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FAA urges airlines to replace altimeters that can’t filter out 5G signals


An airplane flying past a cell tower.

Getty Images | BackyardProduction

The Federal Aviation Administration is reportedly urging airlines to retrofit or replace altimeters that receive transmissions from outside their allotted frequencies. The FAA is meeting Wednesday “with telecom and airline industry officials on a push to retrofit and ultimately replace some airplane radio altimeters that could face interference from C-Band 5G wireless service,” Reuters reported Tuesday.

The Reuters report continued:

The FAA wants to use the meeting to establish “an achievable timeframe to retrofit/replace radar altimeters in the US fleet,” according to a previously unreported letter from the FAA’s top aviation safety official Chris Rocheleau reviewed by Reuters. It also asked aviation representatives “to offer options and commit to actions necessary to meet these objectives.”

Some altimeters used by airplanes to measure altitude apparently cannot filter out transmissions from C-Band frequencies assigned to wireless carriers for 5G. Altimeters are supposed to use frequencies from 4.2 GHz to 4.4 GHz, while wireless carriers’ C-Band licenses are for 3.7 to 3.98 GHz.

Retrofits could be achieved with antenna filters that “are currently in production,” with one key question being “how to determine which planes are most at risk of interference and should therefore get retrofitted first,” the Reuters report said.

FCC urged aviation to act over two years ago

Airplanes’ faulty altimeters recently spurred the Federal Communications Commission to launch an inquiry into poorly designed wireless devices that receive transmissions from outside their allotted frequencies. The FCC could ultimately issue rules requiring better performance for wireless receivers, similar to how the FCC prohibits wireless transmitters from broadcasting outside their licensed spectrum. While the FCC has historically focused on regulating transmitters, the 5G/altimeter debacle showed how poorly designed receivers can prevent efficient use of spectrum.

The FAA’s new move to retrofit or replace altimeters comes more than two years after the FCC urged the aviation industry to fix the problem. In the FCC’s February 2020 decision to reallocate C-band spectrum, the commission said the 220 MHz guard band between altimeters and 5G should be plenty. More “analysis is warranted on why there may even be a potential for some interference given that well-designed equipment should not ordinarily receive any significant interference (let alone harmful interference) given these circumstances,” the FCC said at the time.

AT&T and Verizon were set to deploy 5G on the C-Band in December 2021, but the FAA forced the wireless industry to delay. The rollout eventually went ahead, and the carriers agreed to additional temporary restrictions near airports until July 2022.

The 220 MHz guard band is 400 MHz in practice this year because AT&T and Verizon have not yet deployed above 3.8 GHz, but the FAA has continued to warn of potential problems with altimeters. While the FAA authorized 90 percent of the US commercial aircraft fleet for low-visibility approaches in areas with C-band deployment, there could be another fight coming in July when AT&T and Verizon are scheduled to lift their voluntary 5G restrictions around airports.

Dispute “won’t be completely resolved” this summer

US Secretary of Transportation Pete Buttigieg told the Senate Appropriations Committee last week that the problem “won’t be completely resolved by this summer” and remains a “top concern,” according to CNET.

“We’re in a much better place than we were in December and January,” Buttigieg said, expressing optimism, “largely because we have much better dialogue and collaboration, not just among regulators but among industries, and have been directly engaging with the airlines, the aviation equipment manufacturers and with the telecom carriers to make sure that we’re on a better path.”



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