FAA To Airlines: 5G-sensitive Radio Altimeters Have To Go

The US Federal Aviation Administration (FAA) met with airline and telecom officials yesterday to present its latest solution to the instrument interference problem presented by C-band 5G: replace the affected equipment.

A letter from the FAA's head of aviation safety, Chris Rocheleau, proposed the meeting to establish a timeline for retrofitting or replacing radar altimeters in US airliners that are affected by 5G C-band signals, Reuters reported.

5G C-band was expected to roll out in the beginning of 2022, but was put on hold until July while the FAA, airlines, and jet manufacturers seek a resolution. A number of different planes were affected, including most of the Boeing 737 family, due to their use of radio altimeters, which use radio signals to determine the plane's distance from the ground. 

C-band signals operate between 4.2 and 4.4GHz, while C-band 5G transmits between 3.7 and 3.98GHz. There isn't any overlap, but the FAA isn't taking chances. It claims that interference could still affect altimeters to the point where they become unreliable. Radio altimeters are typically used to land planes in low-visibility conditions. 

The FAA has cleared 90 percent of the US's commercial aircraft fleet for operation in and around 5G C-band antennas, but some Boeing planes, the 737 included, are still under restrictions. 

• FAA now says 5G airports may interfere with Boeing 737s
• Watchdog clears 90% of US commercial aircraft to land in low visibility at nation's 5G C-band airports
• Japan solves 5G airliner conundrum: Keep mobe masts 200m from airport approach paths. That's it
• 50 US airports to be surrounded by 5G C-band-free zones

Details of what was discussed at the meeting have yet to be released, but the FAA did verify its occurrence in a tweet published the afternoon of May 3. According to Reuters, the meeting plans also included a discussion of what to do after the July 5 C-band rollout happens, and whether to prioritize retrofitting radio altimeter antennas with filters to mitigate C-band interference. Whether replaced or retrofitted, the prospect is likely to be expensive for the thousands of affected aircraft.

There may be a simpler, cheaper, and more reasonable response to the problem of 5G C-band interference: just move the antennae to a safe distance.

Researchers in Japan found that 400 meters of physical separation between the focal point of an aircraft's radio altimeter (the ground right below the plane) and a 5G antenna, and 60MHz of unallocated frequency between the two, were all that was needed to prevent interference.

Japan's 5G allocations sit closer to the spectrum used by radio altimeters than in the US. ®