LightSquared booth at Satellite 2011 Convention at Washington Convention Center in Washington,DC (Jeffrey MacMillan/FOR WASHINGTON POST)

The FCC in a letter earlier this week told Grassley that it doesn’t have to reveal documents requested by the Judiciary committee. Only the Commerce committee can force the agency to reveal correspondence between the FCC and LightSquared officials.

FCC Chairman Julius Genachowski instead told Grassley to file a freedom of information act request.

“What is the FCC hiding?” Grassley said in a statement.

Since last April, the lawmaker has asked the FCC to explain why it decided to waive a portion of LightSquared’s licensing process. The decision came amid warnings by the Commerce Department and GPS industry that the satellite firm, financed by hedge fund billionaire Philip Falcone, could interfere with navigation technology for military, aviation, construction workers and other industries that rely on GPS technology.

After recent tests that confirmed interference, LightSquared has proposed GPS device makers use filtering technology to avoid problems.

In his letter to Grassley, Genachowski said the agency “will not make decisions that jeopardize national security, safety of the public, or the very important benefits the GPS industry has brought to the public nad will continue to provide in the future.”