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    Fwd: Marine Electronics Issue #18: Fluxgate Compasses
    From: Aubrey O?Callaghan
    Date: 2002 Sep 12, 07:15 -0400

    Here goes in entirity, I was going to edit it but perhaps the other website
    may be of interest.
    There have recently been discussions on COLREGS.
    It's an interesting subscription.
    Aubrey.
    
    >Date: Tue, 10 Sep 2002 17:00:20 -0400 (EDT)
    >From: Ocean Navigator Online 
    >Subject: Marine Electronics Issue #18: Fluxgate Compasses
    >Message-id: <3278348.1031691619908.JavaMail.javaproc{at}10.76.10.10>
    >
    >OCEAN NAVIGATOR EMAIL NEWSLETTER
    >
    >Topic: Marine Electronics        Issue No.: 18        Date: 09/10/0002
    >Title: Fluxgate Compasses
    >
    >This newsletter is a free bi-weekly resource published by Ocean Navigator.
    >You have received this newsletter as a registered newsletter subscriber. If
    >you wish to unsubscribe or feel you have received this in error, please see
    >the information at the end of the newsletter.
    >
    >***************************** ADVERTISEMENT *****************************
    >
    >Subscribe to Ocean Navigator magazine and receive essential, need-to-know
    >information that will put you in control of your boat and make bluewater
    >voyaging more enjoyable. Navigation, Weather, Electronics, Sail Handling,
    >Provisioning, Route Planning, Personal Safety - it's all in Ocean Navigator.
    >Start your subscription today at:
    >http://www.oceannavigator.com/public/subscription/subscriptionLanding.jsp
    >
    >***************************** ADVERTISEMENT *****************************
    >
    >TITLE: Fluxgate Compasses
    >
    >By: Larry McKenna
    >
    >First off, thanks to all of you who emailed greetings. I greatly appreciate
    >all of the "welcome onboard" messages I received. Also enjoyable, in a weird
    >sort of way, were the many harrowing stories about single-platform
    >navigation. The clearest exposition of that thread was by Peter Savage, who
    >wrote, "All this (electronic) stuff is no good unless you use it and use it
    >correctly."
    >
    >Using "it" correctly is what navigation is all about, and the feedback I
    >received indicates that our readership consists primarily of navigators who
    >want to navigate correctly. To me, this means being able to use any and
    >every tool you have to establish and maintain a position estimate at all
    >times. Using a tool is different than operating one. Anyone can operate a
    >many?buttoned black box -- using one takes intelligence and judgment. At the
    >core of judgment is understanding how something works, and thus we arrive at
    >today's topic: fluxgate compasses.
    >
    >Every boat going to sea should have at least two compasses. One MUST be an
    >analog, or mechanical, unit -- one that requires no batteries or electrical
    >connections, most often this is a steering compass, mounted within easy view
    >of the helm. The second compass MUST be a hand-bearing compass of some kind,
    >Either a mechanical or electronic compass will do. Given that this is a
    >marine electronics column, I thought we'd start with a review of how
    >electronic, or fluxgate, compasses work.
    >
    >For many of us, the Autohelm Personal Compass was our introduction to
    >electronic compasses. It was (and mine is, if I can remember where in the
    >chart table I put it) a great, easy-to-use, ergonomically pleasant machine
    >with one great drawback -- it only worked if you held it absolutely flat. On
    >a rolling deck, you became a human gimbal. If you didn't keep the thing
    >exactly level, your bearing would be off, perhaps even way off. Alas, there
    >was no way you could tell this unless you took multiple bearings of the same
    >target. All fluxgate compasses suffer from this problem, though few as
    >severely as the Autohelm.
    >
    >The reasons for this have to do with how fluxgate compasses are constructed.
    >According to Dr. Bill Lee, of AlphaLab Inc., the core of every fluxgate
    >magnetometer is a loop of iron nickel foil. Approximately 10,000 times per
    >second, this loop is magnetized, demagnetized and then magnetized in the
    >opposite polarity by an excitation coil wrapped around the loop. As the AC
    >current in the coil increases, the loop of foil becomes increasingly
    >magnetized. At some point (fixed by the system's construction), the loop's
    >magnetic field saturates and fails to keep pace with the excitation coil.
    >The loop's field remains saturated even as the current in the excitation
    >coil begins to decrease. Eventually, the loop's field decreases, goes to
    >zero and then strengthens in the opposite polarity as the excitation coil
    >goes through the remainder of its AC cycle. Each episode of saturation or
    >desaturation of the loop produces a brief pulse of current in a pickup coil
    >surrounding the foil loop and excitation coil. If the earth had no magnetic
    >field, the foil loop would always saturate at exactly the same point in the
    >excitation coil's AC cycle, and the pulses would be recorded at exactly the
    >same phase of the AC cycle.
    >
    >But earth does have a magnetic field, about half as strong as the one
    >produced in the foil loop. Earth's field penetrates the loop, inducing a
    >magnetic field in it even before the excitation coil starts its cycle. Once
    >the AC cycle starts, the foil loop saturates at an earlier phase in the AC
    >cycle than it should, an event indicated by the emission of saturation
    >pulses, duly picked up by the pickup coils. On the second half of the AC
    >cycle, earth's magnetic field acts to delay the onset of saturation.
    >Combining the two readings gives the strength of the magnetic field in one
    >direction.
    >
    >To get direction, we need to measure the field strength in at least two
    >perpendicular directions. Modern units do this by using two different pickup
    >coils perpendicular to one another. Comparison of these two-axis
    >measurements provides the orientation of earth's magnetic field and hence
    >the magnetic bearing. Typical units run at 10,000 cycles per second;
    >approximately 1,000 cycles are averaged to give 10 readings per second. Some
    >manufacturers use software to select an update rate, but rest assured that
    >any fluxgate compass you have is going through at least 1,000 measurements
    >every time it shows you a single reading.
    >
    >The problem with the Autohelm was that the foil loop was fixed in the
    >compass' housing. Tilting the compass changed the apparent strength of the
    >magnetic field hitting the loop and hence the apparent direction. The
    >sensitivity at North American (magnetic) latitudes is pretty high -- as much
    >as 3? bearing error per degree of tilt! Newer fluxgate compasses solve this
    >in a number of ways. For example, according to Chris Watson at KVH
    >Industries, their Datascope uses a foil loop that floats in a light oil
    >bath. To first order, the ring is self-leveling. The Datascope can be tilted
    >up to 20? from horizontal and still work within the 0.5? accuracy limits.
    >Beyond this angle, however, beware of any readings you take. One might want
    >to measure the horizontal angle between two marks by holding the Datascope
    >sideways, and that's great -- just don't take a bearing at the same time!
    >
    >For those of you with the interest, you can easily see this behavior
    >yourselves. I know this technique works with Autohelms, and Chris Watson
    >thought it might work with the Datascope. Take the compass to the least
    >magnetically disturbed area you can find. Find magnetic north, and then
    >slowly tilt the compass down while observing the bearing. Eventually you
    >will find a point where the inaccuracy of the reading is maximized. The
    >compass is now pointing exactly parallel to earth's magnetic field. The
    >angle of the compass from the horizontal is the magnetic inclination, or
    >magnetic latitude. It was sometimes used in days of yore as a line of
    >position. I'd appreciate hearing from any readers who try this. Send your
    >name, lat/long, the inclination and the type of compass you have to
    >lmckenna{at}oceannavigator.com. For those of you interested in comparing your
    >readings to the real thing, see http://geomag.usgs.gov/usimages.html#us_i.
    >
    >Thanks again for all your emails -- I haven't come close to answering all of
    >them yet, but keep them coming. Practical Sailor recently rated various
    >hand-held bearing compasses, find them at www.practical-sailor.com.
    >
    >
    >
    >-- Larry McKenna
    >
    >lmckenna{at}oceannavigator.com
    >
    >Use of trade names or manufacturers in this article isn't meant as an
    >endorsement.
    >
    >Related Articles:
    >Title: Four hundred degree compass explained
    >http://www.oceannavigator.com/public/action/ArticleAccess?doc=cbghxzsr
    >Title: GPS compass
    >http://www.oceannavigator.com/public/action/ArticleAccess?doc=epghvuob
    >Title: Trickle-down technology
    >http://www.oceannavigator.com/public/action/ArticleAccess?doc=cpjrhssr
    >
    >Author Bio:
    >
    >
    >Larry McKenna is a former Ocean Star instructor and frequent contributor to
    >Ocean Navigator, as well as other publications. Raised in New England, he
    >sails Restless (and any other boat he can get aboard) along the mid-coast of
    >Maine.
    >
    >
    >
    >
    >
    >***************************** ADVERTISEMENT *****************************
    >
    >Subscribe to Ocean Navigator magazine and receive essential, need-to-know
    >information that will put you in control of your boat and make bluewater
    >voyaging more enjoyable. Navigation, Weather, Electronics, Sail Handling,
    >Provisioning, Route Planning, Personal Safety - it's all in Ocean Navigator.
    >Start your subscription today at:
    >http://www.oceannavigator.com/public/subscription/subscriptionLanding.jsp
    >
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    >If you would like to comment on this newsletter, please email
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    >
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