Radio Altimeter Questions Remain With Vote Pending on C-Band NOI

The NOI asks about spectrum directly below 4.2 GHz, the radio altimeter band, which makes the timing “terrible,” given the deadly crash two weeks ago of American Eagle Flight 5342 and an Army Black Hawk helicopter, said a technologist who has studied the interaction between C-band spectrum and the safety devices. While nothing is certain, the American airliner’s radio altimeter could have been a factor in the accident, the official said: “I would have encouraged [Carr] to hold off until the dust settled on that one.”

Airline industry officials declined comment Wednesday, saying they are studying the draft NOI, which became public last week (see 2502060062). The National Transportation Safety Board said in a Tuesday update that its Airplane Structures and Helicopter Airworthiness groups have “completed their on-scene tasks and are preparing for potential follow-on investigative activities.” None of the updates so far discuss radio altimeters.

Other industry experts defended the NOI, noting that it starts the process of asking questions, including about protection of radio altimeters. Following the C-band auction, carriers agreed to concessions to the airlines, including extended protection for flight operations near airports (see 2304030070). The FAA has also worked with the airlines on installing altimeters that are more tolerant of 5G interference (see 2302130048).

5G transmissions in the C band “are subject to many limitations that were negotiated with the aviation industry and FAA,” said Joe Kane, director-broadband and spectrum policy at the Information Technology and Innovation Foundation. “They are far more protective of altimeters than the aviation industry itself asked for in the FCC proceeding.” He said one of the benefits of the “drawn-out C-band saga” was better-performing altimeters replacing old technology.

An NOI is an “appropriate vehicle” to start reexamining the issues surrounding the upper C band, emailed Cooley’s Robert McDowell. Asking questions now may be “more timely and constructive” as a result of the recent air collision, “whether or not radio altimeters are implicated,” he said. The FCC must “uncover the empirical technical data needed to make a sound decision, one way or another, about the upper C band,” he said. “Hopefully that process will be completed in time for future spectrum auctions, if Congress renews that authority as part of the uncertain reconciliation process.”

The draft NOI proposes numerous questions about protecting altimeters, while acknowledging work the airline industry and carriers have done on more resilient altimeters. “How long would it take to develop, manufacture, and install further improved altimeters on transport aircraft?” the draft asks. “What, if any, technical and service rules for new operations in the Upper C-band could be implemented to maximize the commercial utility of the band while mitigating against any potential for adjacent band harmful interference?”

An NOI provides the opportunity “to learn from everything that went wrong with the first C-band auction and migration and do it right this time,” said Public Knowledge Senior Vice President Harold Feld. If the issue is updating altimeters, part of the auction revenue “can be used to replace old altimeters with modern devices that can reject unwanted signals,” Feld said. If interested parties work “in good faith” they can “develop better mitigation techniques, tighter emission masks, even use AI to come up with complex algorithms for reflection paths around airports that would reduce or eliminate the need for exclusion zones."

ITIF’s Kane noted that receiver performance remains “a major priority in making productive use of spectrum,” and “higher-performing receivers both protect incumbent users and make those users better neighbors to new ones.” He said he expects the FCC to look closely at receiver performance in the proceeding “alongside all the other legal and technical factors that impact more intensive use of the C band.”