if you're so far back in time that the stars have shifted positions.

If you have far-future travellers, you know what I'd have them do? Launch their own GPS. A pocket rocket can put up a microsatellite. You'd still have to give it a precise time, and you might only get a reading from it once every 90 minutes (the rest of the time it'd be over the horizon), but it'd be better than your other options.

[jenandbronze.livejournal.com]

I use cardinal directions to know which direction I am facing, and the sun helps to know which directon I am facing as well. Tha's one way of orientation.