Re: Pulsar Nav accuracy

KH.Ranitzsch@t-online.de wrote:
> 
> Hello everybody
> 
> Somebody mentioned that deep space navigation using pulsars (and
nearby
> stars?) could be accurate to within an astronomical unit.

That was Katy

> Thinking about it, I doubt that this accuracy could be achieved with
> distant objects alone.
> 
> Problem is, we have to know the position of the objects in 3
dimensions
> and the measurement is unlikely to be very precise.
> 
> Assume we know the pulsars' position to within 1/10th of a Lightyear
> (big if) and that no other measurement errors creep in (another big
> if). This means we know our spaceships's position to within 1/10th of
a
> lightyear.

Well, on a galactic scale 1/10 ly ain't anything to sneeze at.	;-)

Actually I think, given interstellar travel, we will be able to nail
the positions of other celestial objects (pulsars, et al) down pretty
darn accurately (okay, within 1/10 ly ;-). Right now we are limited
(see an earlier post in this thread) to approx 100 parsecs for parallax
measurements. That's using a 2 AU baseline. Once we start heading out
to other star systems (e.g., Alpha Centauri, Sirius, even Vega), our
baseline is going to grow massively and we ought to be able to extend
the parallax measurements out pretty far. And pretty accurately.

Of course using a parallax measurement from, say Sol and Alpha Centauri
will only cover a portion of the sky - that part being the toroid per-
pendicular to the line subtended by Sol and A-Cen. It would be more
difficult to the extreme to get the same results when looking along
the line between Sol and A-Cen. So you'd have to use other stars as
your baseline there (yielding a different accuracy rating, of course,
but hey, that's what will keep us astronomers in business 200 years
from now! ;-)

> Using many pulsars won't help that much. If you take N measurements,
> the accuracy of your result improves with the square root of the
> number. 700 pulsars will give an improvement by a factor of 1/26 [
> 1/sqrt(700) ] that is 1/260th of  LY or 1 1/2 lightdays - about enough
> to know in which star's system you are.

Which, for most purposes, is all you'd really need. You can repoint your
ship and zip in closer. :-)

In another post Katy said in response to a Karl question:

>It's not /just/ looking at the pulsars. You also know /which one/
you're 
>looking at because of the pulse rate. So you actually have to get their

>positions and frequencies right. And then there's the fact you're
observing 
>from a moving starship -- at interplanetary speeds you /ought/ to be
able 
>to get a distance to the pulsar from it's parallax in a couple of
hours. >Certainly 
>if its parallax is high (close object) you're going to notice.

Well, that's presuming you're moving at such speeds.  ;-)   I wouldn't
presume that a starship would drop out of FTL and still be moving at
fractional c velocities. But then again, that all works out to whatever
scale you're assuming. I haven't sat down to work out exactly what
"interplanetary speeds" might be. Maybe I'll do that someday. In my
copious free time (maybe this coming weekend...oh, wait, no, can't; got
the ECC to play in :-)