World is too reliant on GPS, warns report
In London, it's compulsory for black cab drivers to pass 'the knowledge' - an exam designed to demonstrate that they know every single street in the capital and can navigate from any one to any other.
And if the Royal Academy of Engineering is to be believed, they're about the only people likely to be able to function properly if global satellite navigation systems go down.
"GPS and other GNSS [global navigation satellite systems] are so useful and so cheap to build into equipment that we have become almost blindly reliant on the data they give us," says Dr Martyn Thomas, chairman of the Academy's GNSS working group.
"A significant failure of GPS could cause lots of services to fail at the same time, including many that are thought to be completely independent of each other. The use of non-GNSS back ups is important across all critical uses of GNSS."
While the report is focused on the UK, most of the findings apply internationally. Societies around the world are getting more and more dependent on GNSS systems.
As well as consumer satnav, the signals are used by data networks, financial systems, shipping and air transport, agriculture, railways and emergency services. Indeed, the European Commission recently estimated that an €800 billion chunk of the European economy is already dependent on GNSS.
And the report points out that such systems are vulnerable to deliberate or accidental interference - both man-made, such as jamming, and natural, such as solar flares.
The biggest threat, says the report, lies in dangerously misleading results which may not seem obviously wrong - a ship directed slightly off course by faulty data, for example.
"The deployment of Europe's Galileo system will greatly improve the resilience of the combined GPS/Galileo system, but many of the vulnerabilities we have identified in this report will remain," says Dr Thomas.
"No-one has a complete picture of the many ways in which we have become dependent on weak signals 12,000 miles above us."