A second round of tests on LightSquared’s proposed land-based mobile data network again showed interference with a majority of GPS devices, except for cellphones, two U.S. federal departments stated Wednesday.

LightSquared wants to build a network of 4G LTE (Long-Term Evolution) base stations around the U.S. that would operate on frequencies close to those used by GPS (Global Positioning System) receivers. But the U.S. Federal Communications Commission (FCC) will not approve the company’s plan unless potential interference with GPS has been resolved. Tests earlier this year showed the network could cripple many GPS devices.

The second round of tests was conducted last month and concentrated on LightSquared operating in a lower block of frequencies farther from those used by GPS.

“Preliminary analysis of the test findings found no significant interference with cellular phones,” the Department of Defense and Department of Transportation said in a statement on Wednesday. “However, the testing did show that LightSquared signals caused harmful interference to the majority of other tested general purpose GPS receivers. Separate analysis by the Federal Aviation Administration also found interference with a flight safety system designed to warn pilots of approaching terrain.”

The test results were presented on Wednesday to a technical steering group representing seven federal agencies, including Defense and Transportation, that make up the Space-Based Positioning, Navigation, and Timing Executive Committee (PNT ExComm).

The latest findings could be a sharp blow to LightSquared, which hopes to launch its network before the end of next year and faces another round of interference testing beginning in January. On Monday, the company proposed a new set of concessions to the FCC.