It wouldn’t be a total catastrophe, but it could certainly cause lots of trouble for transportation systems that rely on GPS, especially passenger airlines.

Existing satellites are ageing, and replacements are behind schedule and over budget, according to a report from the US Government Accountability Office.

Satnavs and other GPS devices calculate their position by comparing time signals from at least four satellites. To keep that many within range at all times requires a fleet of at least 24. For now there are 31 operating, but 13 of them are more than four years past their design lifetime.

The first replacement “block IIF” satellites are not due to launch till November, three years behind schedule, and the GAO predicts a 20 per cent chance that the fleet will drop below 24 at times in 2011 and 2012. That wouldn’t cause GPS to shut down, but its accuracy would drop unpredictably.

And you wouldn’t want to trust an unreliable GPS…