The cause for their concern, Johnson said, was an unexpected solar radio burst on December 6 that affected virtually every GPS receiver on the lighted half of Earth. Some receivers had a reduction in accuracy while others completely lost the ability to determine position, he said.

In addition to the GPS system, the December solar flare affected satellites and induced unexpected currents in the electrical grid, Johnson said.

"The effects were more profound than we expected and more widespread than we expected," added Kintner.

Dale E. Gary, chairman of the physics department of the New Jersey Institute of Technology, said the burst produced 10 times more radio noise than any burst previously recorded.