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    Re: New ways to create false horizons?
    From: Don Seltzer
    Date: 2017 Jul 22, 10:11 -0400
    I should clarify that my remarks about using MEMS accelerometers were about attaching them to the frame of a sextant as an electronic equivalent to a bubble sextant for determining when the sextant was very near level.

    As for electronic measurement of the index arm angle, it is certainly doable with high precision rotary encoders (14 bits or more) mounted at the pivot point, but prohibitively expensive.  There are however some other possibilities regarding using incremental optical or magnetic encoders along the scale arc that might be feasible if the market for such sextants still existed.

    Don Seltzer

    On Fri, Jul 21, 2017 at 7:58 PM, Pete Solon Palmer <NoReply_Palmer@fer3.com> wrote:
    Hi Don,

    MEMS  sensor:

    << Scale factor error is a problem for measuring a wide range of angles.  When simply trying to determine a very small range around 'zero' angle it is of minor concern.  >>

    This changes everything that I had imagined.  Thanks; better to know now before cutting materials.   I was previously thinking to put the inclinometer on the index arm where it would need to read a wide range of angles ( Hs 0 to 90, or 10 to 80 degrees).  In light of what you said, it's not a good idea, so we use the MEMS to indicate level where it is in a comfortable range, and then figure out some other way to read the index arm angle.

    << 1 or 2 arc minutes around zero  >>
    That sounds good to me.  With sight averaging and software signal filtering, it might end up resulting better than 1 minute.  That explains why my phone works fairly well as a level, but no good for measuring the Alt of a body.

    What about using a digital angle meter to read the index arm angle?  If that is not accurate enough then maybe a linear measuring device to indicate where the index arm is?  Please don't feel obligated to answer these questions; just thinkin' out loud. 

    Of course we can read the index manually, but something digital would allow rapid averaging to enhance accuracy.

    Someone explained in a previous post about how they do it on the modern jets;  I think I'll try to find that.

    -----Original Message-----
    From: Don Seltzer <NoReply_Seltzer@fer3.com>
    To: globenav <globenav---.com>
    Sent: Fri, Jul 21, 2017 8:55 am
    Subject: [NavList] Re: New ways to create false horizons?

    On Thu, Jul 20, 2017 at 6:13 PM, Pete Solon Palmer <NoReply_Palmer@fer3.com> wrote:
    Maybe there is an electronic AH solution.  The sensors (mag, gyro, accel, gravity) in my smart phone seem to be less accurate than a ball bearing and a toy compass, so I don't see a solution involving phones soon. 

    Ever since I took Frank's course I have been toying around with this idea.  Some years ago I developed some automated test stations for measuring performance of multi-axis MEMS  sensors such as those used in phones and other devices today.

    A two axis MEMS accelerometer is very plausible for determining the gravitational vertical.  The three main error sources are bias offset, scale factor error, and temperature drift.

    Bias offset is easy to deal with, and is conceptually similar to sextant index error.
    Scale factor error is a problem for measuring a wide range of angles.  When simply trying to determine a very small range around 'zero' angle it is of minor concern.
    The most challenging problem is temperature sensitivity of these devices.  Changing temperatures will cause all the other parameters to shift.

    Analog Devices is a leading manufacturer of MEMS multi-axis accelerometers, some costing less than $20.  I once did a rough error analysis of some of their devices and it seemed that it might be possible to achieve a resolution of about 1 or 2 arc minutes around zero.

    Don Seltzer

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