Top government officials said Friday that there are "no practical solutions" to GPS interference problems caused by LightSquared's planned mobile broadband network, signaling that the venture-backed company may have to abandon its plans to deploy LTE service in spectrum near to GPS bands.

"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 year without significantly interfering with GPS," Defense Department Deputy Secretary Ashton Carter and Transportation Department Deputy Secretary John Porcari said in a letter to NTIA Administrator Lawrence Strickling. "As a result, no additional testing is warranted at this time."

Both LightSquared's original deployment plan and its revised plan - which it said fixed interference problems for 99 percent of GPS receivers - still would cause "harmful interference to many GPS receivers," the officials wrote.

The FCC has said in the past that will not allow LightSquared to turn on its network until it fixes problems with GPS interference. The agency has yet to announce a formal ruling on the issue.

LightSquared quickly fired back, claiming the agencies in charge of the testing "have demonstrated bias and inappropriate collusion" with the GPS industry. Tests conducted by the government have repeatedly found that LightSquared's network causes problems with highly sensitive GPS receivers critical to landing planes, guiding ships and targeting missiles, among other uses. LightSquared's own tests have had different results that show its network is compatible with GPS.

In its own letter to Strickling, LightSquared said it had found "serious anomalies" with the way the PNT Systems Engineering Forum (NPEF) conducted its tests. The forum was put in charge of tests analyzing the impact of LightSquared's network.

"From the outset, the process established by PNT EXCOMM, NPEF and AFSC (Air Force Space Command) was fraught with inappropriate involvement of the GPS manufacturers, lax controls, obvious bias, lack of transparency, and unexplained delays," LightSquared said, claiming many of the devices in the tests affected the most by its network were out of date or niche receivers that would "rarely" come close enough to one of its base stations to be knocked out.

LightSquared wants the government to conduct another round of tests under its own terms.

The Coalition to Save Our GPS, a vocal opponent to LightSquared's proposed network, said the government shouldn't be blamed for the plan's technical problems.

"The technical evidence speaks for itself, and no individual, company or government body can legitimately be blamed for the clear defects of LightSquared's ill-conceived proposal or the failure of that proposal to pass an extensive, fact-based review process," coalition spokesman Dale Leibach said in a statement issued today.

The latest grim news for LightSquared comes nearly one year after the FCC granted a conditional waiver for the network over the objections of the NTIA and GPS industry.

The FCC ruled last January that LightSquared could move ahead with its plan to repurpose satellite spectrum for land-based wireless service if it was able to address potential problems with GPS interference, but tests soon showed that LightSquared's network knocked out GPS receivers. The test results prompted the company to come up with an array of increasingly complex solutions, which the government has now dismissed.

LightSquared continued to sign up more than 30 customers for its planned network even as it scrambled to fix problems with GPS, including Leap Wireless International and Best Buy.

LightSquared's primary backer, Philip Falcone's hedge fund Harbinger Capital Partners, has sunk $2.9 billion into LightSquared. The company launched its first satellite in November 2010, signed up Qualcomm to develop chips and hired Nokia Siemens Networks to construct its network. LightSquared also signed a network hosting deal with Sprint that has been put on hold.