Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    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.
    >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@>
    >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:
    >***************************** 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
    >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
    >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@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
    >Use of trade names or manufacturers in this article isn't meant as an
    >Related Articles:
    >Title: Four hundred degree compass explained
    >Title: GPS compass
    >Title: Trickle-down technology
    >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
    >***************************** 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:
    >***************************** ADVERTISEMENT *****************************
    >To unsubscribe from this newsletter or change your email address, please go
    >to http://www.oceannavigator.com/members/subscription/newsletterUpdate.jsp
    >To subscribe to additional topically-focused free email newsletters from
    >Ocean Navigator, including Weather and Offshore Safety/Medicine, please go
    >to http://www.oceannavigator.com/members/subscription/newsletterUpdate.jsp
    >If you have subscription questions, please email
    >If you would like to comment on this newsletter, please email
    >If you would like to provide editorial suggestions, please email
    >For information on advertising in this newsletter, please email
    >Ocean Navigator is committed to protecting your privacy. To view our Privacy
    >Policy go to http://www.oceannavigator.com/public/abouton/privacyPolicy.jsp
    >Please visit Ocean Navigator online at http://www.oceannavigator.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site