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Satellite expert Roger McKinley warns that “we’ve become overdependent” on GPS, in an interview with Vox. The former president of the Royal Institute of Navigation says that our navigational capabilities are atrophying and recently wrote in the journal Nature that “automatic wayfinding is eroding [our] natural abilities.”

McKinley’s interest now is in those rare instances when satellite-dependent navigation fails. While over 99.9 percent of the time it works exactly as expected, the few instances when it doesn’t can mean seriously bad news.

One of the issues that impedes the reliability of GPS is inherent in the system. Thrity-one GPS satellites orbit the Earth, and devices need to receive four signals to triangulate properly. These signals, however, are fairly faint. Moreover, they can be bounced around and distorted by large buildings or other structures. That’s why GPS can fail so spectacularly in urban environments. “There are plenty of cases of drivers in built-up urban areas suddenly wishing they had a map,” McKinley says. That’s the same reason why GPS barely works indoors.

When self-driving cars (that heavily rely on GPS to move even a few metres) finally hit the roads in large numbers, these unreliable signals could spell disaster. Working well 99.9 percent of the time just isn’t good enough when the cars drive themselves. With thousands of vehicles, accidents (and bad ones) are bound to happen.

Unreliability is one thing, but GPS disrupting devices are a whole other kettle of fish, and they’re becoming quite common.

“Back in 2009, engineers were testing a GPS system at Newark airport and kept encountering regular interference,” McKinlay says. “They finally figured out, through closed-circuit cameras, that it was a passing truck driver who was using a GPS jammer so that his employer couldn’t track him. There are also cases of criminals using jammers — which are handy for stealing cars — around ports, which can affect maritime navigation. This still isn’t common, but it’s being observed more often.”

This doesn’t affect just the regular folks. Ships are ultra-reliant on GPS these days, and the US Navy is highly concerned about the trouble signal-jamming can cause. An unnamed US East Coast port was left paralysed for seven hour in 2014 because of interference with GPS signals.

Luckily, officials are looking at ways to overcome these issues. Europe’s Galileo will consist of 30 satellite when its fully operational, and that will go a long way to providing redundancy. China is working on a similar system, BeiDou. But this solves only part of the problem – jammers remain.

Several US agencies have augmented GPS with additional systems, air and ground-based, like the WAAS and NDGPS. And the US Department of Defence is working on eLORAN, a ground-based system that would assist GPS with unjammable signals.

Until these systems are running, however, we remain vulnerable. And even when they’re all set up and functioning, problems may creep up and malicious actors could find ways to stifle them as well.