Out-of-date satellites could leave motorists in the lurch, a report by the US Government Accountability Office (GAO) warns.
It's warning of a GPS meltdown, as the replacement of in-orbit units is months behind schedule, and millions of dollars over budget.
The GAO said "It is uncertain whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption."
The report goes on to criticise the expense and delay of the new satellites, and advises that "If the Air Force does not meet its schedule goals for development of GPS satellites, there will be an increased likelihood that in 2010, as old satellites begin to fail, the overall GPS constellation will fall below the number of satellites required to provide the level of GPS service that the U.S. government commits to. Such a gap in capability could have wide-ranging impacts on all GPS users"
Sat-navs and other GPS devices use satellites to calculate a motoristís position, often triangulating the location using 3 or more military or civilian satellites to do so. Most satellites have an average life expectancy of 8 years, but some 'geostationary' satellites can last up to 15 years.