The United States Air Force (USAF) has been telling the world that its global positioning system (GPS) will be working well for a long time to come.

The reassurance came after a US General Accounting Office review expressed concerns at the state of the network of satellites behind the system, suggesting that new satellites were not being launched fast enough to replace those lost to age and technical failure.

The report said that Boeing, which is building the next generation of satellites, had suffered from major cost over-runs and years of delay.

However, the USAF maintained that 24 satellites are currently operational, and that others can be activated as needed.

"The GPS will not go down," wrote Colonel Dave Buckman, of the USAF Space Command, in a post on Twitter. "The issue is under control. We are working hard to get out the word. The issue is not whether GPS will stop working. There's only a small risk we will not continue to exceed our performance standard."

Buckman added that a new satellite would be launched in August, and another next year.

GPS systems need to have line-of-sight contact with at least four satellites to establish a good positional fix, and academics have warned that the system is operating at the limits.

"The GPS constellation is skinny compared to what it should be," Per Enge, a professor of aeronautics and director of the GPS Research Laboratory at Stanford University, told Associated Press.

"No-one can complain or state that the sky is falling right now. At the same time, delays in launch schedules and funding are difficult to account for, and it is possible that new satellite models could have problems that are not discovered until they are in orbit."