What would happen if GPS - the Global Positioning System - stopped working?

For a start, we would all have to engage our brains and pay attention to the world around us when getting from A to B. Perhaps this would be no bad thing: we'd be less likely to drive into rivers or over cliffs through misplaced trust in our navigation devices.
With no GPS, emergency services would start struggling: operators wouldn't be able to locate callers from their phone signal, or identify the nearest ambulance or police car.

There would be snarl-ups at ports: container cranes need GPS to unload ships.

Gaps could appear on supermarket shelves as "just-in-time" logistics systems judder to a halt. Factories could stand idle because their inputs didn't arrive just in time either.

Farming, construction, fishing, surveying - these are other industries mentioned by a UK government report that pegs the cost of GPS going down at about $1bn (£820m) a day for the first five days.

If it lasted much longer, we might start worrying about the resilience of a whole load of other systems that might not have occurred to you if you think of GPS as a location service.

Consider phone networks: your calls share space with others through a technique called multiplexing - data gets time stamped, scrambled up, and unscrambled at the other end.

A glitch of just a 100,000th of a second can cause problems. Bank payments, stock markets, power grids, digital television, cloud computing - all depend on different locations agreeing on the time.

If GPS were to fail, how well, and how widely, and for how long would backup systems keep these various shows on the road? The not very reassuring answer is that nobody really seems to know.

No wonder GPS is sometimes called the "invisible utility".
Trying to put a dollar value on it has become almost impossible. As the author Greg Milner puts it in Pinpoint: How GPS is Changing Our World, you may as well ask: "How much is oxygen worth to the human respiratory system?" It's a remarkable story for an invention that first won support in the US military because it could help with bombing people - and even it was far from sure it needed it. One typical response was: "I know where I am, why do I need a damn satellite to tell me where I am?"

it wasn't until the first Gulf War, in 1990, that the sceptics came around.

As Operation Desert Storm ran into a literal desert storm, with swirling sand reducing visibility to 5m (16ft), GPS let soldiers mark the location of mines, find their way back to water sources, and avoid getting in each other's way.

It was so obviously lifesaving, and the military had so few receivers to go around, soldiers asked their families in America to spend their own money shipping over $1,000 (£820) commercially available devices.

The American taxpayer puts up the billion-odd dollars a year it takes to keep GPS going, and that's very kind of them. But is it wise for the rest of the world to rely on their continued largesse?

In fact, GPS isn't the only global navigational satellite system.
There's a Russian one, too, called Glonass - although it isn't as good. China and the European Union have their own well advanced projects, called Beidou and Galileo respectively. Japan and India are working on systems too. These alternative satellites might help us ride out problems specific to GPS - but they might also make tempting military targets in any future conflict, and you can imagine a space war knocking everyone offline. A big enough solar storm could also do the job.