I HAVE a meeting this morning. Here are the particulars:

Time: 10 a.m. Place: 39 degrees 08" 39 seconds North, by 77 degrees 15" 17 seconds West. That's it; that's all I know. I have to be at this precise spot on Earth in about one hour, and all I have by way of an address is a set of coordinates in latitude and longitude.

As an experiment, I'm turning over my schedule this morning to a little black box about the size and shape of a decent cell phone. It's a Magellan GPS Blazer, one of the hand-held navigational devices that has caught the fancy of gizmo-minded backpackers all over thecountry. Gear magazines, catalogues and outfitters are filled with glossy hype for GPS (Global Positioning System). Proponents claim the technology, which lets you fix your position anywhere on the globe by locking on to the constellation of satellites orbiting above, is nothing less than the long-sought cure for being lost.

I've been dubious. Trail partners of mine are already on their second or third generation of GPS units (like all electronics, these are getting better and cheaper all the time). But I've resisted, in part because I have a Luddite resistance to flavoring my time outdoors with too much high-tech complexity -- the opposite is what I seek in the wilderness. But also, I've had my doubts about how useful GPS really is for the kind of hiking most people do -- that is, straightforward walks over well-marked trails.

"Oh, no question that half the people we sell them to use them as high-tech toys," says Don Rhodes, a GPS guru at Hudson Trail Outfitters in Gaithersburg. "They're fun. It's your space technology at work. But you'd be surprised how many people go out and are awfully glad they've got one of these along. They work."

Rhodes offers to lend me a Magellan Blazer for a few weeks of field testing during two outings I had planned this spring: four days of kayaking in northern Florida and a week's hiking and car camping in southern Utah. After that, we agreed to meet for a debriefing. Specifically, we agreed to meet this morning at 39 degrees 08" 39 seconds North, by 77 degrees 15" 17 seconds West.

I turn on the Magellan, and the screen -- a bit smaller than a playing card -- lights up. A small radar circle appears within the screen, sweeping the sky for satellites. Within four minutes, it has locked on to three of them and immediately tells me exactly where I am on the planet. I punch in Rhodes's coordinates, and the unit tells me it's a spot 17.7 miles to the north, northwest of me. A small arrow points the way. I climb in my truck.

Now, if I follow the arrow exactly, I'll drive through my neighbor's living room and then into Sligo Creek. When driving -- and even when walking -- you often have to override the GPS's literal instructions with a little common sense, and/or some additional information. In this case, it's not hard to plot the points on a map and see that they fall somewhere in the 6,500-acre Seneca Creek State Park. I can drive normally to the park and then turn myself over to technology at the front gate. (I still keep the machine on, watching the miles fall away in the "distance to destination" box as I zoom up I-270).

Inside the park, I plug in a second set of numbers supplied by Rhodes -- his recommended parking spot. I follow the arrow -- and the road -- until the unit tells me I've arrived. It's a perfect landfall -- an obscure wide spot on the road not far from Clopper Lake. I climb out, and reprogram for the meeting point. It's about seven-tenths of a mile away, into the woods. I head in, with the little digital arrow as my North Star. It tells me how fast I'm walking (about 4.3 miles an hour), my exact bearing and heading at any given second and the distance to my destination. Mainly, I just follow the little arrow. When it says veer left, I veer left. When it says veer right, I veer right. Soon, it has me going in circles, and I realize I'm here. 39 degrees 08" 39 seconds North, by 77 degrees 15" 17 seconds West -- a lovely spot on the lake shore. I wait.

A few minutes after 10, a figure appears over the low hill. He's doing the distinctive GPS walk: head down, staring at the box in his hand, swerving this way and that. It's Rhodes. He looks up and catches my eye with a broad I-told-you-so grin.

"You made it," he says. "It works."

I admit to being impressed. It's accurate, and it's fun. I've never enjoyed finding my way to a meeting so much. But this is the play part. I want to talk about real-world, outdoor applications. He plies me with questions about my recent experiences.

First, in Utah I used GPS not at all. My hiking there was always with my young kids in the frontcountry of the big national parks. Walking those well-worn, well-marked trails is like walking a sidewalk. Getting lost didn't seem remotely possible.

Rhodes made the obvious point. "Ah, but if you had ventured off the trail and into the bush, where the landmarks are hard to find, GPS can be extremely useful." Rhodes himself had that experience on Mount Rainier, when a GPS unit allowed him to get oriented in near white-out conditions. I've done plenty of bushwacking without a GPS and never had problems. But that's luck. If things did get ugly, I can see that being able to call on NASA technology would be comforting.

Second, on my southern paddling trip, I actually found GPS capacity to be genuinely useful -- not critical, but useful. I was on the upper Suwannee River, a 30 mile-stretch that was completely free of road crossings or other clear landmarks. I wasn't lost, per se, but I couldn't tell where I was on the river, how much time I was making, or whether I was going to be grossly late or grossly early for my rendezvous in two days. Sure enough, the Magellen gave me exact coordinates, pinpointing my position on the chart and allowing me to track my progress.

In other applications, I've seen GPS perform with mixed results. Traveling with a team of biologists in the Amazon basin, I saw them use GPS to record exact spots along their survey routes. They were delighted. But in northeastern Cambodia, I saw an older GPS unit fail to even spit out our location. The person using the unit never could identify the bug.

Even when consumer GPS units are working perfectly, they're not. That is, the military insists that these civilian devices come with a degree of built-in inaccuracy. In theory, the technology can pinpoint a spot to within centimeters; in the reality allowed by the Pentagon, it will put you within a few dozen feet. That's almost always good enough for backpackers.

Rhodes, a former Army artillery surveyor himself, says all of today's consumer units are more than precise enough for outdoor navigation, even the cheapest. Prices range from about $100 to $320, with the Magellan Blazer costing about $130.

"All of them are equally accurate," Rhodes says. "All of them use the same satellites. For the extra money, you get more features -- more memory, an external antenna. But they all work, and they've all gotten better at picking up weak signals. I locked onto six satellites in my bathroom the other day."

The day I need help navigating in there, I'll get a walker, not a GPS. But I take his point. The GPS is easier to use and less intrusive than I thought. It takes a few minutes to initialize every time I turn it on, and every now and then it goes crazy. The military-mandated variance, the hand-held wobble and the sporadic interruption from trees and other obstacles makes the thing's head spin every now and then. But it always seems to right itself quickly and give basically sound guidance.

Rangers have mixed feelings about high tech on the trail. Cell phones have added significantly to their caseloads as people call for help at the slightest hint of being lost. GPS, too, can lead to overconfidence if you let it take you too deep into the wilderness before you really know how to use it.

I still see no real need for one on the most common kind of garden-stroll hike along the trails of Shenandoah National Park. But for more challenging trips into the trackless wilderness, I would consider packing this wickedly clever device along.

It's a sick feeling when you think you might be lost. If you're going some place where that's a real possibility, this might be the cure.