He also said it is "very unlikely" that users will notice a decline in GPS accuracy. The next satellite designed to maintain the current group is set to launch in August, he added.

The GAO reported that 31 satellites are being used when only 24 are required to keep the GPS service for civil, commercial and military users above a 95% probability of staying within acceptable performance standards.

Defense Department officials predict that several satellites in the constellation will reach the end of their operational life faster than they will be replaced, meaning the probability of maintaining 24 operational satellites falls below 95% during fiscal year 2010 through the end of 2014, the GAO said.

From October 2011 to October 2012, the probability of maintaining 24 GPS satellites drops to as low as 80%, the GAO said.

In her testimony, the GAO's Chaplain said there could be "wide-ranging impacts" on GPS users with gaps in capability, including a decrease in the accuracy of precision-guided munitions using GPS to strike their targets. That would require using larger munitions or more munitions to hit the same target, GAO said. Also, airlines might have to delay, cancel or reroute flights, and E-911 services could lose accuracy.