The weak signal can cause a ship's instruments to gradually drift out of true, without setting off any alarms, leaving the ship in a different position to where it "thinks" it is.
The Foreign Affairs Select Committee, which has recommended that that the government should review how it handles the kidnap of British nationals abroad, said there was limited guidance on the use of lethal force against pirates trying to ambush vessels, and that ship captains were left in a difficult position trying to decide what action to take.