The gadget in our picture...

The gadget in our picture may look unassuming, but it could sound the death knell for national road pricing plans. That’s because it’s a sat-nav jammer, and could render the most suitable technology for a pay-as-you-drive charging scheme useless.

It costs ?700 and plugs into a car’s cigar lighter socket. We were offered one of the ‘cloaking devices’ by a retailer, which said it wasn’t strictly supposed to sell them in Europe. And that’s not surprising, as thieves could also use the gadget to foil certain GPS-based stolen car tracking devices.

How does it work? Trackers – similar to those Norwich Union uses for its pay-as-you-go insurance – rely on sat-nav technology to log vehicle movements. This data is stored on a ‘black box’ in the car, then relayed to a mon­itoring station via mobile phone. The jammer stops both processes.

It emits a frequency that blocks the signal from global positioning satellites. As a result, the black box can’t record movement data. The jammer unit also has a mobile phone blocker to stop the black box sending information back to its base. Essentially, it has the same effect as leaving a vehicle in a lock-up or underground car park.

A Department for Transport spokesman said: “We have asked firms to suggest ways to operate a road pricing scheme, with a view to trialling some systems next year. But this could only be introduced if we can find technology that’s robust against attack.”

Would you use one of these GPS jammers? Tell us what you think on Mat"s blog here!