Concern over the future performance of the US Global Positioning System (GPS) went up a notch last week as a government watchdog official warned that the US Department of Defense faced substantial challenges meeting its space-programme commitments.

A report from the Government Accountability Office (GAO) on 30 April had cautioned that new GPS satellites might not be launched in time to replace the ageing constellation that is currently in orbit. And on 20 May, Cristina Chaplain, GAO director of acquisition and sourcing, told the Senate committee on armed services that cost overruns of space programmes are part of the problem.

Dave Buckman, of the US Air Force Space Command, quickly replied on a Twitter feed that "GPS isn't falling out of the sky". Still, a temporary decline in performance might cause a problem for scientists who rely on GPS-positioned equipment that cannot be easily upgraded, such as low Earth-orbiting satellites, says Marek Ziebart, a space geodesy researcher at University College London.