Last week Richard Brill in his science column explained the world's modern navigation technique, GPS, Global Positioning System. This is rocket science at its best, and I enjoyed learning more about how it works.

Still, the system feels like magic. Push a button and an oracle from space shows me where I am, how fast the boat is going and what time I'll get to my destination.

I would never have sailed off to the South Pacific without my user-friendly, color-coded, GPS-map at the wheel. But it has a downside: co-dependency.

Several times during my offshore passages, I heard a crew member call out, “;Susan, the GPS is dead.”; It's scary enough out there with everything working, and this shout used to unnerve me.

GPSs, however, are addicted to electricity and my boat has a design flaw. The master instrument panel is located on a wall people have to pass on the way to the head. In a pitching, rolling boat it's easy to bump the exposed toggle to the GPS, cutting off its power supply.

This has happened so many times that now when I hear a GPS call, I go to the panel below and flip its switch back to the “;on”; position. The GPS gets its fix and everyone is happy.

We sailors have additional problems. One dark and stormy night, somewhere between Fiji and New Caledonia, I was asleep in my bunk when the boat's motion made a dramatic change. I jumped up and rushed to the cockpit where my friend Scott was peering at the GPS and pushing its keys. “;Something's wrong with the GPS,”; he said. “;It's gone crazy.”;

Since he knew the machine better than I did and it still had power, this was bad news. My heart sank. But when I moved to the wheel next to Scott, I noticed something he had not: We were going the wrong way. During a passing squall, the swirling wind had turned us around. The GPS was correct.

GPS stories are common in cruising guides these days. While sailing in northeast Australia, a guide to the area came with a surprising warning: Do not think your GPS guarantees a safe passage across the Torres Strait.

This 93-mile-wide gap between Australia and Papua New Guinea is a maze of reefs and islands, and notorious for shallow water and strong currents. It's also an international sea lane, being the route freighters, oil tankers and container ships use in passing between the Pacific and Indian oceans.

What? I thought. How can GPS not work there?

It works just fine there. The problem lies in its users. Since GPS units have become cheap and easy to operate, inexperienced sailors are nonchalantly crossing the strait, causing near-collisions and giving ship captains fits. Australian maritime officials are not amused.

I didn't cross the Torres Strait, but I did cross the Sea of Cortez and the GPS in some places there is off up to 2 miles. It's not the GPS's fault. The coordinates are correct. It's the charts that are wrong. These charts haven't been updated since they were drawn in the 1890s.

I use eyeball navigation there and it's a good thing. Once after we were safely anchored, Craig and I looked at the GPS. It showed Honu sitting on a mountaintop.

The GPS is a remarkable invention, and its pluses far outweigh its negatives. Still, another piece of space magic gives me even greater peace of mind: my satellite phone. That oracle speaks.