|Jack Ganssle's Blog
This is Jack's outlet for thoughts about designing and programming embedded systems. It's a complement to my bi-weekly newsletter The Embedded Muse. Contact me at firstname.lastname@example.org. I'm an old-timer engineer who still finds the field endlessly fascinating (bio).
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Over-Reliance on GPS
March 15, 2021
Scott and I were aboard my sailboat Voyager approaching Bermuda. On a small boat the island is a week's sail from Norfolk. If your course is off even a single degree you'll miss it, so navigation is critical. We've sailed there many times in the past, but on this passage decided to leave the GPS turned off, relying on celestial instead. (With celestial you measure the angle of the sun or other object over the horizon using a sextant, and then can compute your position). We got within five miles of Gibb's Hill and could easily make out the lighthouse. I turned the GPS on to be super careful navigating the reefs.
It told us we were on the other side of the island, 20 miles away.
Bermuda Harbor Radio told us the US Navy was conducting some sort of GPS jamming drills. Had we relied on that system we could have lost the boat.
I get the weekly notice to mariners and it's rare there's an issue which does not detail some planned GPS jamming, at times generating expected errors of 50 or more miles.
A recent NY Times article details tens of thousands of GPS incidents of this nature. Yet our entire infrastructure is dependent on the system. Even ATMs will fail to function if they can't get timing info from the satellites. An IEEE Spectrum article describes how airliners are facing problems when the military purposely degrades or jams the signals.
It is the DoD's baby, after all. I guess if they want to mess with it, that's their right. However, we have become so dependent on the system that purposeful jamming, which can drive airlines miles off course, seems criminal.
Cheap jammers are available on eBay. Malevolent actors can cause much disruption.
The problem really isn't GPS or the jammers. It's that we've become entirely dependent on the system. Sure, there are other satellite-based navigation aids, but their signal levels simply invite bad actors. GPS can be -150 dBm or lower; it doesn't take much to wipe that signal out.
The oldest rule of navigation is to never depend in a single aid to navigation. That buoy might have drifted off course. The radio beacon could be out. Not long ago we sailors had many electronic aids. There was Loran, which transmitted strong signals long distances. Many lighthouses broadcast morse code beacons that we could triangulate on. These are all gone. We're left with satellite navigation which, when it works, is breathtaking in its accuracy and ease-of-use. But that's about it. There's talk of enhanced Loran, but that seems to be more talk than action.
Me, I'll continue to carry a sextant at sea.
With a sextant there's a lot of plotting to get a position. This is typical over a three day period.
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