The GPS Broke. Now What?
Though I've never had the system fail me, when I began to delve into all the things that can completely screw up the ability of a GPS to give a good fix, I grew a little nervous about trusting it much farther than I can throw one. Because of the very low power of the signal, GPS systems are vulnerable to interference from such natural causes as solar flares and a whole range of human-generated broadcasts, including intentional jamming. Add the fact that on a boat, anything can-and will-break, often at the worst moment, and you'll see why it's important always to have a backup plan.
To illustrate the frailty of your GPS, consider the story about the United States, Britain, and France trying to sell Greece some tanks in 2000. According to press accounts that cited the U.S. Department of Defense, the Americans and the British had continual problems with the guidance systems on their tanks failing. It turned out that the French had a little portable GPS jammer with a foot-high antenna that they hid behind their vehicle and used whenever another country's vehicles were being demonstrated. The Greeks were reportedly amused, but had it been you trying to navigate along an inhospitable shore, you might not have been.