Long before GPS, ships have been using a system called “LORAN” to navigate the worlds oceans. LORAN, or Long-Range Aids to Navigation is a land-based positioning system run by the Coast Guard to allow ships to track their positions at sea. (Both GPS and LORAN work on the same principles - you can triangulate your location based on two or three fixed points provided you know how far from each one you are.) The 30-year-old system will get a $34.5 million upgrade from the Department on Homeland Security to upgrade the transmission equipment, and add a data channel that will transmit detailed information and a constant time-keeping signal.

Popular Mechanics says that LORAN is a good backup for GPS because :

Story and quote from Popular Mechanics
http://www.popularmechanics.com/technology/industry/4266972.html

I mean, think about what would happen if GPS failed. It’s everywhere. Its used to locate millitary airplanes, ships, and weapons. Hikers and other outdoor professionals use it to navigate rough terrain and not get lost. We’ve got it in our cars to give us directions on how to get from here to there. It’s used to track the location of other commercial equipment and packages. A major GPS failure could take quite a while to recover from. A backup plan is always a good idea.