How to measure Arctic sea-ice

A British team has begun a gruelling trek to the North Pole to discover how quickly the Arctic sea-ice is melting.

During their 1,000km journey, the group plans to take measurements of the thickness of the ice.

It will be the most detailed survey of its kind this season, and should be completed in late May.

Pen Hadow explains how a radar and a hi-tech sledge are key to the expedition.