Outdoor enthusiasts have been warned military jamming exercises will affect Global Positioning System devices in East Anglia.
Ofcom, the Government’s communications body, has given advance warning of the exercises, which will take place in September and October.
The daytime jamming will affect GPS signals within 5km (3 miles) of the Stanford military training area, north of Thetford in Norfolk.
Walkers and cyclists will need to use manual navigation rather than relying on GPS units during the jamming, which is due to take place between 9am and 5.30pm between 19 and 23 September and again during the same hours between 3 and 7 October.
Although the military training area is not open to the public, the Peddars Way and Norfolk Coast path run along its eastern boundary.