Cell phones are giving employers new ways to check up on employees in the field–and raising fresh workplace privacy concerns as a result.

On the leading edge of the trend is Nextel Communications. The wireless provider began selling its Mobile Locator service last November, giving bosses an easy way to find employees who carry GPS-equipped cell phones.

Earlier this month, mobile tracking firm Xora showed off the latest version of its Nextel GPS (global positioning system) phone software. The company says 1,600 corporate customers have signed up for its services, including “geofences” technology that sets off an alarm at the office when field workers go to preprogrammed off-limits sites, such as a bar or a park.

“There’s no electro shock–yet,” Xora CEO Sanjay Shirole said.

Employee-tracking devices are gaining steam thanks to ever-more-accurate GPS technology and a U.S. mandate requiring wireless companies to develop ways for emergency workers to find the physical location of people who dial 911 on a cell phone.

Developed in the 1970s by the U.S. military, GPS uses signals from low orbit satellites to triangulate the position of a ground-based receiver. GPS trackers were once an expensive luxury, but costs have plunged with the expansion of cellular-phone services.

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