What is WAAS? It is a bit of high-tech wizardry that improves GPS accuracy to around five feet. In principle it is not unlike DGPS (Differential GPS) in that the position based on GPS satellite transmissions is compared to a known position. However, DGPS simply calculates the amount and direction of the error at that location at that moment and notifies all "local" DGPS receivers. The DGPS system uses pre-existing radio beacon towers—the old RDF system—as the reference locations, and transmits a correction factor from each tower. DGPS receivers within range assume that applying the same correction will better reflect the actual position of the receiver and they shift their read-out positions accordingly.
WAAS is much more sophisticated. It doesn’t just measure the position error and send it out as a kind of electronic "local knowledge." It actually identifies various causes of GPS inaccuracy and broadcasts a stream of data that lets the software in a WAAS-capable GPS receiver calculate an extremely accurate local position. WAAS transmits this data from an array of geostationary satellites (not the GPS satellites). This allows virtually worldwide coverage (when fully operational)—in contrast with the near-coast coverage of DGPS.
Since Selective Availability was turned off last year, a regular GPS position typically locates you within 30 feet of your actual position. If you plunked down an additional $300 for the receiver and antenna required for DGPS signals, this combination cuts the position error in half—to around 15 feet. Now we have five-foot accuracy available using WAAS.
The yawn that you no doubt perceive in that last sentence is because I can’t help but wonder if we sailors really need five-foot accuracy. And an even more troubling question for me is whether sailing is enhanced by our increasing attempts to reduce its uncertainties.
|"Sailing is a determinedly uncertain activity...if the wind blows hard, we sail fast."|
This is acceptable because we expect it. That is the game. Take whatever wind we get and adjust the sails to keep the boat moving efficiently/comfortably. If we can’t sail in the direction we want to go, then we tack, sailing about three miles for every two made good—but who’s counting. Or we yield to Mother Nature and embrace an alternate destination. Sailing defined this way is fun. It is usually relaxing. It is often surprising. It is never disappointing.
The capabilities of modern electronics change that equation. They have a disturbing tendency to embolden sailors, often unwisely. Radar, for example, can "see" through fog, so if you have these electronic eyes, why let a blanket of fog defer a planned run?
Cartographic GPS offers us another example. If you cannot determine your exact location because of haze or rain or fog or gloom of night, not to worry. The unit’s postcard-size screen graphically displays your exact position in relation to nearby hazards. The unblinking certainty of this electronic "navigator" can lull you into a patently false sense of security. Other vessels and floating debris are not on your display. Nor recent underwater obstructions. And rocks, markers, and shorelines may not be where the display depicts them. (Even the manufacturers of this equipment warn that the charts lack the detail and accuracy of a real navigation chart.) If it causes you to relax your vigilance, a cartographic GPS puts your vessel at increased risk, hardly an enhancement of the sailing experience.
Charting software has become the rage among those with deep enough pockets. With your GPS connected to an onboard computer you can watch a boat-shaped icon trace your track on a real chart, one identical to the printed variety. There is little need to practice any traditional piloting skills. Your position is plotted automatically by the computer. Add a link to your autopilot and you can do all your navigation before you leave the harbor. Point and click a few waypoints and the computer will do the rest, steering the boat and taking up new headings as you reach each waypoint.
|"I don’t want to go for a boat ride. I want to go sailing. It's the tactile experience of sailing that attracts me. "|
I don’t want to go for a boat ride. I want to go sailing. It is the tactile experience of sailing that attracts me. I am attracted to the boat as a living thing as it dips and swoops, as the sail expands and deflates like an air-pumping heart, as the rudder reacts to the passing water like a child flying her hand out the window of a speeding car.
I am no Luddite. I have a GPS and I use it. The irrefutable value of a reliable fix that is not dependent on clear skies is not lost on me, but the thing is, GPS removes nearly all doubt—exactly what it was conceived to do—about whether the island I am sailing for is going to pop up over the horizon. This may not detract from the satisfactions of the passage, but it does diminish the sense of accomplishment. Making an accurate landfall is no longer an affirmation of my skill as a navigator; the credit belongs to a faceless computer programmer in Kansas.
Still, the days that you really need your electronics are not likely to be among your fondest sailing memories (would the day have been better spent doing something else?), and when the horizon is hard, the ocean sparkling, onboard electronics are simply a distraction.
So here is my suggestion. The next time you go sailing for fun, leave all the screens dark and turn all of your attention outward, to the water, the sky, and the changing face of the shoreline. Dig out your hand-bearing compass. Estimate your speed. Find your way based on your own skills and abilities. You just could rediscover some satisfactions that you had forgotten sailing could provide.
Advanced GPS Interfacing by Jim Sexton
What is Differential GPS by Jim Sexton
Navigating with Radar by Jim Sexton
Buying Guide: Chartplotters
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