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    Re: Question Re: Position by Calculation
    From: Gary LaPook
    Date: 2014 Jan 6, 16:51 -0800
    Except when you use HO 249 volume 1 in the way flight navigators do. Plan your shots to be taken exactly four minutes apart which makes the GHA of Aries increase exactly one degree so the minutes of LHA Aries will be the same for both shots so you can then use the same assumed position. ( I know, that to a purest, GHA Aries does not change by exactly one degree in four minutes, it's actually 1°00.2'. If that isn't good enough for you then take the shots 3m59s apart in which time GHA Aries changes by 59.9', one-tenth of a minute of GHA greater accuracy.)

    gl



    From: Lu Abel <lu{at}abelhome.net>
    To: garylapook---.net
    Sent: Monday, January 6, 2014 2:37 PM
    Subject: [NavList] Re: Question Re: Position by Calculation


    Sean:

    It would be exceeding rare to discover that two sights reduced using the NA tables would use the same Assumed Position. 

    A set of tables that would work for every tenth of a minute of dead reckoning latitude and longitude and of body GHA and declination (the things we need to reduce a sight) would be impossibly large.   Even HO229, the largest of the traditional sight reduction tables, doesn't do that.

    The way to make sight reduction tables "concise" is to reduce the number of possibilities.   With the NA tables we do that by reducing accuracy to a full minute, not 0.1 minutes but, more important, using not a DR position but an assumed position that is (a) an exact number of degrees of latitude (ie, 00.0 minutes of lat) and (b) a longitude that makes LHA an exact number of degrees.

    So if I had to reduce two different sights using the NA tables to get a fix, it's highly likely I'd use APs with the same latitude (they'd be different only if I were doing a running fix and even then only under rare circumstances) -- and highly unlikely I'd use the same longitude for the APs.   (The only circumstance that I'd have the same AP longitude is if the two bodies had the same exact number of minutes (down to tenths) in their GHA if both were either east or west of me, or with "complimentary" GHAs (ie, the two bodies GHA minutes added up to 60') if the bodies were on opposite sides of my meridian.

    Hope this helps.

    Lu


    On 1/6/2014 12:57 PM, Sean C wrote:

    I was just using my new Kolbe LTA to complete a fictitious sight reduction problem of my own creation. I was pleased with the result which was 4.5 nm away from the actual position. (Bear in mind I purposely threw in a little random error.)
    However, as is my usual practice I also ran the numbers using other methods to cross check my results. As one might expect Rodger Farley's "Teacup Celestial" program produced a fix much closer to the actual position. But I was curious about which part of my original calculations contributed most to the error. Was it the limitations of the concise sight reduction tables or maybe a plotting error on my part or just the random error I threw in? (I did remember to *add in the refraction and other corrections to the "raw" sight data.)
    So, I decided to work backwards and start by checking the plot. I figured I'd cross check it by using my azimuths and intercepts to calculate the fix directly, which brings me to my question: Can the method described in the "Position from intercept and azimuth by calculation." section of the Nautical Almanac be used when you have different APs for each sight, as is required when using the concise tables, or is it necessary to have one single AP?
    I'm no mathematician (to be honest, most of this seems like voo-doo to me), but my intuition tells me the latter would be the case. I say this because I notice there is only one instance of an AP lat/lon in the equations. Also, when I tried it I got a longitude that was very close to the actual, but the latitude was even farther off.
    Thanks in advance for any insights!
    Regards,
    Sean C.
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