Tech: The fabulous fluxgate compass

Compass rose for southern Spencer Gulf, South Australia.
Can you imagine sailing without a compass? Perhaps it’s conceivable in the modern era of GPS navigation, but only an unprepared sailor heads out without a compass (or two) on board. The compass is one of humankind’s oldest navigational tools. Before compasses, sailors only had local landmarks and the stars to navigate by, which would have made venturing forth into unknown waters very daunting.

Undoubtedly you learned in school that a compass works by detecting the Earth’s magnetic field, but here’s a refresher. The Earth has an iron core, the inner part of which is solid (due to the higher pressures down there) and the outer part of which is liquid. Remember how magnetic fields are produced by moving electric charges? It is the rotation of the liquid outer core that produces Earth’s magnetic field. It’s worth noting that there is no guarantee that a planet (or moon) should have a strong magnetic field, however most planets do since the forces that created them tend to involve lots of rotating mass (technically, “angular momentum”). Interestingly, Earth’s nearest neighbors, Venus and Mars, are notable exceptions, so it’s just as well we don’t live there. Besides, the sailing is limited on those worlds.

Side note: Earth's magnetic field also protects us from the ravaging solar winds and its absence would result in the loss of most of our atmosphere, a fate which has already befallen Mars.

Like all magnetic fields, Earth’s magnetic field has two poles, north and south. As navigators know, these magnetic poles are not aligned with Earth’s axis of rotation, which defines “true” north/south, but they are close enough to be useful. The difference between true north and magnetic north is called variation (or declination) which is always published on charts on the compass rose, such as shown above. NOAA has this neat magnetic field calculator.

Note: The north magnetic pole is moving slowly, which will also be indicated on the chart. For example, the chart above shows that it is moving 1'W every year. 

In my local waters magnetic variation is approximately +7, which means true (T) north is 7° east of magnetic (M) north., i.e. 0°M = 7°T. Conversely true north (0°T) is 353°(360 - 7) on the compass (ignoring compass errors - more on that later).

Tip: The mnemonic I like is “Correcting add east” but “Compass least, error east; Compass best, error west” is another one.

A traditional, mechanical compass is just a lightweight, magnetized needle, on a freely rotating pivot (suitably protected). The pivot allows the needle to react to nearby magnetic fields - hopefully Earth’s magnetic field and not some spurious field produced by nearby equipment. Since opposite poles attract, the southern pole of the needle is attracted to Earth’s magnetic north pole.

Side note: The earliest compasses were water compasses invented by the Chinese (circa 206 BC), used for divining not navigating. They were simply a magnetized piece of metal floating in a bowl of water.

The traditional compass has one limitation for the modern sailor. It is an analog device and cannot be interfaced with other instruments on board.

Raymarine fluxgate compass.
Enter the fluxgate compass, which is an electromagnetic device that produces an electrical signal output that can be digitized. The fluxgate compass consists of two or more small coils of wire wound around an electromagnetically permeable core (as opposed to a permanent magnet). An alternating (AC) input electrical current is passed through the first coil which magnetizes the core in corresponding alternating cycles of magnetic saturation. (Remember that magnetic fields are produced by moving electric charges, including Earth’s magnetic field.) This changing magnetic field in turn induces an output electrical current in the second coil.

Now for the really clever part. If the input electrical current was the only thing that influenced the magnetic field of the fluxgate compass core, the output current would be absolutely identical to the input current. How boring! In reality, planet Earth supplies a background field which combines with the induced magnetic field, so the core is more easily magnetized when aligned with the background field and less easily magnetized when opposed to it. As a result, the core’s alternating magnetic field, and the induced output current, will be out of step (phase) with the input current. The difference between the input and output currents is a measure of the strength of background field, with the largest difference corresponding to the magnetic poles. Voila!

There’s only one gotcha. Fluxgate compasses can’t tell north from south until calibrated. So when you turn on a new fluxgate compass for the first time, the displayed heading will be incorrect. Follow the manufacturer’s procedure for calibrating the compass. This typically entails going round in circles a couple of times, which is known as “swinging the compass.” This also enables the software to produce a compass deviation table to compensate for differences between compass readings and corresponding magnetic headings. These small differences are known as the compass deviation (or magnetic deviation), not to be confused with the aforementioned magnetic variation (declination). The infamous TVMDC formula expresses the relationship between these values.

One caveat is that it's important to only measure the horizontal component of Earth's magnetic field, so the fluxgate must be kept as horizontal as possible. Typically this is achieved means of gimbals or by suspending it in a fluid. It's also important to install it properly (shown below).
Leveling fluxgate compass.
That might have sounded complicated but fluxgate compasses are actually simple, reliable devices. There are a few things to keep in mind though.
  • Locate the compass as close as possible to the pitch and roll center of your vessel (admittedly less of an issue on multihulls which don’t roll). 
  • Avoid locations close to other electrical equipment and magnetizable objects, such as pots, pans and iron keels.
  • Recalibrate your fluxgate compass from time to time, since several factors could cause it to lose calibration. These could be as benign as a crew member inadvertently stowing metal objects nearby, or as serious as shore power surges or nearby lightning strikes. 
I think it's pretty neat that you can make a reliable and accurate compass from a couple of coils of wire and some basic electronics. Compasses have come a long way since that Chinese water bowl.