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    Re: Do We Still Need to Use Sextants?
    From: Brad Morris
    Date: 2013 Mar 28, 16:44 -0400
    On Mar 28, 2013 4:31 PM, "Don Seltzer" <timoneer---.com> wrote:

    Brad wrote:
    If all it took was a 0.1 hertz low pass filter (which could easily be realized with digital software) then cell phone inertial navigation would be resolved. I think there's much more to it than that.

    A 0.1 hertz low pass filter also means a very low frequency response. Hold that device very steady for a long time.
    ----------

    Brad, I didn't mean to imply that low pass filtering was all that it took. Rather that low pass filtering was one of several requirements for any chance of performing useful celestial navigation.

    To summarize,
    For a MEMS dual axis accelerometer to determine a vertical reference from gravity with sufficient accuracy for useful celestial navigation, I estimate that the basic sensor accuracy needs to be on the order of .15 milli g's of acceleration (g for gravity, not grams).

    The most significant errors that must be dealt with are:
    1. Temperature sensitivity of bias offset and scale factor
    2. Inherent random electrical noise related to operating bandwidth
    3. External forces/vibration

    Other less important errors also exist (cross-axis sensitivity, axis scale factor mismatch, etc) but these can be corrected for in straightforward factory calibration.

    #1 might be corrected for with individual unit calibration over the entire temperature range, and subsequent active temperature compensation during use.

    #2 and #3 require low pass filtering of the sensor output, probably to a bandwidth of .1 Hertz or less, implying sighting times of 10 seconds or more. Hand held operation does not seem practical to me. A steady, well-damped tripod would likely be necessary.

    I think that it would make a great project for a college student.

    Don Seltzer

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