# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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3 Lop's
From: Alexandre Eremenko
Date: 2004 Nov 14, 23:01 -0500

```Some books consider finding a fix from 3 lines
of position. This case also popped up in our infinite
discussion of the Averaging.

Can anyone explain, under what circumstances one could
possibly want to determine her position from 3 LOP's?
It is hard to imagine (for me) such circumstances.
(Hard but possible).

1. Three stars (or two stars and the Moon) taken on a short intevral.
Why would you want to do this?
If there are two bodies whose azymuths make a decent angle
(well away from 0 and 180d), and whose altitudes
are reasonable (away from 0 and 90d) why don't you concentrate
your attention on these two bodies?
Twilights are short. Is not it better to take several
altitudes of these two bodies, rather than distruct your
attention for the third one?
If all pairwise angles are "bad", three stars will
not help much.

The only case where the choice seems hard is when you see 3
bodies with all asymuth differences about 120d, and on approx
the same altitude:-)
(In which case I would probably still restrict myself with two,
taking their altitudes carefully).

2. Sun. We are talking of "running fixes" then.
If the first two observations are well-spanned,
why not to derive a fix from these two,
without waiting for the third?

The only case when you
may want to use 3, seems to be the following:
Suppose 30
minutes
after the first observation, (which was made in some
"windiw in the clouds") the sky is still cloudy,
you are afraid you won't be able to get a second LOP today,
and you don't want
to wait longer,
so you make the second observation, 30 minutes after the first.
The angle difference is too small for a reliable fix.
Then, 2 hours after, the clouds disappear and you decide to take
the third altitude. But 1 hour after that, the sky is again
completely covered by clouds:-)

Alex.

```
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