WAAS Up? - SailNet Community
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post #1 of 1 Old 05-15-2001 Thread Starter
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With WAAS-enhanced GPS, every nav station can be as up to date as this one.
The sailing press is abuzz with a new acronym—WAAS. It stands for Wide Area Augmentation System. If WAAS isn’t already familiar to you, it soon will be. Before this year ends, it’s not likely that you’ll see much advertising for GPS units that doesn’t also proclaim WAAS.

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.

WAAS-capable GPS units can give you position information within five feet, but is that entirely necessary?
While "Wide Area" coverage may be the defining feature for the system’s designers, the most endearing characteristic for sailors is that WAAS does not require any special equipment. No supplementary receiver. No additional antenna. Augmentation data is transmitted from overhead on the same frequency as the GPS signals. The only real difference between a standard GPS and one that is WAAS-capable is the internal software. And because this is a government-funded (FAA) project, the software is in the public domain. Translation: GPS receivers with WAAS capability cost no more than those without it.

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.

That’s nice.

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."
Sailing is a determinedly uncertain activity, beginning with which tack we are going to find ourselves on when we drop the mooring. Once underway, we are in concert with—some some would say at the mercy of—the wind. If it blows hard, we go fast—unless it blows too hard. If it dies, we stop. If it changes direction, we may be forced onto a new course. If the shift is enduring, it can lead us to change our destination. We start out for Point B but end up at Point C.

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?

Is digital dependence prompting us to go offshore at times when we shouldn't, leading us to fates like the one above?
The prudent question, it seems to me, is why not wait? To press on suggests that where you want to go is better than where you are, that a tense day in the soup is better than a relaxing day on the hook, that being wet all day will be better than being dry, that reading a good book, baking bread, or catching up on maintenance promises less satisfaction than a few more clicks of the log.

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. "
Is it just me, or does this feel less like sailing and more like a video game? No dinghy sailor would willingly give up steering and hiking and playing the shifts, but the willingness to become a passenger rather than a sailor seems to increase with the size of the boat.

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.

Once in a while, says the author, leave the digital screens dark  enjoy the simple pleasure of navigating the old way.
I am not arguing against electronics. Interfaced charting is—well—at the very least, tres cool. Radar can save your bacon when fog rolls in. And when the night is dark or the weather malignant, who wouldn’t be happy to have aboard a positioning system so accurate it can tell you whether the receiver is on the port or starboard side of the boat?

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.

Suggested Reading:

Advanced GPS Interfacing  by Jim Sexton

What is Differential GPS  by Jim Sexton

Navigating with Radar  by Jim Sexton


Buying Guide: Chartplotters


Don Casey is offline  
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