Outdoor pursuits enthusiasts have been warned about military jamming of GPS services during UK-led Nato war games later this month.
The Mountaineering Council of Scotland has flagged up information from communications watchdog Ofcom about the denial of GPS during Joint Warrior.
Jammers will be in place at Faraid Head at Durness and Loch Ewe.
Jamming during Joint Warrior in October 2011 was suspended after complaints from Western Isles fishermen.
Relying on GPS for navigation in Scotland's hills and mountains is usually discouraged by the mountaineering council.
It would rather people attained skills in the use of a map and compass.
Ofcom said signals would not be jammed if GPS was required by the emergency services.
Joint Warrior, which involves army, navy and air force personnel, ships and aircraft from the UK, USA, Canada and European nations, is held twice a year.
The first exercise this year, in April, was the biggest in its history.
This month's exercise will be held from 4 to 16 October, with jamming happening periodically from 5 October.