Re: Pulsar Nav accuracy

At 11:10 PM 2/26/02 -0500, Hal wrote:
[snip Roger's explanation]
>Lets say for the sake of argument, that I attempt to take a bearing on
>Pulsar A.  I get that bearing.  At the same time, I have someone else,
or
>the computer take readings automatically) that gets the bearing on
Pulsar
>B.  For this "exercise" I have the bearings on both known Pulsars,
along
>with their *known* 3d co-ordinates.  From those two known co-ordinates,
I
>should be able to compute the third co-ordinate (my location).  This is
why
>I am confused as to why it should require more than *two* pulsars... 
Mind
>you, I'm not saying "exact" co-ordinates down to precision values, but
>general ball park at least.

Nope.  Your assumption above is that you know where you are relative to
the 
origin of the "known" coordinate system, which provides you with the
third 
point required to relate two volumes.  But if you're lost, how do you
know 
that relation?

For a simplified case, say you are right on the line between the two 
pulsars, so one is dead ahead, the other straight aft.	How do you know 
what angle to rotate the ship so that the origin of the "known"
coordinate 
system is directly overhead, without a third reference point?

Now, I know that in straight-line cases some examples break down, but
this 
holds more or less true as you get further away from the line between
the 
two pulsars.  You can establish where you are relative to the two
pulsars, 
and you know where the origin of the "known" coordinate system is
relative 
to the two pulsars, but you can not establish a relationship between the

two without a third point to locate by, otherwise you're on a circle of 
possible locations around the line between the two pulsars.

I hope this helps, though given the hour I'm not so sure....