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George,
I wondered if this was the same algorithm as in the Almanac, it was
something I was going to check tonight, but now it's confirmed
That being said, I remember plugging this formula into a spreadsheet (on a
Psion 3A - I was at anchorage by some Caribbean island at the time)
I then placed my initial position nowhere near where I was. After about 3
iterations it converged to my actual position. I was quite surprised as I
had thought that one's initial guess should be approximately where one is
(at least within a few degrees). I even tried different hemispheres as
starting position.

Aubrey.

At 15:52 16-02-04, you wrote:

>This is a response to a message from Paul Hirose-
>
> >George Huxtable wrote:
> >>
> >> [Using the newly updated values for B(F) and L(F) as Lat and Long, rework
> >> the calculated altitudes and azimuths, in 7.2.2, 7.2.3, and 7.2.4, and
> >> obtain a new intercept p from (observed alt. - calculated alt.). For this
> >> to be possible, the original values for dec., GHA, and observed
> altitude of
> >> the body must have been retained.]
> >
> >George, that algorithm sounds identical to the direct computation
> >method in my 1998 Nautical Almanac (the newest one I have). The
> >paragraph that begins "If d exceeds about 20 nautical miles..." is
> >reproduced verbatim in the Almanac.
> >
> >It includes a fully worked reduction, which makes clear that some
> >values are retained: "The table shows the intermediate values of the
> >calculation for the second iteration. In each iteration the quantities
> >Ho, GHA, Dec and t do not change."
> >
> >Computation of standard deviations and error ellipses are not
> >mentioned in the Almanac.
>
>====================
>
>Paul is correct. The same algorithm is in the Nautical Almanac, under- "11.
>Position from intercept and azimuth by calculation"
>
>This is followed by "12. Example of direct computation". This example, in
>the almanac, is different from, and better-chosen than, the example used to
>illustrate the method in the AstronavPC booklet.
>
>In the almanac, the first-chosen value for lat and long is "improved" by
>the initial operation, but the change made exceeds the chosen criterion of
>20 nautical miles. Therefore a complete reiteration is needed, in which new
>values for intercepts and azimuths are calculated. By following the steps
>in the example, it's reasonably clear what needs to be done.
>
>In contrast, in the example shown in the AstronavPC booklet, first-chosen
>values for lat and long are such that the first operation of "improvement"
>is all that's needed, as the resultant change in position is less than 20
>miles. As a result, no reiteration is shown, so that example fails to give
>any guidance about how to reiterate, in cases where that's needed.
>
>As Paul says, the Almanac does not proceed to deducing an error ellipse,
>but the AstronavPC booklet does.
>
>George.
>
>================================================================
>contact George Huxtable by email at george@huxtable.u-net.com, by phone at
>01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
>Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>================================================================

   

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