Global Positioning Systems are built into millions of cellular phones - 175 million worldwide, according to the research firm Berg Insight. And that number will be well over half a billion in another five years.

In the United States, the GPS phone boom was kicked off by the Federal Communications Commission, which wanted to ensure that emergency workers could find people who place 911 calls from their pocket phones. Two of the nation’s leading cellphone providers, Sprint Nextel Corp. and Verizon Wireless LLC, decided to comply by putting GPS chips in their phone handsets.

At first, this phone-based GPS was good only for calling the cops; consumers couldn’t make use of the technology. But now a number of software firms make downloadable programs that convert GPS-capable phones into powerful navigation tools.

Cellphone companies that once rejected GPS technology, including AT&T Inc. and T-Mobile USA Inc., now offer GPS-capable handsets so their customers can run these programs.

But not every GPS program can run on every phone, so consumers should contact their phone carriers to confirm compatibility. Also, ask if you need to subscribe to a data plan along with the standard voice service. For example, Verizon doesn’t require a data plan subscription for GPS service, but Sprint does.

Verizon’s VZ Navigator was among the first phone GPS programs aimed at consumers when it was rolled out two years ago. The software, developed by Networks In Motion Inc., of Aliso Viejo, Calif., provides turn-by-turn driving directions delivered in a natural-sounding human voice. It also lets you quickly locate the nearest bank, movie theater, or hotel, and even serves up the phone number so you can call ahead.

Boston’s uLocate Communications Inc. has developed one of the most engaging applications for phone-based GPS. It’s called WHERE, and though it lacks the sophistication of competing products, it’s just about irresistible.

WHERE is actually a software development platform that lets companies write mini-GPS programs, called widgets, which you can then download onto your phone. Each widget does one or two simple things.

For instance, a widget called GasBuddy will direct you to neighboring service stations with the lowest prices. (New York Times News Service)