WASHINGTON (AP) -- President Barack Obama has signed legislation Tuesday that modernizes the nation's aviation system, speeding up the nation's switch from radar to an air traffic control system based on GPS technology. The law also opens up the skies to military, commercial and privately-owned unmanned drones.

The legislation faced opposition from some labor unions because it set new rules governing union organizing elections at airlines and railroads.

The Senate passed the bill last week, completing action after a struggle that shut down the Federal Aviation Administration for two weeks.

The law authorizes $63.4 billion for the FAA over four years, including about $11 billion toward the air traffic system and its modernization. It sets a deadline of June 2015 for the FAA to develop new arrival procedures at the nation's 35 busiest airports.

"This critical effort to shift from our antiquated air traffic control technology to a GPS-based system will improve air traffic efficiency and safety, reduce fuel burn and pollution from aircraft, and bring costs down for consumers," said Republican Rep. John Mica of Florida, the chairman of the House Transportation and Infrastructure Committee.

In a compromise, the legislation set some new requirements for union organizing elections. While some unions accepted the changes, others called for it to be rejected. Among those opposing the legislation were the Teamsters, Communications Workers, Machinists and Flight Attendants.

The president of the National Air Traffic Controllers Association, Paul Rinaldi, praised the new law.

"This new technology will help reduce delays, give controllers better tools with which to perform their jobs even more efficiently and provide a platform for further technological and safety enhancements," he said in a statement.

Under the new law, the FAA must by Sept. 30, 2015, begin permitting unmanned drones controlled by remote operators on the ground to fly in the same airspace as airliners, cargo planes, business jets and private aircraft.