A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert VanderPol II
Date: 2015 May 6, 16:11 -0700
A Coronal Mass Ejection (CME) large enough and hitting the earth directly would probalby knock out the GPS system and the electrical and telecom grids on the earth. The Carrington Event of 1859 was such an event, it caused damage to the telegraph systems of the time and auroas were visible as far south as the caribbean and as far north as Queensland.
In July 2012 there was a similarly sized CME that missed earth but hit a Solar Observatory satellite (Stereo-A) which suffered no apparent damage. My understanding is that survival was partly due to the design of the satellite and partly due the fact that it would outside of Earth's magnetic field which would have induced electrical currents that would have damaged or destroyed it. All of the GPS satellites are within earth's magnetic field.
This link : http://science.nasa.gov/science-news/science-at-nasa/2014/23jul_superstorm/ says there's a 12% chance of getting hit with a large enough CME in the period 2012-2022.
Other research I have found indicates that getting hit with a big enough CME is a 500-yr event though the validity of the underlying assumptions is disputed.