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    Re: The development of bubble sextants
    From: Gary LaPook
    Date: 2009 Aug 18, 12:06 +0200

    I've never seen one of these for sale on ebay nor have I ever seen a
    mention of it in any navigational manual. Just because you can get a
    patent for something doesn't mean it will work.
    
    gl
    
    
    Hanno Ix wrote:
    > Gentlemen:
    >
    > Please refer to:
    >
    > US Patent 1,912,358
    > V. Bush: Apparatus for establishing an artificial datum
    > Filed April 8, 1928
    >
    > The inventor means an artificial horizon. Please refer to the
    > attachments for the drawings.
    >
    > He has implemented a mechanical lowpass filter. We have discussed LP's
    > before. Better yet: This filter simultaneously works in 2 coordinates,
    > roll and pitch.
    >
    > He also points out that the natural oscillating frequency of the
    > mirror should be as "low as possible" but he doesn't give any data.
    > Since the roll frequency of ships is rather low to begin with (judging
    > from you-tube: about. 0.1 Hz = 1 period per 10 sec for an oil-tanker )
    > the filter has to have a cut-off frequency of about 0.01Hz  which
    > amounts to 1 period per 100 sec. The filter also needs to be
    > sufficiently damped. The reason is simply avoiding resonant
    > oscillations of the filter/mirror in response to the ship's movements.
    > To build an LP of this kind is the challenge!
    >
    > The inventor has made every effort to decouple the housing of the
    > mirror from the body of the sextant. So to speak, he created something
    > like a bubble level floating within bubble level.
    >
    > With a lowpass of this kind, influences from pitch/roll would be
    > reduced by a factor of 100, possibly more. So, a 10 degree roll would
    > create 0.1 degree (6 arc-min) deflection of the mirror. Smaller ship
    > will have higher roll/pitch frequencies than oil-tankers, maybe 0.5
    > Hz. Accordingly, on smaller ships 1 arc-min oscillation of the mirror
    > might be possible. Is that error sufficiently low given the
    > circumstances? It is certainly much less than I saw once in a bubble
    > sextant.
    >
    > Perhaps most importantly, this patent points out how to separate
    > accelerations of the sextant which are instantaneous from gravitation
    > which is constant in time.
    >
    > When I made my proposal with the tubular ring the other day I had
    > similar ideas in mind, however I had not fully understood the
    > interaction between bubble and spirit.
    >
    > Regards
    >
    >
    >
    > >
    >
    > ------------------------------------------------------------------------
    >
    >
    > ------------------------------------------------------------------------
    >
    >
    > ------------------------------------------------------------------------
    >
    
    
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