When the first of the Boeing-built Global Positioning System 2F satellites blasts off toward orbit May 21, the launch will close a long chapter of delays for Boeing and the U.S. Air Force on the program.

The satellites will bring modest improvement to the constellation’s sub-meter positioning accuracy, and their cores are similar to the Boeing GPS 2A satellites launched in the 1990s, an Air Force official said.

Boeing and the Air Force discovered engineering flaws during tests late in development of the first satellites. The Government Accountability Office last year declared the program $870 million over budget and three years behind schedule, and made the 2F program the basis for its controversial warning that the GPS constellation could grow unacceptably thin in coming years.

In a prelaunch press briefing, Col. Dave Madden, commander of the Air Force’s GPS wing, said the delays were one-time events that should not happen again. He said the program’s history is sometimes misunderstood by critics.

“A lot of people when they pick on the 2F program, they’ll pick on it for how long it took,” Madden said, “but a lot of it had to do with the fact that we changed requirements right in the middle of the program.”

The government added a new civilian signal, L-5, and a new military-only signal, M-Code, that is being used for tests at this point, he said. M-Code is not scheduled to be used operationally until 2013, when the first receivers are fielded.

There were engineering mistakes, Madden said, but those were the result of a failed acquisition strategy called total system performance responsibility, under which contractors were given latitude to meet broad design requirements, he said.

“We removed all the [military] standards, and the system engineering practices that we believed were best practices all got removed during that era,” he said.

Once mistakes were uncovered, the Air Force restored independent oversight of technical work and Boeing set up a series of technology gateways to spot problems early.

During the briefing, Madden’s staff said the average cost of a 2F satellite is $121 million.