Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Problems with AstronavPC
    From: Aubrey O?Callaghan
    Date: 2004 Feb 16, 17:11 +0000

    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.
    >================================================================
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site