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    Re: The development of bubble sextants
    From: George Huxtable
    Date: 2009 Aug 25, 10:10 +0100

    Peter Hollings wrote, in [9616],-
    "I think electronics has opened up better and cheaper solutions than
    contemplated by Vannevar Bush.  Here is a project in the field of robotics
    from a few years back that addresses a similar problem of maintaining
    vertical orientation: http://www.dprg.org/projects/2003-01a/ . A Google
    search for "integrated circuit accelerometer" turns up a large number
    interesting and relatively inexpensive devices. Similarly, there are
    compasses, etc. In fact, I believe a whole Sextant might be automated and
    probably the military has done so."
    Response from George-
    I have severe doubts about that. There's a world of difference between an
    application that needs to know little more than "which way is up", and an
    instrument which is capable of sensing the vertical to a minute, or a very
    few minutes, of arc. In respect of the stability of the output, in terms of
    sensed angle of a piezo accelerometer, particularly against changes in
    temperature, these devices are at present woefully lacking. They are, of
    course, getting better all the time, but I suggest they have a long way to
    go yet before doing the same job as a precise bubble.
    The same fundamental problem, in distinguishing gravitational acceleration
    from the unwanted accelerations caused by wave disturbance, faces such
    electronic devices, just as it challenges mechanical contrivances such as
    that of Bush. The combination of accelerometer with a vibrating "gyro" can
    indeed help to filter out the wheat from the chaff. In that respect
    electronics does have a distinct advantage over mechanics, in that precise
    filtering is easier to implement in circuitry than it is by weights,
    springs, and viscous damping.
    Such combinations could do a useful job, aboard, in applications where less
    precision is required. I'm thinking particularly of providing compensiation
    for ship's motion for a magnetic sensor, in making a strap-down magnetic
    steering compass that avoids gimballing. We've discussed such matters
    before, on this list.
    If more sophisticated technology (the ring-laser "gyro"), ever comes down
    from present stratospheric prices to those that the rest of us can
    contemplate, then indeed a precise electronic artificial horizon could
    become very practicable.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
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