Re: Pulsar Nav accuracy

B Lin wrote:

> I wrote:
> > True. But it should not be difficult at all to determine this quite
> > precisely. We already know to 1/10 ly or better the distances
> > to nearby
> > stars. Once we actually *travel* to them, all we have to do is turn
a
> > sensitive receiver back toward Earth and pick up signals.
> > From this we can
> > determine *extremely* well the distance (you know when a given
signal
> > was transmitted - such as, oh, a Weather Channel show which
> > carries the
> > timestamp in the program - and with calibrated time
> > instruments you can
> > determine to with lightseconds or tighter accuracy how long
> > it took the
> > signal to reach your location, and from that, derive the distance to
> > some small number).
> >
> >
> There are some issues with that.

There are always issues with everything.  ;-)

> a) what if you're going to place that doesn't have any signals yet -
i.e. 500LY or more away from Sol?  Even early signals won't have a time
stamp (at least I don't think the orignal episodes of I Love Lucy don't)

I was not addressing jumps of that magnitude. I was addressing the
problems and accuracy range of parallax measurement to other stars.
Karl had stated in order to use stars as a baseline you need to know
*exactly* (or damned close) the distance between your baseline stars.
I proposed a method in which this baseline could be determined. This
would be with relatively nearby stars, not stars like Betelguese.

> b) Assuming advanced tech drives, non-FTL speeds could reach an
appreciable portion of the speed of light, how do you correct for the
time shift of the internal chronometers compared to the rest of the
universe? Differences of micro or milli seconds could occur if you
stayed at "high" velocities long enough.

Not being addressed in the recent thread/posts.

> c) Stars, systems and galaxies are all moving - tens, even hundreds of
thousands of miles per hour which would mean some systems are moving a
few light-seconds every day away from Sol.  After a year, a decade or
even a century they could be as far 2 light hours per year or 8.3 light
days per century. You'd have to measure the velocity of the star or
system to correct for any red-shift (or blue-shift for that matter)in
your timing signal.  The shift in wavelength might add to the
imprecision of the measurements. These velocities would be in refrence
to Sol, other systems might see a different velocity.

Again, not an issue really addressed in the thread/posts. You are
starting to go out well past what is, er, "reasonable" for space nav
in the vicinity of Sol. IIRC, laserlight postulated a range of about
20 parsecs. I indirectly expanded on this by stating parallax
measurements
are [currently] only good out to 100 parsecs, but in the future we will
be able to go further. How far? Don't know. Don't think it really
matters
that much for where the discussion is at this time. Yes, your points are
valid, but they take on a whole new scope and scale that I do not think
is easily addressed or dealt with right now. I say, let the astronomers
of 200 years hence worry about it.  ;-) ;-)

> d) For a standard map to work, we'd need a good, fixed reference point
to start from - Are we going to use some sort of Galactic North Pole? or
perhaps a "True Galactic Blackhole Center" at or near the center of the
galaxy?  Does the Tufflyverse have humans getting to the center of the
Galaxy?  Or would we be Sol-centric and make Sol our reference point?

I don't think you can have a "good, fixed reference point" in a universe
that is constantly moving. Any map you make is going to be out of date
the moment you stamp "FINI" on it (much like computers as soon as you
buy
them ;-). Whatever reference point you pick is going to have to be a
relative one.

The way I see it (and this is obviously speculation), maps may be
modular
in nature. You would create a map of a stellar neighborhood (say, oh, 50
parsecs on a side). Using a particular star (oh, let's say Sol :-) as
the
relative center point. Next, make additional map cubes that surround the
local stellar neighborhood cube (I'm stretching the currently accepted
definition of "stellar neighborhood by going 50 pc, btw; change it to 
50 ly if that feels better to you). Each map cube would be more or less
centered on a particular star, whose relative distance away from Sol is
well determined (in some way, shape, or form; if we are mapping the
galaxy,
a 50-100 pc distance between stars is a drop in the ocean). After this
next batch of map cubes is done you start on the next "ring" of cubes.
Then the next, then the next, building around the previous. In the end
you've cubed up the galaxy and can make a [temporary] galactic map (I'm
guessing also by this point maps will be 3-D hologram type things that
can evolve as you look at them, not the fixed 2-D pieces of parchment
we have now).

Ya know, thinking about this, I think I have to attribute some of
this cubic mapping to FGU's Space Opera game. IIRC they had their
stellar atlases put together very much this way, with each sector
denoted by the relative center star (Sol Sector, Antares Sector, etc).

> I think that we sometimes forget that we live in a relative world.  If
you stand at the equator, you don't feel motion, even though you are
traveling 1,000 miles per hour as the earth rotates.  In addition, you
are moving 67,0000 miles per hour (+/- a thousand) as the Earth orbits
the sun.  Since everything around you is moving at the same rate, you
don't notice the effect. What happens when you jump to another system in
another part of the galaxy?
> 
> Some more material to chew on...

I don't [forget about the relative world, that is; my dice remind me all
the time]. I took all that into account in my earlier postings, but
often
the motion of Earth around Sol was such a minute quantity in the scheme
of
things that it is lost in the noise and can be ignored. Again, it's all
a
matter of scale. If you are worried about a star's position shifting 2
lighthours a year, the velocity of Earth around the sun is not going to
be
a significant factor by any stretch of the imagination. However, a
couple
lighthours is not that far. You will already be inside the system of the
star - Uranus, for example, is but 2.6 lighthours from Sol

But, yes, after a while one does have to take into account stellar
motions
all around when doing mapping and space navigation. Which is what will
make
FTL travel fun. Your database has to continually be updated (or be
updating
itself) if you are to not get lost in the vastness of our galaxy. But
for
most things within 100 parsecs, it's not going to be that critical an
issue
in the space of a dozen years. By then you'll have updated star charts,
anyhow (unless you went "lowest bidder" again!).