November 14, 2013

No need for navigation precision

THERE MUST BE VERY FEW of us who don’t use GPS for navigation these days, but I get the feeling that we often wonder what would happen if those faint electronic signals from those distant satellites suddenly stopped coming.  Would we be able to navigate with sufficient precision to get safely into port?

I was often bothered by that nagging thought until I realized that yacht navigation wasn’t what you might call precise in the days before GPS, and still doesn’t need to be now.  The very movement of the boat rules out 100 percent precision at sea, whether land is visible or not.

At first, you might be dismayed and frustrated at how difficult it is to get decent cross-bearings of features on shore, for example.  Working with a hand bearing compass in a seaway can drive you crazy.  Then, gradually, you realize that whatever you can get is good enough, and you relax, just as the old-timers did.

You develop a kind of feeling for it, and if you suspect your bearings aren’t accurate within 10 degrees either way, you sort of adjust them before you plot them on the chart, and you use with animal cunning the scraps of information they provide. This is why navigation is as much an art as a science.

The point is that you’re not kidding yourself, whether you’re working with a hand bearing compass on a costal passage or a sextant at sea. Any fix is better than no fix — but at the same time, you know that only a fool would trust it implicitly.  You, on the other hand, not being a fool, would interpret your fixes in the most pessimistic fashion and set a course that might not be the shortest or most convenient, but should certainly be the safest.

I often hear long-distance cruisers say that their back-up, in case of GPS failure, is another GPS receiver.  But the problems that might lead to a lack of navigation data do not necessarily stem only from hardware or software glitches on your boat.  The U.S. military has shut down the system for a short period before now, and who knows when a GPS satellite will develop a fatal wobble, and crash into the others, and bring the sky down on our heads?  A second GPS isn’t going to help in that case.

When all is said and done, you can still trust your arm, your eye, your compass, your sextant and good old man sun far more than you can trust a silicon chip or a bunch of wire and solar cells hurtling around the ionosphere.

Today’s Thought
Science is some kind of cosmic apple juice from the Garden of Eden. Those who drink of it are doomed to carry the burden of original sin.
— Lewis M. Branscomb, Director, National Bureau of Standards, 9 Apr 71

Tailpiece
“What’s that bite mark on your arm?”
“Oh,  I was at the vet’s office when a man came along with a basket and said: ‘Watch out, there’s a 10-foot snake inside.’ ”
“What did you do?”
“I laughed and said: ‘You can’t kid me. Snakes don’t have feet.’ And I opened it.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

4 comments:

biglilwave said...

One thing is for sure John, and that is no matter where you go there you are.
By the way, what's a compass? ;)

Ken said...


It used to be knowing exactly where you "were not" to remain safe, now you always know exactly where you are.
The new fun of modern sailing, following a pretty line with a little boat icon on a color screen
two feet from your face. Very challenging ...not to fall asleep.

Matthew Marsh said...

The GPS orbits at 20,200 km are pretty much free of other satellites, so they'd likely be OK if a collision cascade were to occur. And, at least officially, selective availability has been removed from the newest satellites- they can't be shut down without also crippling the US military tht pays for them.

But you're right, there's a lot of other stuff that could make the signal unavailable. Actively jamming the GPS signal over a few square kilometres is trivial. Interference, a loose wire, a corroded solder point, or a dozen other things could make your receiver cut out. And if lightning takes out all the electronics on the boat, your spare GPS / GLONASS / Galileo / Beidou receiver might be useless even if all four constellations are still transmitting.

GPS is, I think, best used as one navigation aid among many. It gives you a starting position that you then confirm with compass, eye, sounder and perhaps radar. Placing blind trust in it can be dangerous.

57 degrees North said...

I realize I'm late to the party here, but nonetheless...

It fascinates me how many people these days are simply terrified of leaving the harbor without GPS, chart plotter, radar, satellite weather download thingy for their laptops... Back in my salad days we used to work offshore in the Gulf of Alaska all the time. This was pre-GPS of course, and while there was Loran-C it was notoriously wonky, and generally no better than dead-reckoning anyway so many folks didn't bother.

To get home, you simply steamed a reciprocal heading, which was usually in the ball park. After all, missing the coastline of Alaska would be no mean feat of navigation. Coastal charts and a smattering of local knowledge usually did for the rest.

One of those odd geographic features is on clear days the tallest mountains always pop over the horizon first, which gives you a nice visual fix on position. Obviously not as accurate as that electronic thingummy, but plenty good enough for us Neanderthals.