When traveling over land, Delta usually relies on radar to track planes, but when flying over water or across the Arctic Circle, the airline depends on satellite surveillance systems. These include Automatic Dependent Surveillance, or ADS-B and ADS-C, and the Aircraft Communications, Addressing and Reporting System, or ACARS.
The ADS-B system works by triangulating a signal between two GPS satellites, a communication satellite, the ground command center and a plane's cockpit. The "B" system signals are sent to air traffic control and other aircraft. The ADS-C is similar, but also sends information to the Air Traffic Service Unit, which tracks the airplane's position. ACARS is a digital data link that tracks messages between planes and ground control.