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    Re: automatic celestial navigation
    From: W F Jones
    Date: 2007 Dec 06, 22:20 -0500

    I have briefly looked at the preliminary technical datasheet for the
    ADIS16209 device and found the following information:
    
    1)  The tilt sensing system uses gravity as its only stimulus, and
    a MEMS accelerometer as the sensing element.  This results in
    responses to dynamic forces associated with acceleration and static
    forces, such as gravity.
    
    2)  Corrections are included for several error sources so that
    high accuracy is maintained.  Although the device is factory
    calibrated, user offsets may be applied for calibrated acceleration
    data and accurate incline angles.
    
    3)  Claims are made that dual-axis tilt error is not more than
    0.1 degree across a +/- 30 degree range in a horizontally mounted
    setup..  14 bit inclination resolution is  0.025 degree.  14 bit
    acceleration data is 0.244 mg resolution (gravity = 1 g).
    
    4)  Applications include platform control, stabilization, and tilt
    sensing, inclinometers, leveling motion/alarm devices (security,
    medical, safety), navigation and surveying equipment, satellite-
    antenna-stabilization systems, automotive-wheel alignment and
    autonomously piloted farm equipment, etc.
    
    I would of course like more technical data and even several
    application notes describing useage but it is early for such
    documents since this device hasn't been around long.  In fact, some
    of the data sheets indicate "TBD" for the perfomance charactistics
    portion.  I have not been able to discern just how fast the data
    throughput is since it will be dependent upon the chosen
    clock/sample rate but it is very likely a matter of milliseconds.
    Usually, sample rate, serial transfer rate and power dissipation are
    related and the designer has to determine what combination will
    best do the job.
    
    This device might be a key item in the electronics package of any
    futuristic sextant.
    
    Frank J
    Rochester, NY
    
    
    =================================================
    From:               "George Huxtable" 
    To:                 
    Subject:            [NavList 4240] Re: automatic celestial navigation
    Date sent:          Thu, 6 Dec 2007 15:45:07 -0000
    Send reply to:      NavList@fer3.com
    
    [ Double-click this line for list subscription options ]
    
    
    Frank Jones wrote-
    
    | I recently noted in the November 22, 2007 issue of EDN (Electronic
    |
    Design News - <>), page 19 a brief announcement
    |
    regarding a MEMS-based (microelectromechanical-system) |
    inclinometer
    apparently capable of accurately measuring the | deviation from
    vertical by sensing the downward G force.  The | device/s is
    described
    as an Analog Devices (<>) | ADIS16209. | | All
    the key
    elements are obviously available to designers for | construction of an
    advanced sextant now, even one that | automatically tracks objects
    day
    or night.  I suspect a gimballed | arrangement would make the
    software
    engineering easier although | it might be avoided at the expense of
    much more complicated | tracking schemes.  I recently saw optical
    sensors for similar | systems go for several hundred dollars each on
    eBay.  The cost of a | 'new' sensor like these in small quantities
    would be much pricer. ====================
    
    Comment from George-
    
    If they have somehow succeeded in disentangling the "downward G
    force"
    from all the other accelerations that apply to a sextant in a vessel
    in waves on the sea, then there may be a future in it. But I doubt if
    they have. In which case, it would suffer from all the defects of a
    pendulum or spirit level or other form of artificial horizon, that
    render them all unusable at sea.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
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