WASHINGTON — An expert panel tasked by the U.S. government to test the revised plans of a company hoping to deploy a hybrid satellite-terrestrial mobile broadband network serving North America has concluded that the revised operating scheme will still cause harmful interference to GPS satellite-based navigation applications, including aviation safety.

The finding by the nine U.S. federal agencies comprising the National Space-Based Positioning, Navigation and Timing Executive Committee (EXCOM) is a major blow to LightSquared, which has invested $3 billion in its proposed L-band network and launched one large satellite. The Reston, Va.-based company, backed by hedge fund billionaire Philip Falcone, is planning to augment that satellite with a nationwide network of ground-based signal repeaters.

“It is the unanimous conclusion of the test findings by the National Space-Based PNT EXCOM Agencies that both LightSquared’s original and modified plans for its proposed mobile network would cause harmful interference to many GPS receivers,” Deputy Defense Secretary Ashton Carter and Deputy Transportation Secretary John Porcari, co-chairmen of the EXCOM, said in a Jan. 13 letter to Lawrence Strickling, assistant secretary of commerce for communications and information. “Additionally, an analysis by the Federal Aviation Administration (FAA) has concluded that the LightSquared proposals are not compatible with several GPS-dependent aircraft safety-of-flight systems.”

LightSquared offered to modify its operations scheme after a similar working group concluded this past summer that the original plan would cause unacceptable interference to GPS applications. Under the modified plan, the company’s ground stations would broadcast at reduced power levels and in its allocated frequencies that are farthest away from the GPS bands.

In their letter, Carter and Porcari wrote that “there appear to be no practical solutions” to the problem and that no further testing is planned.

LightSquared officials could not be reached by press time for comment. LightSquared has maintained that the manufacturers of GPS equipment, and not the LightSquared network, are responsible for the problem.

Warren Ferster is the Editor-in-Chief of SpaceNews and is responsible for all the news and editorial coverage in the weekly newspaper, the spacenews.com Web site and variety of specialty publications such as show dailies. He manages a staff of seven reporters...