Avoid a GPS-induced incident Page 2

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• Become proficient with your GPS before you need to rely on it. Although most units are reasonably easy to operate, certain functions, such as establishing waypoints and routes, can be tricky if you are not familiar with them. Leafing through the operator’s manual for the first time when conditions are challenging is not good seamanship. Raise your competency level when you really have time to study and learn—when conditions are favorable or when you are ashore.

• A GPS position may be more accurate than the chart on which it is plotted, particularly if the chart was created with sextants, triangulation, and other “manual” devices. The position of an object on these charts can be substantially different—1,000 yards or more—from the GPS position, and the consequences of such a difference can be significant (Fig. 2.)

This point was driven home on a recent cruise I made in the Caribbean when I discovered that the GPS coordinates and those on the (old) chart I was using differed from each other by more than 500 yards. Fortunately, I quickly noticed the discrepancy. If I hadn’t, I could have been in serious trouble when navigating through, say, a narrow channel.

Although charts are constantly being updated, especially those used in popular sailing areas, you should always check the chart’s accuracy against the GPS with visual bearings on fixed objects. Also note the datum printed on the chart. If it says NAD 83 or WGS 84, it is relatively new and you are less likely to have problems.

• Carry a backup GPS unit, because all electronics will eventually fail. I like to carry a spare that is similar or identical to the main unit; if I should have to activate the spare, there’s no learning curve involved. Also be aware that most units go through a cold start the first time you turn them on in a new area; this means it may take several minutes, or longer, to establish a fix while the unit is searching for the satellites. That’s why, if you are passing through navigationally tricky waters, it’s a good idea to start up the spare beforehand; once the spare unit has a fix, you can turn it off. Periodically turning on the spare also allows you to check that its batteries are strong and functional.

• Know how to navigate without having to rely on GPS. Because a GPS unit continually displays your position and other relevant navigational data, it’s easy to get overconfident. However, any unit might stop working. One friend of mine was on a delivery last year when suddenly the GPS no longer displayed the boat’s position. No problem, he thought, and he turned on a spare unit. But it wouldn’t display a position either. Two additional units also failed to give a position. The situation could have been serious because he was approaching the entrance to Chesapeake Bay in near-gale-force conditions. Fortunately, he is a proficient navigator and had been keeping a log; he was able to use dead-reckoning to plot and track his position on a chart, and he made a safe landfall. Twelve hours later, all the GPS units began working again.

Although he never got a definitive explanation, it’s likely he passed through a military exercise; GPS units are very easy to jam.

• Never rely exclusively on GPS. Learn piloting and dead-reckoning techniques and maintain your proficiency through practice. You can also use these techniques to double-check your electronics. During a delivery from Maryland to Newport, Rhode Island, we were sailing offshore near Cape May, New Jersey.

It’s a tough stretch of water, and full of shoals. Although the boat had a differential GPS and a state-of-the-art chartplotter, we used four different data sources simultaneously: the GPS/chartplotter, the depthsounder, radar, and visual observations that included watching for breaking waves. We had a paper chart laid out and plotted our position on it frequently. Overkill? Perhaps. But if the GPS had somehow malfunctioned, we would have known immediately. And, more important, we would have known exactly where we were.

It’s true that GPS has revolutionized navigation; when you use it, you know exactly where you are. But this has created another navigational challenge: You need to use GPS properly, as well as traditional piloting and dead-reckoning techniques, to get you where you want to go both safely and efficiently.

Larchmont, New York–based Steven Henkind, M.D., Ph.D. is a marine safety consultant who has served as captain, navigator, and crew on sailing vessels up to 295 feet. He also holds a 200-ton U.S. Coast Guard license.

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