As navigational devices get smaller and more affordable, those who are lost may soon be found.

For just over $100, you can purchase a handheld, waterproof Garmin e-Trex GPS receiver (www.garmin.com) or the company's NavTalk, a combination cell phone and GPS receiver. For about twice as much, you can wear a GPS receiver embedded in a Casio watch (www.casio.com).

These gadgets are fruits of the Global Positioning System, developed by the Department of Defense at a cost of $12 billion. Today there are about 10 times as many civilian receivers as military ones, and commercial uses for GPS are still expanding. In May President Clinton announced the federal government was discontinuing Selective Availability, which intentionally degraded the GPS signal for nonmilitary users in the interests of national security. That change means the margin of error for determining position has dropped from 100 meters to 10, enabling a host of new opportunities for localized digital services.

Before they can become widespread, says Mark Zohar of Forrester Research, location-based services require two conditions: An affordable system has to be in place, and always-on networks must be the norm. The Federal Communications Commission, as part of its E911 program, is mandating that cell phone companies be able to locate their callers to within about 100 meters by October 2001. The excitement over providing localized, wireless advertising is already on the rise. But Zohar says that both critical pieces won't be in place until 2002, when the market will really take off.

Zohar predicts widespread use of GPS will also "create opportunities for pay-per-use applications." It could improve the sales of related technologies too. Parents might give children cell phones more willingly if the devices doubled as emergency locators.

While businesses ponder new location-based services, consumer GPS technology is also marching ahead. Freeplay Energy (www.freeplay.net, formerly BayGen), a manufacturer of wind-up devices including flashlights and radios, is developing a human-powered generator that can provide the energy for a GPS receiver. Alex Makarow of Innovative Technologies, distributor of the Freeplay devices, anticipates that the users of wind-up GPS will be "the recreational market—hikers, campers, kayakers, small boaters, and other people who travel without power."

The maker of another self-powered GPS device hopes it will fit into a completely different niche. The Digital Angel (www.digitalangel.net), acquired by Applied Digital Solutions last December, initially will be a wearable patch and, following Food and Drug Administration approval, will be available to implant in the human body. It requires no energy source other than the body itself, and it both receives and transmits GPS information along with medical diagnostic data including heart rate and blood pressure. Dr. Peter Y. Zhou, the project's chief scientist, foresees medical, personal security, and military applications. He expects to complete a prototype by the end of the year.

Zohar cautions that manufacturers need to be aware of the privacy implications of location transmitters. Zhou seems well aware of the potential misuse. The Digital Angel, he says, "is very tightly encoded, so you will not be tracked by [unauthorized] people." He adds, "You can turn it off by yourself. It's only for your safety and health. It's not for monitoring. Except for criminals."