• 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.

Watch: GPS jamming (& spoofing) explained, Flightradar24

Our data comes from volunteers' aircraft receivers, shared through adsb.lol and adsb.fi. Empty areas have few volunteers or no civil flights.