Conflict jamming. Armed forces jam GPS around war zones and sensitive sites, such as the Baltic, the Black Sea and the eastern Mediterranean.
Interference by bad actors. Illegal jammers, often in vehicles around busy cities, and fake signals that trick receivers into reporting the wrong position.
Space weather. Solar flares and coronal mass ejections (CMEs) disturb the upper atmosphere and weaken signals, most of all near the poles.
Tests and exercises. Authorised jamming trials and military exercises, usually announced in advance.
Aircraft and equipment. Faulty receivers, or the aircraft itself blocking its antenna during sharp turns.
