At the Naval Medical Center, emergency pagers used for summoning doctors stopped working. Chaos threatened in the busy harbour, too, after the traffic-management system used for guiding boats failed. On the streets, people reaching for their cellphones found they had no signal and bank customers trying to withdraw cash from local ATMs were refused. Problems persisted for another 2 hours.
It took three days to find an explanation for this mysterious event in January 2007. Two navy ships in the San Diego harbour had been conducting a training exercise. To test procedures when communications were lost, technicians jammed radio signals. Unwittingly, they also blocked radio signals from GPS satellites across a swathe of the city.
Why would a GPS outage cause such disruption? These satellite signals now do a lot more than inform your car's satnav. GPS has become an "invisible utility" that we rely on without realising. Cellphone companies use GPS time signals to coordinate how your phone talks to their towers.
Energy suppliers turn to GPS for synchronising electricity grids when connecting them together. And banks and stock exchanges use the satellites for time-stamps that prevent fraud. Meanwhile, our societies' reliance on GPS navigation is growing by the year.
Some are worried that we are now leaning too heavily on a technology that can all too easily fail – and it doesn't need a freak navy training exercise to cause havoc. Their biggest concern is a GPS jammer – a plastic device that can sit on car dashboards.
These can be bought on the internet, and tend to be used by say, truckers who don't want their bosses to know where they are. Their increasing use has already caused problems at airports and blocked cellphone coverage in several cities.
One jammer can take out GPS from several kilometres away, if unobstructed. No surprise, then, that researchers across the world are scrambling to find ways to prevent disastrous GPS outages happening.
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