Captain Ron's

Sailing and Cruising Pages

GPS Navigation (continued)

Continuing the discussion of navigation and GPS units; I’m surprised and amazed at the number of sailors out there that have little or no understanding of the navigation instruments and what they are telling you. Perhaps it’s because we assume we know what they’re saying when we’ve never really thought about it. So in the category of ‘there are no dumb questions only dumb answers’, here’s an answer to a question nobody asked.

I don’t know how many sailors have said to me, ” I have quite a discrepancy between what my knotmeter says and what the GPS says my speed is.” Well, yeah, they measure different things. No, they don’t measure speed. The readings they give may be in terms of speed but they are different measurements. The GPS measures your position on the face of the planet, compares that to the last position it had and implies speed from the distance between the two positions and the time it took to cover that distance. In other words, it is using the old time-distance-speed formulas to imply over the ground progress. Some GPS units may have slightly more advanced calculations where they average a certain set of speed computations and report that number just so the reported speed doesn’t fluctuate so much. It is not measuring velocity at a given moment in time although the distances measured can be very short.

On the other hand, your knotmeter is measuring the flow of water over the hull of the boat. In other words it measures water speed. It also does not measure velocity. It is akin to an airplane measuring airspeed. You can have knotmeter readings when you’re firmly anchored provided there’s enough current. Knotmeters are a variation of the ancient mariner’s chip log. I am old enough to remember when we actually had an improved chip log; that is, a propeller driven cable that we dropped over the side to read speed on a dial indicator. It wasn’t all that long ago! And no, I never owed a flintlock rifle either.

Very often the difference between the knotmeter and the GPS reported speeds will be the current either for or against which you are attempting to make progress. Of course, currents rarely flow in the same direction as the boat is moving but still the knotmeter measures water flow past the paddlewheel. A 1.5 knot difference between the knotmeter and the GPS means that there is an ‘apparent’ current of 1.5 knots assuming proper calibration of the knotmeter. Sideways slip and eddies caused by the keel also impact the knotmeter readings not to mention vertical velocity changes as the boat plunges through waves.

To measure actual velocity at a moment in time, you could ask a local gendarme to take up a position on the nearby breakwater and aim a radar gun at you as you streak by. Again, that would be measuring something different.

Different compasses likewise measure different things and it can be helpful to understand the different measurements they take. I’ve often heard sailors tell me about the differences they experience in heading between their steering compass, GPS and ‘autopilot’. That is as it should be. They measure different things.

The steering compass and the autopilot’s fluxgate compass ostensibly measure the same thing; The earth’s magnetic field. They do it in different ways however. The flux gate compass senses the horizontal component of the earth’s magnetic field by it’s changing relationship to 2 or more electromagnetic fields (coils) in the unit. The coils must be gimbaled to keep their horizontal plane and then they must be dampened just like a magnetic compass to prevent wildly swinging readings. Remember too that fluxgate compasses were invented to provide an electronic reading rather than a visual one. A fluxgate compass likely controls your autopilot. Magnetic flux, in case you were wondering, is the earth’s magnetic field. Remember the classroom experiment with a magnet and iron filings? Remember the pattern the filings made on the paper? That pattern is called the flux. That’s what a flux gate compass measures in a relative way. That’s why they must be calibrated to give reasonable approximations of direction, it senses changes in the flux plus or minus but it really doesn’t know what direction that is.

Meanwhile, the old standby, mechanical compass has a magnet attracted (or repelled) by the earth’s magnetic field. In our hemisphere, the card of the compass remains fixedly pointed at the earth’s magnetic north pole; about 82 degrees north or somewhere near Ellsmere Island in northern Canada; meanwhile the boat turns underneath the card. It’s not the compass card that moves, it’s the boat that changes it’s relative angle to the pole. The fact that magnetic north is in Canada and not that close to the true north pole is a clue; no offense. Navigational charts express the difference between the true north and magnetic north (declination) as variation (usually it’s printed in the compass rose). The earth’s magnetic field does move somewhat over time, hence the variation expressed on navigational charts must be adjusted every year for progression changes. The earth’s magnetic field even reverses itself periodically although not predictably, yet.

The GPS implies the boat’s heading from changes in geographical position, without regard to the bow of the boat. The GPS doesn’t care which way the boat is pointed and in fact, can’t tell. It reports simply the pure direction of travel over the face of the earth. It bears no relationship to magnetic north but since it is an electronic computerized unit, computations can be introduced that compensate for declination (the difference between true north and magnetic north) so that some units can be switched from reporting true to magnetic direction; But it is only relative to the GPS unit’s progress over the face of the earth. There is no heading if the boat is stopped.

Differences between the three carefully calibrated units can be fairly extreme given wind and current conditions. A boat heading west according to the GPS, could be pointed north as reported by the steering compass and north-northwest as reported by the fluxgate compass. As a side note, small aircraft (at least) also have a gyro-compass which measures changes in the airplane’s horizontal aiming direction since calibration (calibration is usually performed in preflight when the gyro compass is set to match the take-off runway’s direction).

Anyway, the difference between a boat’s heading on the steering compass and the GPS can represent a measurement of the angular effect of wind and current applied to the hull at that moment. From a practical navigator’s point of view it is helpful to remember that the steering compass tells you where the bow is pointed while the GPS reports your angular progress over the planet’s surface. The fluxgate compass, while it may report differences with the steering compass, should be relatively close to the steering compass if it is calibrated correctly. Mainly the difference between the two is likely the result of deviation or the influence of ferrous metals and magnetic fields in the boat. This can be affected by the location of metal masses vs. the difference in location of the two compasses. In my boat, the steering compass is in the binnacle just behind the engine/transmission mass and centered in the boat while the fluxgate compass is located on the V-berth bulkhead about 5 or 6 feet ahead of the mast and offset maybe 4 or5 feet to port. So some small differences are to be expected.

Keeping the magnetic compass and the fluxgate compass away from iron masses is important although fixed iron sources such as the engine can be compensated for. It is the portable or intermittent ferrous metal/field sources that are problematic. We all know about the effect of a screwdriver carelessly stowed on the binnacle but things such as a strong magnetic field near the fluxgate compass can throw it off too. That’s why my fluxgate compass is located so far forward. In fact, we moved it there after we discovered that the boat’s autopilot would make a 40 degree course change when someone flushed the toilet. The Lectra-san system was throwing a strong enough field to move the fluxgate compass from 6 feet away! It’s hard to beat that for fun things to do in the toilet! Well, maybe not so hard.

Also consider things like your handheld VHF radio. Is it hanging on a bracket near the binnacle? The speaker magnets may be enough to disturb your steering compass.

I know that you all know these things but sometimes it helps to remember them or think about it again. I think it might be neat to integrate the fluxgate compass with the GPS such that GPS headings could be computed relative to the direction the boat is pointed. This might allow a user of a chart based nav system to set a real anchor alarm that measures not just a change in position but a change in position along a heading. That way a change of 100 feet swinging along the anchor rode’s arc would not trigger an alarm but an increase in the arc radius of 6 feet would set it off. Oh well, that’s probably been invented already.

Meanwhile we should all get ready for the next geomagnetic reversal.