Solar flare activity can disrupt GPS signals also, as it did last December over a good part of the U.S.
"More seriously, magnetic field disruptions corrupt signals between Global Positioning System satellites and receivers, creating measurement inaccuracies. According to NOAA scientists, a solar flare last December caused large numbers of receivers to stop tracking the GPS signal altogether.
"A small measurement mistake probably isn't crucial to somebody with GPS in their car," said Bogdan. "They will still know generally where they are. They're not going to drive into a ditch."
"But such disruptions "can be crucial for industries and activities that require precise positioning, something within a few meters or less," he added. That includes "everybody from farmers to offshore oil drillers to soldiers on the battlefield."
From this link:
http://www.bendweekly.com/Science/10278.html
That is why there will certainly have to be a position source backup when ADS-B is fully deployed….perhaps the leading candidate for that is Loran E.