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    Re: The development of bubble sextants
    From: Hanno Ix
    Date: 2009 Aug 17, 20:57 -0700
    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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