CHICAGO — A World War II-era air traffic network that often forces planes to take longer, zigzagging routes is costing U.S. airlines billions of dollars in wasted fuel while an upgrade to a satellite-based system has languished in the planning stages for more than a decade.

The $35 billion plan would replace the current radar system with the kind of GPS technology that has become commonplace in cars and cell phones. Supporters say it would triple air traffic capacity, reduce delays by at least half, improve safety and curb greenhouse gas emissions.

An Associated Press analysis of federal and industry data found that if the new system were already in place, airlines could have saved more than $5 billion in fuel this year alone.

But funding delays and the complexities of the switchover have kept the project grounded. The government does not expect to have it up and running until the early 2020s, and without a major commitment, supporters warn that even that goal might be unattainable.

"The United States has been to the moon and back. I think the public deserves that same level of effort for our national airspace system," Robert Sturgell, the acting administrator of the Federal Aviation Administration, said in a recent interview.

The planned satellite-driven network, dubbed NextGen, would save fuel by ditching radar technology that is more than 50 years old and enabling GPS-equipped planes to fly the shortest route between two points: a straight line.

NextGen could save airlines at least 3.3 billion gallons of fuel a year — or more than $10 billion annually by 2025, based on today's fuel prices, according to FAA projections obtained by The Associated Press.

'DRAG ON PRODUCTIVITY'

Currently, jetliners move in single-file lines along narrow highways in the sky marked by radio beacons. Many of the routes gently zigzag from one beacon to the next, sometimes forcing cross-country flights to follow sweeping arcs and waste hundreds of gallons of fuel.

It's "the equivalent of using an electric typewriter when others are using computers," said David Castelveter, a spokesman for the Air Transportation Association. "It's a huge, huge drag on productivity."

Some private and commercial aircraft already are equipped with GPS devices that pilots use to determine their position, but the NextGen system would dramatically expand use of the technology by creating a nationwide GPS network for air traffic.

Building the network involves gradually putting together the new system while still relying on radar for day-to-day operations.

Gerald Dillingham, director of civil aviation issues at the U.S. Government Accountability Office, likened the process "to changing a tire on a car that's going 60 miles an hour."

Hank Krakowski, the FAA's head of the air traffic system, called it "one of the largest project management challenges the federal government has had since we put somebody on the moon."

Airports also have to make improvements to accommodate the expected increase in air traffic.

U.S. airlines are struggling, in part because of high fuel prices. Ten airlines have shut down and others are facing bankruptcy. Their financial troubles mean less-frequent flights and fewer amenities for air travelers, who must pay more for tickets, luggage, drinks — even pillows.

MORE PLANES IN AIR

Airlines are expected to contribute $15 billion toward the $35 billion project, and must equip their fleets with GPS at a cost of more than $200,000 per plane.

But most carriers — which are otherwise enthusiastic about NextGen — are reluctant to retrofit planes years, maybe decades, before the satellite network is fully operational.

"It's like you buying a new car and the dealer saying, 'How would you like to buy this nifty GPS technology — but it won't be available in your area for years,' " Castelveter said.

The NextGen system could offer airlines a 10 percent savings in fuel costs per year. If the network were in place today, it would essentially pay for itself in seven years.

At least one major U.S. carrier, Southwest Airlines, says it's investing $175 million to equip 500 planes with GPS within a few years. That will allow pilots to fly more efficiently even before the full NextGen system is in place, including quicker landings that burn less fuel.

Getting each of its planes on the ground just one minute faster, Southwest says, would save $25 million in fuel a year.

Cargo carrier UPS has also installed GPS gear on hundreds of aircraft for use at its Louisville, Ky., hub, saying the technology will save nearly a million gallons of fuel a year, as well as reduce noise and emissions by around 30 percent.

NextGen would also help airliners fly, land and takeoff closer together, minimizing delays. Even though the technology would allow more planes into the sky, the FAA and pilots agree that the technology would actually reduce the risk of accidents such as in-flight collisions and runway incursions.

NextGen would let flight crews view precisely where other aircraft are using a cockpit monitor. The current radar system takes more than 10 seconds to scan an area, so controllers keep aircraft separated by several miles as a precaution.