US wholesale LTE prospect LightSquared has received further setbacks to its goal of building a national mobile data network on airwaves previously used by satellite phones. Federal officials – including the Department of Transportation and the Department of Defense – said yesterday that tests showed the network would still knock out a “majority” of GPS devices.

Separate testing by the Federal Aviation Administration (FAA) found LightSquared’s network would interfere with a flight-safety system designed to keep planes from crashing into mountains or other terrain.

In a statement, LightSquared CEO Sanjiv Ahuja said the company is “eager to continue to work with the FAA on addressing the one remaining issue regarding terrain avoidance systems,” but it “profoundly disagree[s] with the conclusions drawn with respect to general navigation devices.”

Ahuja’s statement went on to claim that it “has had the legal and regulatory right to use its spectrum for eight years over two administrations,” and that the interference issues “are not caused by LightSquared's spectrum, but by GPS devices looking into spectrum that is licensed to LightSquared.” It added:  “We have taken extraordinary measures — and at extraordinary expense — to solve a problem that is not of our making. We continue to believe that LightSquared and GPS can co-exist. And we will continue to work with the federal government on a solution that will allow us to begin investing US$14 billion in private money into the infrastructure of America to create jobs, competition and increased access to technology to the nation.”

Despite these major obstacles, LightSquared continues to invest in its business plan. Last week it announced it has already signed more than 30 wholesale customers, including operators Sprint and  Leap Wireless, device vendor Sharp and retailer Best Buy, as well as non-mobile players such as Smarter Car.