The U.S. National Space-Based Positioning Navigation and Timing Committee (PNT), an advisory board made up of representatives from nine federal departments, reached the “unanimous conclusion” that the LightSquared network could “cause harmful interference to many GPS receivers” as well as the GPS-powered Ground Proximity Warning System (GPWS) maintained by the FAA.

“Based upon analysis, there appear to be no practical solutions or mitigations that would permit the LightSquared broadband service, as proposed, to operate in the next few months or years without significantly interfering with GPS. As a result, no additional testing is warranted at this time,” the committee wrote. The committee is made up of representatives from departments including the Commerce Department and Air Force. The letter was signed by Ashton Carter, Deputy Secretary of Defense and John Porcari, Deputy Secretary of Transportation.

LightSquared countered that the test results reflected a “bias and inappropriate conclusion” because the committee included non-governmental employees with ties to the GPS industry. “We are confident that the tests, when the protocol is disclosed and the details are examined, will be shown to be invalid. The devices chosen by GPS manufacturers to be tested were selected to ensure failure. They included obsolete GPS receivers (some a decade out of production) and niche-market receivers. This is nothing more than a thinly-veiled attempt by government agencies to protect the interests of the GPS industry who are unauthorized users infringing on spectrum licensed to LightSquared,” said Harbinger Capital, primary financial backer of LightSquared.

The group’s findings increase pressure on the FCC to scale back the license granted to LightSquared in 2005 to build a combined ground/satellite communications network with an unlimited number of ground-based towers. LightSquared maintains that the interference issue can be corrected with the addition of a “relatively inexpensive” filter to current GPS systems. If forced to upgrade, GPS manufactures could face millions in liability.