How Did the 5G C-Band Threaten to Ground Thousands of Flights?

January 19, 2022 by Jake Hertz

AT&T and Verizon agreed to delay a mass 5G launch near airports. But major airlines are still concerned about how the 5G C-band may affect their aircraft altimeters.

This week, major airlines have filed an emergency request with the FCC to delay the impending rollout of 5G C-band technology, fearing that this cellular network will cause significant interference to commercial airplanes.

This has resulted in a heated battle between leading airlines and wireless providers Verizon and AT&T, leaving the FCC with a difficult decision to make.

In early January, AT&T and Verizon rejected a proposal from the U.S. Department of Transportation and the Federal Aviation Administration to delay widespread commercial 5G deployment. The two telecom giants argued that the rollout of 5G's C-band is essential to remain competitive with China and support remote work. 


What is the 5G C-Band?

5G technology often refers to wireless communications that occur in the 25–39 GHz frequency range. The 5G C-band, however, is a unique 5G band that exists between 3.7 GHz–4.2 GHz, which is considerably lower than most 5G offerings. The 5G C-band still offers significant data rate improvements over 4G (which operates between 700 MHz–2500 MHz), but its lower frequency band allows it to offer a higher communication range. 


Common U.S. spectrum bands

Common U.S. spectrum bands. Image used courtesy of PC Magazine


Last year, during a government auction of this band, AT&T and Verizon cumulatively purchased almost all of the rights to the C-band for a combined total of $80 billion. This week the wireless carriers were to begin their deployment of the technology, which includes areas around many major U.S. airports.

Radar altimeters on commercial and military aircraft function within 4.2 GHz–4.4 GHz, which overlaps with the C-band's frequency ranges of 3.7 GHz–4.2 GHz. Airlines fear that this crossover may detrimentally impact altimeter performance, making it unsafe to fly.


Fears of 5G C-Band Interference

As with all wireless communications, frequency bands must be closely regulated and monitored to avoid interference in channels. 

Airlines claim that the 5G C-band occupies a segment of the radio spectrum that is very close to airplane altimeters. These radar altimeters are typically attached to the bottom of the aircraft, where they transmit RF signals to the ground. The altimeter measures the time it takes for the signal to reach the terrain and reflect back to the aircraft to determine the aircraft's height from the ground. This process occurs multiple times per second to give pilots an accurate measurement.


Altimeters and the 5G C-band

Altimeters and the 5G C-band operate at close frequencies. Image used courtesy of the BBC


For months, the FCC has claimed that a buffer exists between the usable 5G C-band and the frequency band used by the altimeters. Still, airlines are erring on the side of safety, claiming that the risk of interference is still too great. 


Unknown Effects on Altimeter-dependant Systems 

Wireless carriers argue that this interference is not a legitimate concern for multiple reasons. Not only is there a guardband between the 5G C-band and the altimeter band, but the 5G C-band has already been rolled out in over 40 other countries and has not shown any issues of interference.

While the Federal Aviation Administration (FAA) is undergoing pressing investigations on how the 5G C-band impacts altimeter readings in low-visibility conditions, the administration admittedly does not yet have enough data on how other automated safety systems relying on altimeters may be affected. Missing or incorrect altimeter inputs could cause these systems to function in an unexpected manner during critical stages of flight, like takeoff, approach, and landing. 


C-band-effected altimeter readings

Automated safety systems that could potentially be affected by C-band-effected altimeter readings. Image used courtesy of the FAA

This risk is only compounded in older planes that don't have the most efficient band-pass filters on their receivers, according to Ted Rappaport, an NYU electrical engineering professor, in an NPR report. Rappaport compares the C-band/altimeter interference to the way CB radios could interfere with outdated TV sets before cable and digital signals came along. 

In the same report, Randall Berry, an electrical engineering professor at Northwestern University explained, "The underlying issue here is that radio signals cannot be perfectly confined to their assigned frequency band and will result in some energy being transmitted in neighboring bands." He adds, "Also, radio receivers cannot perfectly filter out the signals transmitted in neighboring bands and so will receive some out-of-band interference. Both of these effects result in interference from a neighboring band."


The FAA Asks for More Time to Investigate

As a precaution, the FAA has asked Verizon and AT&T to hold off the deployment of their 5G C-band technology near any high-priority airports for the next two weeks while the FAA conducts a study.

Now the airlines have until this Friday to provide a list of those 50 high-priority airports; until July 5th, wireless companies will agree to a two-mile buffer zone from any airport runway in that list.

The wireless companies have also agreed to decrease the power of the 5G towers, as was done in other countries like France, to help minimize the risk of interference. France uses 5G spectrum situated further away from the radio altimeter spectrum. It also uses lower power levels for 5G than those permitted in the U.S. 


5G fundamental and spurious emission levels

5G fundamental and spurious emission levels. Image used courtesy of Aviation Today


In the meantime, the FAA will only allow planes with accurate, reliable altimeters to operate around high-power 5G. Planes with older altimeters technology will be asked not to make landings when visibility is low. 

In a statement from the White House, President Bident thanked AT&T and Verizon for cooperating with the Department of Transportation to limit locations of 5G deployment amid ongoing investigations. 

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    drjohsmith January 20, 2022

    Other parts of the problem I have heard, but have no proof, would like to know thoughts

    a) US is using 6 or 12 dB more transmit power than rest of world
    b) US is allowing beam forming “upwards” near airports, so planes can receive 5G, whilst Europe is not
    c) US has a lot more “small” aircraft, and private pilots, who might be using “less well designed” radar altiminiters than say UK
    d) 5G was not invented in US

    Contentious I know ,
      any thoughts

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    drjohsmith January 20, 2022

    one last one
      as the band around the altimeter frequency has only had low power adjacent, the front ends might not have been designed as well as they could be, and with the higher power in US, it might be a problem,

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    chrisbaron January 21, 2022

    The auction for this frequency band happened almost 2 years ago and was identified for years before that.  Why is this just coming to light now?  Why is anyone surprised? 

    Is this incompetence or some kind of political flex?  I vote for both.

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