At the Clay Maitland blog, Michael Grey has a good post - The dangers of GPS reliance about the vulnerabilities of GPS. He relates the story of a test of a GPS jammer and says:
The result was pandemonium, with virtually every system aboard this exceptionally well-equipped (and well-prepared) ship gave up the ghost in a simultaneous burst of loud alarms. Only the vessel’s enhanced Loran was unaffected by the interruption.
eLORAN would have been a good back-up for GPS but it seems it is a dead issue in the U.S., the towers are being destroyed. There is one method even more robust then a GPS/eLORAN system however - dead-reckoning, at least on paper charts that is.
Here is an article from BBC News about GPS vulnerabilities to solar activity -
Sat-nav devices face big errors as solar activity rises