Solar Flares and GPS

Newspaper articles have depicted the wave of solar flares that have recently developed, usually accompanied by some oblique reference to their ability to disrupt "satellite based navigational systems." Does anyone know exactly how and to what degree a solar flare and those beautiful northern lights will screw up a GPS unit?

Solar flares can prevent your unit from picking up any satellites at all, or you could pick up satellites and get readings that are slightly off or readings that say you are hundreds of miles from your current position.

The problem is with the satellites, not with the ground unit.