At the Congressional hearing, representatives of federal agencies and industry groups testified that LightSquared's current network plan would hobble GPS use for aviation, navigation, agriculture, defense and other purposes. Some witnesses also were skeptical about the carrier's proposal to . Some also slammed the FCC for giving LightSquared a conditional waiver with too little time for debate. The oversight hearing was held by the Subcommittee on Aviation and the Subcommittee on Coast Guard and Maritime Transportation.

At the hearing, the U.S. Coast Guard and departments of Defense and Transportation reported that tests showed LightSquared's current approach caused unacceptable interference with GPS, endangering a system vital to national defense and commerce. Witnesses from the GPS industry were even more critical.

"All of the testing performed to date confirms that the LightSquared system, as currently proposed, will result in a widespread degradation of GPS receiver performance and severely limit the GPS utility as we now know it," said Philip Straub, vice president of aviation engineering at GPS equipment maker Garmin International, in written testimony. RTCA, a nonprofit advisory commission on aviation, presented the results of tests conducted with LightSquared. From those tests, RTCA . Other tests have led to similar conclusions.

Last week, LightSquared proposed a new plan in which it would set aside its 10MHz spectrum band that is closest to the frequencies used by GPS and instead start out by using a lower band and reduced transmission power. It has not formally submitted this new plan to regulators. Some testimony on Thursday was skeptical of this plan.

"Significant research and modeling is required to fully define this mitigation and conclusively prove whether it would achieve the desired effect," said Thomas Hendricks, senior vice president of safety, security and operations at the Air Transport Association of America.