Airline industry executives have ramped up their warnings that an upcoming type of 5G service will interfere with radio altimeters.

Radio altimeters measure the altitude above the terrain beneath an aircraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft. They are, of course, a key safety feature.

In February, the FCC completed a “C-band” (3.7–3.98 GHz) auction and issued licenses to several wireless network providers to deploy 5G services. Radio altimeters operate between 4.2-4.4 GHz.