The Senate authorized $15.6 billion for the nation's airports and air traffic control system in an effort to ease congestion in the skies that is expected to get worse. The legislation, passed 96 to 1, would guide federal spending for the next three years for improving airports and modernizing aircraft-tracking equipment. It would also help to pay the government's 15,400 air traffic controllers, its 1,900 aircraft inspectors and other workers. The number of passengers has increased from 278 million in 1978 to an estimated 450 million this year, and is projected by the Federal Aviation Administration to reach 700 million by 1998. In addition, the number of takeoffs and landings is expected to grow from 61 million this year to 79 million in 1998. The measure directs that about $1.7 billion be spent annually to expand and modernize airports, up from $1 billion this year.