Is hardware that looks to the sky worth buying for the road? Used by the military for more than a decade, satellite-based global positioning system (GPS) technology has now hit the car market. GPS is being offered on some luxury vehicles. And aftermarket GPS systems, which can be put on any car, have grown in number and dropped in price.

We tested three types of aftermarket GPS systems that cost between $200 and $2,300--portable models, systems you can use with a laptop computer, and models that are permanently installed in your car. The test results suggest that only the most expensive systems are notably more helpful than a paper map.

The cheapest GPS receivers--handheld units that cost $150 or so--show little more than your latitude and longitude, which you must then locate on a paper map. The priciest models can show both your location and destination superimposed on a street map displayed on a color video screen. Such receivers also automatically tell you how to get where you're going: The receiver chooses a route, displays your progress, and guides you to each turn by displaying the distance to the turn, its direction and the name of the road.

Some models also issue those directions in a computer-generated voice. A few even respond to simple spoken inquiries.

Still, GPS help is no substitute for knowing the territory, as our testers discovered when they drove in familiar surroundings. Some devices chose roads that would have made the trip longer than necessary. (They have a preference for numbered and other major roads, and avoid the shortcuts you often find when you frequently drive the same route.) If you deviate from the receiver's suggested course, it will typically recalculate the trip, incorporating your deviation--as long as the road you're driving is in its memory.

Also, you may lose satellite reception when traveling through underpasses or along roads lined with trees or tall buildings. But the receiver picks up the signals again once it's in the clear: GPS systems we tested worked fine at intersections in the skyscraper canyons of midtown Manhattan, for example.

GPS use for road navigation is still evolving, and future receivers will likely be easier to use and less expensive than those we tested. Most people will want to wait before buying a GPS device--if they ever do.

If you can't wait, here are some suggestions:

* Portable models. We'd choose the Garmin StreetPilot ColorMap ($700). The unit uses preprogrammed MetroGuide cartridges costing $100 to $200 each, depending on the metropolitan area they cover. Its color screen was much easier to read than the monochrome displays of other portables we tested.

* Laptop models. A laptop-based GPS receiver is somewhat impractical--even potentially unsafe--for a solo driver. Of the two we tested, the $350 TravRoute CoPilot was the better.

* Installed models. If you take frequent trips to unfamiliar places, an installed GPS unit could be helpful. The best of the models we tested were the Alpine NVA-751AS and the VDO Carin 522--$2,250 and $2,300, respectively, including installation. These are systems similar to those available in expensive cars. Data come from CD-ROMs costing $150 to $170 each; you need seven to nine to cover the whole country.

Consumers Union Inc.