These portable machines help you get your bearings and direct you to your destination by having you select from a list of city names or look at a series of maps presented on a tiny screen. If you get lost, a backtrack function will show you the way back to your starting point -- like leaving a trail of electronic breadcrumbs. The machines can even tell you your altitude, your speed and your estimated arrival time.

In many ways, these machines are marvels. As you drive, walk or otherwise move about, an arrow on the screen follows your motions. But don't throw away your maps or charts just yet. As with many new technologies, the reality falls a bit short of the promise.

The devices work on signals from the satellites in the government's Global Positioning System. The GPS consists of 24 satellites that circle the Earth,transmitting high-frequency radio signals.

The GPS gadgets, which resemble overgrown pocket computers, use built-in antennas to pick up the signals from at least three of the satellites.Using triangulation, the devices calculate their location to within as little as 60 feet. They then display the position on-screen using a built-in map .

I tried out two handheld GPS receivers: the Garmin GPS III Plus, from Garmin Corp., priced at about $370, and the $349 Magellan Map 410 from the Magellan division of Orbital Co. Several other companies, including Lowrance Electronics Inc., have their own entries in this market. Some have bigger screens and are designed to be mounted on a car's dashboard or on a boat; others offer screens in color.

The Garmin and Magellan models I tested are among the most portable. Both weigh well under a pound, have black-and-white screens and run on four AA batteries. The screens can be illuminated for night use. The Garmin fits in a shirt pocket, while the Magellan, which has alarger screen, is a bit bulkier, at 6.9 inches long and 2.5 inches wide. I would give the edge to the Garmin for easy use.

Here's how they work. First, you need a clear view of the sky. If you are in a car you must aim the antenna near a window. You turn on the machines and they go through a setup process to determine your location based on the satellite signals. Once they've fixed the location, a map of the region pops up on the screen. By pushing a button, you can zoom in or out, to give varying levels of map detail.

While testing the devices on a bike trip, I kept the detail at the highest zoom level and watched my movements tracked on-screen. When I made a right turn while biking down a rural road, the arrow on the screen turned right. When I turned around, the arrow followed me a moment later. When I stopped, the arrow stopped. It's uncanny, and a bit creepy, as if you're being watched from the sky.

In the car, the devices allow you to track your progress by showing you where you are on the map inrelation to "waypoints," or electronic markers that you set up.

You can see upcoming exits and figure out what roads you are passing over or under and what landmarks are nearby. You can set alarms to warn you if you get off course. With the Garmin, you can even see what services, such as gas stations and restaurants, are available at exits of interstate highways.The Garmin also allows you to load in detailed road or topographical maps from special CD-ROMs, which cost an extra $100 to $130.

MAGELLAN'S current built-in maps have less detail than the Garmin CD-ROM maps, making them less useful at the highest zoom levels. But the company promises to offer similar CD-ROMs in the near future. With the Garmin maps, I could see small back roads, ponds and other features. And because the Garmin is smaller, I could use the wrist strap and hold the machine in my hand while peddling.

But there are disadvantages. At the top of the list: the size and quality of the screens. For one thing, they are too small to showmuch of a map a once. The Garmin's screen is about the size of a matchbook that is opened up; the Magellan's is a bit longer.

When you zoom in to the point where you can see details, you can't view enough of the map at once to get a feel for where you are going. If you zoom out, you lose too much detail and the map becomes just a grid of major highways.

Moreover, in these days of crisp color screens on laptop computers, the GPS screens are a throwback. They are fuzzy by comparison, particularly the Garmin's, and don't display map detail well. This, combined with the fact that they don't display color, makes them far inferior as a map-reading experience to the average $7.99 atlas from a gas station. It would be tough to rely on a GPS device as your only guide.

Yet for many people, these GPS machines could be a godsend. No paper map can tell you where you are, or point you in the right direction when you are lost.

And with the technology industry continuing to produce ever-better stuff at lower prices,it may be only a short while before low-cost GPS receivers feature crisp color screens and higher memory capacity to store more map detail.

(www.ptech.wsj.com)