The Federal Aviation Administration this week gave its imprimatur to the Cape Wind development on Nantucket Sound, but with expensive strings attached.

In its decision, the FAA determined the 130-turbine wind farm would have “no substantial effect” on air traffic, but also insisted the project developers pay for the upgrading of radar covering the area.

The upgrade to the radar system at Otis Air Force Base, intended to ensure the spinning blades of the 440-foot-tall structures do not cause interference, is likely to cost the developers some $1.5 million.

But they are being required to deposit 10 times as much — $15 million — in an escrow account for two years, just in case the planned mitigation does not work. Plan B would call for even more expensive new equipment.

The developers immediately welcomed the decision and promised to provide the funds sought by the FAA.

Opponents of the development accused the FAA of making a political decision which put public safety at risk. The Alliance to Protect Nantucket Sound said it would challenge the decision through the FAA’s appeals process.

The FAA decision, announced on Monday, was one of the final two regulatory hurdles faced by Cape Wind. The other approval still outstanding is that of the Army Corps of Engineers, relating to potential navigation hazards.

The long-awaited study considered the effect the turbines could have on aircraft movements at all public and military airports in the region.

It was the third time the FAA had approved the development, but the previous determinations expired as the rest of the approvals process dragged on.

In response to the FAA approval, Cape Wind issued a brief media statement: “We are pleased the FAA has approved Cape Wind so that we can get to work on building America’s first offshore wind farm that will create jobs, increase energy independence and contribute to a healthier environment.

“Cape Wind will provide the FAA with the funds they need to modernize and enhance their radar facility at Otis Air Force Base,” it said.

The project’s opponents were much more expansive in their comments. “Today’s FAA decision confirms that the proposed turbines would interfere with radar, but relies on unproven theoretical mitigation to resolve this serious safety issue,” said the Alliance’s Monday statement.

“FAA has also dismissed the known impacts to the many flights in the area operating by Visual Flight Rules (VFR) and, in so doing, has failed to follow its own clear rule dictating that structures causing VFR flights to change their regular course or altitude, are in fact a de facto hazard.

“This is an entirely political decision that flies in the face of public safety and the recommendations of the pilots who use this airspace every day. It is one thing to ignore the public’s opposition to Cape Wind, it is another altogether to ignore public safety.

“Federal agencies such as the Minerals Management Service (MMS) and the FAA should not be the ‘rubber stamps’ for the energy industry — not when lives are at stake. We are prepared to file a petition seeking review of these determinations,” the Alliance statement said.

The Alliance has until June 16 to file such a petition.

If it goes ahead, it will add to a long list of legal and procedural challenges the Alliance and its affiliates are planning.

But the developers, who are building a significant record of success in defeating such challenges — they claim a 12 for 12 record before courts and administrative appeals bodies to date — remain confident.

Cape Wind spokesman Mark Rodgers said they were more concerned with other challenges.

“We are expecting soon to get our lease from the Department of the Interior, with the attached terms and conditions. So we are looking forward to that.

“And we’re looking to sell the other half of our power output. We’re looking to secure commercial finance for the project. We’re in a bidding process with various ocean construction firms to build the project. We’ve got a lot happening right now,” he said.

All those things, he said, should be completed within the next six months — a relatively short time compared with the nine years the approval process has dragged on.

As for the last remaining regulatory approval, that of the Army Corps, Mr. Rodgers said they expected no trouble.

He noted that early in the history of the proposed development, some five or six years ago, the Army Corps had been responsible for drafting an Environmental Impact Statement, before the government shifted bureaucratic responsibility for the task to the Minerals Management Authority.

The corps and the Coast Guard at that time had come down in favor of the project.

Also still outstanding is a Massachusetts Supreme Court decision on a case that challenged the state review of the wind farm. That decision is expected before summer.