Even as our nascent eLoran system is being shut down here in North America, concerns over the vulnerability of GPS and other sat-nav systems are growing elsewhere. Late last month in the U.K., at a symposium hosted by the Royal Institute of Navigation and the Digital Systems Knowledge Transfer Network, several scientists and engineers discussed the increasing threat posed by those seeking to intentionally interfere with GPS signals. Just Google the term "GPS jammer" and you'll get a sense of the scope of the problem. Once GPS jamming equipment was used solely by government and military types for discrete purposes, but now it is commercially available, at prices starting below $100, to anyone who can think up a use for it. Most of these units are quite small and are only powerful enough to mask a car or truck with a GPS transponder aboard, but others are quite powerful, with effective ranges exceeding 300 meters at ground level.
How likely are you to have the GPS on your boat knocked out by someone operating a jammer? Not very, unless say you're passing under a highway bridge just as a delivery truck driver decides to flip on his portable jammer so he can slip off the company nav grid long enough to stop for a beer somewhere. Bear in mind, too, it is technically illegal in the U.S. to sell or operate such equipment. Still, in tests conducted by the British General Lighthouse Authority on England's east coast, engineers found that jammers used in a nautical context are very good at destroying position accuracy (degraded fixes ranged from Scandinavia to west of Ireland), as well as AIS and chartplotter functions.
The larger point is this: GPS and any other sat-nav network, like the Russian GLONASS, European Galileo, and Chinese Compass systems, can never be bulletproof. As was pointed out at last month's symposium, GPS signals are incredibly weak, no stronger at a receiver's antenna here on Earth than a 25-watt lightbulb viewed from a satellite in orbit. It takes very little intentional, or unintentional interference (like from a solar flare) to mess them up.
Not surprisingly, one of the most reliable back-up systems discussed at the symposium was… eLoran. Loran signals are quite strong, hard to fool with, and in eLoran configuration are accurate to within less than 10 meters. Of course, our government knew this already, having conducted its own research, so--for some reason--decided it would be best to turn Loran off.
Sorry for bringing this up again, but it really ticks me off.
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