- 5G in the C-band frequency range can interfere with aircraft altimeters, which help pilots see better when visibility is low.
- For that reason, a new rollout of the wireless technology caused airlines to cancel or delay flights across the U.S. this week.
- C-band 5G bridges the gap between low-band 5G and high-band 5G for "an optimal mix of fast speeds and broad coverage."
International airlines halted or postponed some flights in airports all over the U.S. this week over fears that cell phone companies’ new 5G service in the C-band frequency range would interfere with radar altimeters aboard aircraft. Altimeters play a critical role in determining an airplane’s height from the ground, and pilots rely on them when visibility is poor. While some airlines have resumed flights as of Friday, there is still uncertainty about how the new 5G deployment will affect airports.
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In a letter sent to the White House on Tuesday, CEOs from ten airlines warned that airports were not equipped to handle 5G service on the C-band frequency range, which was set to go live on January 19. Airlines would have no choice but to ground some flights, the letter said, which would result in wide-reaching passenger and cargo flight cancellations, as well as halting medical supply shipments. As a result, big wireless companies agreed to postpone the 5G launch near airport runways until they can work out a compromise with the airline industry, the Federal Aviation Administration (FAA), and other stakeholders.
With coverage in two other frequency bands, 5G service is already in use nationwide, including in airports. These other spectrums do not interfere with airplane instruments, so 5G rollout has not been problematic until now.
As potential solutions to this complex problem unfold, here’s what you need to know about the conflict between emerging wireless communications technology and aircraft safety requirements.
What Is C-Band 5G Service?
“C-band provides an optimal mix of fast speeds and broad coverage, complementing other 5G technology. In a nutshell, it fills the gap between our low-band 5G and our high-band 5G+,” Alexander Byers, AT&T’s director of communications and PR, tells Popular Mechanics in an email. It operates in the 3.7-gigahertz (GHz) to 4.2-GHz range and supports fast performance speeds over a wider geographic area.
What Are Telecom Companies Actually Doing With 5G When They Do Roll It Out?
Companies like Verizon and AT&T have invested heavily in licenses from the FCC to use 5G in the C-band. Those two telecom firms, which won most of the $81 billion auction bid for 5G last year, agreed to create two-mile buffer zones around 50 airports in the U.S. to reduce interference risks for six months. Though 5G service has already been delayed twice before Wednesday, more measures are needed before flight safety is a certainty, airline CEOs say.
Because 5G is already in use, “C-band deployments would not impact 5G availability inside airports or elsewhere where 5G is already deployed using a different spectrum band,” Byers says.
Seventy-eight percent of the U.S. commercial fleet has altimeters approved to work in low visibility during the 5G deployment, according to the FAA’s 5G and Aviation Safety website. C-band is used in over 40 countries around the world, according to the Wireless Association, a wireless communications trade group.
“Our deployments are essentially the same as those abroad—for example, the power levels at which we are authorized to operate are lower than those authorized most recently in the European Union,” AT&T’s Byers wrote.
How Does C-Band 5G Interfere With Airplanes' Instruments?
The problem is that onboard instruments function at the same radio frequencies as 5G. About 220 megahertz (MHz) separates cell phone transmitter signals from the portion of the electromagnetic spectrum aircraft use for communication. However, radio signals don’t stay in neat little frequency bands; they spread into neighboring frequencies, and no radio receiver can filter a signal perfectly to prevent this bleed-through.
FCC regulations in the U.S. allow the cellular systems to operate with higher power, which generates a stronger signal. By contrast, the antennas for cellular systems in France are tilted down, so that only weak signals reach nearby aircraft.
“You can deploy these networks in all kinds of different, complicated ways, and if you put constraints on the deployment, it can help control the amount of interference that these radar altimeters observe,” Nick Laneman, director of SpectrumX, tells Popular Mechanics. “Of course, that limits the 5G performance, or increases the cost of the cellular industry. So there are tradeoffs that have to be explored. It’s very complicated. Even expert researchers in the field are pretty perplexed by the complexity of it all.” SpectrumX is the first National Science Foundation Spectrum Innovation Center, operated by Notre Dame University and composed of an interdisciplinary group of scientists, engineers, and educators around the nation. The organization investigates wireless technology advances and partners with industry, government, and academia to expand wireless communications knowledge.
What Kinds of Solutions Could Allow 5G C-Band Cell Service and Maintain the Safety of Flights?
Radar altimeter manufacturers must provide more radio receiver filtering so that altimeters can effectively ignore 5G signals from telecom transmitters. This means either replacing or retrofitting instruments in the aircraft with more fine-tuned filters, Laneman says. “That’s no small matter. That will take time, and there will be costs associated with it.” While C-band 5G rollout may be delayed around airports, it’s crucial to pause and ensure the safety of flights, he says.
Changing the tech on the aircraft is more of an operational challenge than a technological one, Laneman says. A retrofit would involve inserting a filter somewhere between the airplane’s antenna and the rest of the radio that’s processing the signal. The filter must do a better job suppressing, or attenuating, the 5G signals. Conceptually, it would be like inserting an amplifier between the antenna and the TV receiver to enhance the signal, Laneman says.
The initial C-band plan was to use signal frequencies of 3.7 to 3.98 GHz, but that’s too close for comfort to the operating radar altimeter frequency band of 4.2 to 4.4 GHz. Cell phone carriers have made some concessions as a result, scaling back the frequency to a maximum of 3.8 GHz, which European countries are also using. “Now that kind of equalizes the playing field between the two continents,” Laneman says.
Laneman says SpectrumX is not taking any position in the current quandary, but will help inform and educate the public by analyzing the problem and providing some technology and policy options. “If you want more and more of this 5G technology, and it’s gobbling up spectrum, then it’s creating a bunch of these neighbor issues,” he says. “At a macroscopic level, this is a gigantic negotiation between two major industries and three regulatory bodies. But it is solvable. It’s just going to take time, coordination and sharing of information, and some critical thinking,” Laneman says.
How Do We Know Two Miles From Airport Runways Is an Optimal Distance for C-Band Transmitters?
A transmitted radio signal weakens with distance. The farther the receiver is from the transmitter, the weaker the signal is at the receiver. The 1.2-mile exclusion zone that the wireless companies previously proposed was based on the distances used around French airports, says Randall Berry, a Northwestern University electrical engineer who specializes in wireless spectrum policy. The main reason for extending it to two miles is that there are some differences between the U.S. C-band rules and those in France. The other reason is that by the time the signal reaches radar altimeters on aircraft, it will be too weak to interfere, Berry tells Popular Mechanics.
It’s important to know that the C-band had a former life in satellite systems, Laneman adds (engineers designed radar altimeters in 1938). When satellites became the only other users of the C-band, their weak signals from the sky didn’t interfere with altimeter radio waves. Altimeters are located at the bottom of the aircraft and bounce radio waves off the ground to detect the airplane’s distance at any moment.
What’s changed is that the FCC has authorized a powerful C-band signal that originates on the ground and points up. These signals can reach an aircraft’s altimeter and prevent its use. The signal is not only much closer to planes at airports, but also much more powerful, Laneman says. “You need to go to great lengths to either suppress the signal, with filtering in the receiver, or reduce the power of the signal on the transmitting side and push it farther away, or tilt down the antennas.”
What Can You Do Now?
In the short term, it’s likely that thousands of passengers will be affected by paused flights. Before you fly, check your flight status for any cancellations.