Ten years after Congress gave the go-ahead to modernize the nation’s air traffic control system, one of the government’s most ambitious and complex technology programs is in trouble.

The Next Generation Air Transportation System, or NextGen, was promoted as a way to accommodate an anticipated surge in air travel, reduce fuel consumption and improve safety and efficiency. By shifting from radar-based navigation and radio communications — technologies rooted in the first half of the 20th century — to satellite-based navigation and digital communications, it would handle three times as many planes with half as many air traffic controllers by 2025, the Federal Aviation Administration promised.