A $14 billion plan that would use satellites and 40,000 new ground towers to dramatically expand wireless communication would overpower the GPS signals that drivers, pilots and mariners use to navigate, according to testimony at a House subcommittee Thursday.

Opponents of the plan by Reston-based LightSquared described in stark terms the impact it would have on GPS, endangering rescues at sea, adding risk to planes landing in bad weather and garbling dashboard-unit reception.

At stake are billions of dollars in investment by two companies in the most explosive industries of the 21st century, a wireless business with more than 300 million users and a navigational network of more than 400 million receivers.

“LightSquared’s proposal is sort of like driving a lawn mower in a library,” Philip Straub, a vice president of GPS manufacturer Garmin, told a joint House subcommittee.

LightSquared responded that it won’t drown out GPS signals.

“LightSquared has no intention of operating a system in any way that will compromise government or commercial aviation operations,” LightSquared Vice President Jeffrey J. Carlisle said.

The two industries came onto a collision course nine years ago, when LightSquared began working on a plan that seeks to provide uncompromised wireless service virtually everywhere in the nation.

Both systems operate on adjacent portions of the L band, in the upper spectrum at 1559 megahertz, where LightSquared’s authorized bandwidth ends and the GPS band begins. When the Federal Communications Commission granted LightSquared a waiver that authorized it to proceed with a more powerful transmission scheme, the GPS industry balked.

“LightSquared’s proposed broadband terrestrial network will cause catastrophic and perhaps life-threatening harm to reliable GPS services,” Straub said.

“The impact of the LightSquared upper-channel spectrum deployment is expected to be complete loss of GPS receiver function,” Margaret T. Jenny, president of the Radio Technical Commission for Aeronautics told the committee.

Jenny said aviation would be less affected if LightSquared restricted its operations to the portion of the bandwidth farthest from the juncture with GPS.

Carlisle said that, in light of recent studies that showed potential interference, that is just what LightSquared plans to do.

“The vast majority of GPS receivers look only at that portion of LightSquared’s spectrum that is immediately adjacent to GPS,” he said. “Our operation in the lower portion of our band, furthest from GPS, does not cause interference. Operation at the far end of the spectrum will avoid overload for 99 percent of the receivers, including those used for aviation and maritime operations.”

Carlisle proposed that LightSquared would operate at lower power levels than those authorized by the FCC. He said the company would not use the upper-spectrum 10 megahertz adjacent to GPS until the FCC determines that it would not disrupt GPS.