Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: bubble sextant test results
    From: Paul Hirose
    Date: 2003 Jan 2, 21:30 -0800

    [About half a second before my reply went out, too late to stop it, I
    saw that it was going to Jared Sherman only. Since he did ask the
    question on the Navigation list and it's on-topic, I will repost my
    reply to the whole list.]
    Jared Sherman wrote:
    > Paul, how did your position fixes agree with where you actually were?
    I didn't take any fixes, just shot the Sun when convenient. There was
    no attempt to plot LOPs or cross them for a fix.
    Since I shot from a known position, the observed altitudes should have
    matched the calculated ones obtained from the USNO Web page. Any
    discrepancy would be due to nonstandard refraction, personal error
    (i.e., systematic tendency to position the body off center with
    respect to the bubble), instrumental error, or random error.
    Refraction I'm disregarding, so that leaves the last three. The
    personal and instrumental errors can't be distinguished by any simple
    test I know, and since I'm the sole user of these instruments, there's
    really no reason to do so. I simply took the mean discrepancy (Ho - Hc
    for all observations) and called it "index error". That was +12
    minutes for the A-12 and -1.4 for the Kollsman. These are the amounts
    my observations were offset (on average) from the expected values.
    After subtracting this bias, the individual observations will still
    have random plus and minus discrepancies with respect to the expected
    values. The size of these is expressed by the standard deviation: 1.8'
    for the A-12, 1.7' for the Kollsman. What this means is that after I
    subtract 12' to remove the index error, 2/3 (practically) of my A-12
    LOPs would lie within 1.8 miles of my position, if I plotted the
    observations. And that is the bottom line.
    It's surprising a near-novice can do so well, since my A-12 requires
    centering a 30' solar disk in a bubble image, the inner ring of which
    is about 120' in diameter, and doing it without the aid of any
    magnification. Furthermore, the Sun and bubble don't move together as
    in a marine sextant. The Sun moves around with respect the bubble, so
    you must mentally average out the wobbles.
    I made a special effort to work fast, terminating each observation as
    soon as the coincidence looked correct. "Miss 'em quick" was the
    saying among the old U.S. Coast & Geodetic Survey triangulation
    observers. They found that fussing with the theodolite added no
    accuracy, wasted time, and tired the observer.
    Of course when using the Kollsman's averager you do have to sustain
    the coincidence. But on the other hand it runs for two minutes so any
    given instant is not critical. You just maintain a good overall
    picture and trust the pluses and minuses to cancel.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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