Patent 6,947,978, granted Tuesday, describes a way to discover someone's physical location by comparing it to a "map" of Internet addresses with known locations.

The NSA did not respond Wednesday to an interview request, and the patent description talks only generally about the technology's potential uses. It says the geographic location of Internet users could be used to "measure the effectiveness of advertising across geographic regions" or flag a password that "could be noted or disabled if not used from or near the appropriate location."

Other applications of the geo-location patent, invented by Stephen Huffman and Michael Reifer of Maryland, could relate to the NSA's signals intelligence mission--which is, bluntly put, spying on the communications of non-U.S. citizens.

"If someone's engaged in a dialogue or frequenting a 'bad' Web site, the NSA might want to know where they are," said Mike Liebhold, a senior researcher at the Institute for the Future who has studied geo-location technology. "It wouldn't give them precision, but it would give them a clue that they could use to narrow down the location with other intelligence methods."

The NSA's patent relies on measuring the latency, meaning the time lag between computers exchanging data, of "numerous" locations on the Internet and building a "network latency topology map." Then, at least in theory, the Internet address to be identified can be looked up on the map by measuring how long it takes known computers to connect to the unknown one.

The technique isn't foolproof. People using a dial-up connection can't be traced beyond their Internet service provider--which could be in an different area of the country--and it doesn't account for proxy services like Anonymizer.

Geo-location, sometimes called "geo-targeting" when used to deliver advertising, is an increasingly attractive area for Internet businesses. DoubleClick has licensed geo-location technology to deliver location-dependent advertising, and Visa has signed a deal to use the concept to identify possible credit card fraud in online orders.

Digital Envoy holds a patent on geo-location, and Quova, a privately held firm in Mountain View, Calif., holds three more, one shared with Microsoft.

"It's honestly not clear that there's anything special or technically advanced about what they're describing," Quova Vice President Gary Jackson said, referring to the NSA's patent. "I'd have to have our technical guys read it, but I don't think it impacts us in any way."