The Federal Communications Commission’s initial approval of a troubled satellite venture came under fresh criticism Thursday as a key Republican lawmaker questioned whether the agency’s actions could lead to billions of dollars in costs for taxpayers.

LightSquared is trying to build a cellphone network out of satellites, but the technology interferes with Global Positioning System technology used by pilots, consumers and meteorologists. LightSquared wants the military and other federal agencies to refit its equipment with filters.

But the cost to the government could be substantial, officials have said. And in a letter to FCC Chairman Julius Genachowski, Sen. Charles E. Grassley (R-Iowa) questioned whether the agency considered that cost when it allowed LightSquared to take the next step in its plan in January.

Federal officials warn that LightSquared’s interference with their GPS technology could lead to airplane crashes and failed military operations. Air Force Space Command Gen. William Shelton has said it would cost the U.S. military in the “B’s — billions of dollars” to accommodate LightSquared’s proposed technical fixes.

“At no point has the FCC addressed who will pay to retrofit every single federal, state, and local government GPS receiver, which will require a filter as a result of interference” caused by LightSquared’s network, wrote Grassley, the ranking Republican on the Judiciary Committee.

The FCC declined to comment about Grassley’s letter. Agency spokeswoman Tammy Sun pointed to a recent letter to Rep. Michael R. Turner (R-Ohio), chairman of an Armed Services subcommittee, in which the agency said it doesn’t have the authority to force other federal agencies to spend money.

LightSquared has said that the military is exaggerating the cost and has offered to pay $50 million for federal agencies, including the Defense Department and the Federal Aviation Administration, to retrofit or replace their GPS devices.

Grassley’s letter is the latest criticism by Republican lawmakers of the process at the FCC that some say fast-tracked a private company’s ambitions to become a broadband Internet service provider that would compete with giants such as AT&T and Verizon Wireless. Several news outlets have reported that LightSquared enjoys close connections to Democrats.

LightSquared chief executive Sanjiv Ahuja said during an interview on C-SPAN’s “The Communicators” that the company is offering federal agencies “a sufficient amount of money to replace most receivers or fix most receivers out there.”

He added that the company received no special favors from the FCC.

“The process was robust,” Ahuja said. “These are technical and engineering issues that should be resolved by engineers.”

Under mounting pressure, Ahuja and billionaire financier Philip Falcone have been publicly defending the company in recent days. LightSquared has taken out full-page ads in national newspapers touting the discovery of what it says is a technical solution to its problems. Falcone has appeared on Fox News to defend the company’s practices, saying it in no way used political connections for business favors.

The National Telecommunications and Information Administration, the communications policy arm of the White House, this month ordered LightSquared to retest whether the company’s proposed fixes will work.

Aside from Grassley, House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.) is also probing LightSquared’s connections to the White House and FCC.