As of 10 p.m. EST, there had been at least 850 flight cancellations and more than 28,000 delayed flights Saturday. American, Southwest, Alaska, Frontier and United say all of their planes have height-measuring devices, called radio altimeters, that are protected against 5G interference. FlightAware listed nine Delta flight cancellations Saturday. The Delta planes that have not been retrofitted include several models of Airbus jets: all of its A220s, most of its A319s and A320s and some of its A321s. Wireless carriers including Verizon and AT&T use a part of the radio spectrum called C-Band, which is close to frequencies used by radio altimeters, for their new 5G service.