NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Reality check
From: Greg R_
Date: 2006 Jun 6, 10:44 -0700
> Ilya wrote
> > You may chose any position as starting position.
> > The process to calculate a LOP is an iterative calulation.
> > Most navigator make only one step but even with an error of 600 NM you kan
> > get your position in 2 or 3 iterations.
>
> This is quite true. Let's say you are all at sea, with no idea what to use
> as a DR. We'll assume you do know which ocean, so choose a spot in the
> middle of it as a DR. What you are likely to get is enormously long
> intercepts. No worries, take the calculated fix position and redo the sight
> reduction, using that fix as a new DR. That should shorten the intercepts
> dramatically.
>
From: Greg R_
Date: 2006 Jun 6, 10:44 -0700
Peter Fogg wrote:
> We'll assume you do know which ocean, so
choose
> a spot in the middle of it as a
DR
I'm really glad this topic has come up, because
this very same question came from a colleague at work while I was extolling the
virtues of knowing how to do celnav even in this wonderful electronic age of
10-foot accuracy GPS gear (i.e. "So if I blindfolded you and dropped you off
anywhere in the world, you could figure out where you are just from
celestial?").
I really didn't have a good answer for him, but
said (without using the technical language) that I assumed that the az/int data
from a sight reduction would still work no matter how far off from an actual
position you started the process from.
But as a practical matter (assuming that we've
got a current almanac and can get a reasonably accurate time reading), even if
we had no idea at all about a DR position couldn't we at least narrow down the
starting point to ± 90 degrees of LHA (or maybe slightly more from an elevated
position)? Seems like anything beyond that could be ruled out as being
below the local horizon of the observer at that particular location, right?
--
GregR
----- Original Message -----
From: "Peter Fogg" <fthre@OPTUSNET.COM.AU>
Sent: Tuesday, June 06, 2006 3:10 AM
Subject: Re: Reality check
> > You may chose any position as starting position.
> > The process to calculate a LOP is an iterative calulation.
> > Most navigator make only one step but even with an error of 600 NM you kan
> > get your position in 2 or 3 iterations.
>
> This is quite true. Let's say you are all at sea, with no idea what to use
> as a DR. We'll assume you do know which ocean, so choose a spot in the
> middle of it as a DR. What you are likely to get is enormously long
> intercepts. No worries, take the calculated fix position and redo the sight
> reduction, using that fix as a new DR. That should shorten the intercepts
> dramatically.
>
