The question of whether Google is liable for damages for secretly intercepting data on open WiFi routers across the United States is boiling down to the definition of a “radio communication.”

That appears to be the legal theory embraced by the Silicon Valley federal judge presiding over nearly a dozen combined lawsuits seeking damages from Google for eavesdropping on open WiFi networks via its Street View mapping cars. The cars had been equipped with WiFi-sniffing hardware to record the names and MAC addresses of routers to improve Google location-specific services.

But those cars were also capturing the contents of internet packets that were sent over unencrypted WiFi as they drove by, something the company said was an accidental leftover from testing.

While the company quickly admitted that it had made a mistake and temporarily grounded its fleet of mapping vehicles last year, the company was confronted with a number of investigations around the world, as well as class-action lawsuits that were joined in San Jose, California. The lawsuits are being heard by US District Judge James Ware.

At the center of the legal flap is whether Google breached the Wiretap Act. The answer is important not only to Google, but to the millions who use open, unencrypted Wi-Fi networks at coffee shops, restaurants or any other business trying to cull customers.

Google said it is not illegal to intercept data from unencrypted, or non-password-protected Wi-Fi networks. Plaintiffs’ lawyers representing millions of Americans whose internet traffic was sniffed by Google think otherwise, and are seeking unspecified damages.

Judge Ware, however, suggested the answer to the far-reaching privacy dilemma lies in an unanswered question. He has asked each side to define “radio communication” (PDF) as it applies to the Wiretap Act, and wants to know whether home W-Fi networks are “radio communications” under the Wiretap Act.

In response, Google wrote last week that open WiFi networks are akin to “radio communications” like AM/FM radio, citizens’ band and police and fire bands — and are “readily accessible” to the general public. Indeed, packet-sniffing software, such as Wireshark and Firebug, is easily available online.

Hence, because unencrypted WiFi signals travel over the radio spectrum, they are not covered by the Wiretap Act, (PDF) Google responded.

“There can be no doubt that the transfer of any sign, signal, writing, images, sound, data, or intelligence of any nature transmitted over the radio spectrum constitutes a ‘radio communication.’ Indeed, there is nothing in the text or legislative history of the Wiretap Act that would exclude any transmission sent over the radio spectrum from the definition of ‘radio communication,’” Google wrote.

The plaintiffs’ lawyers countered that the communications in question started on a computer and only briefly were relayed on radio waves “across the living room from the recipient’s router to her laptop.”

“The fact that either the first or final few feet of the electronic communication may have gone via wireless transmission ['Wi-Fi'] does not transform the communication into a ‘radio communication’ broadcast similar to an AM/FM radio or a CB.,” (PDF) plaintiffs’ lawyer Elizabeth Cabraser wrote. “Nor is there anything in the statute to define ‘radio communications’ as synonymous with anything sent on a radio wave, however briefly and without regard to the entirety of the communication system at use.”

Both sides agree, however, that it’s illegal to listen in on cordless phones.

According to the Wiretap Act, it’s not considered felony wiretapping “to intercept or access an electronic communication made through an electronic communication system that is configured to that such electronic communication is readily accessible to the general public,” according to the text of the federal wiretapping statute.

The Federal Trade Commission closed its investigation into the brouhaha in October, without imposing any sanctions on the Mountain View, California internet giant. The Federal Communications Commission commenced a probe in November, but has not announced a conclusion.

The FCC’s government affairs chief wrote last year that “Google’s behavior also raises important concerns. Whether intentional or not, collecting information sent over WiFi networks clearly infringes on consumer privacy.”

Several state attorneys general are also looking into the debacle.

Google said it didn’t realize it was sniffing packets of data on unsecured WiFi networks in about a dozen countries over a three-year period until German privacy authorities began questioning what data Google’s Street View cars were collecting. Google, along with other companies, use databases of WiFi networks and their locations to augment or replace GPS when attempting to figure out the location of a computer or mobile device.