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What’s behind the 5G controversy at airports, and how is Boston Logan affected?

Several airlines, including the long-haul carrier Emirates, have rushed to cancel or change flights heading to the US over an ongoing dispute about the rollout of 5G mobile phone technology near American airports.Jon Gambrell/Associated Press

Let’s get one thing straight: The dogfight between the FAA and FCC over the safety of wireless technology at airports isn’t about 5G networks per se, which have been around for years.

Instead, they’re at odds over an improved 5G network that could finally make the technology truly useful. This network will use “C-band,” a set of radio frequencies that deliver longer range and faster data speeds. But C-band frequencies are also used by aircraft radars that are essential for safe flying. And for months, airlines and the Federal Aviation Administration have warned that interference from C-band phone networks could scramble aircraft radars, making it too risky to land planes at night or in bad weather.

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The FAA found that 50 US airports are especially vulnerable to this risk. Boston Logan International Airport is not on the list. Neither is T.F. Green International Airport in Providence.

The FAA did not respond to inquiries about why Logan and Green didn’t make the list. But a fact sheet published on the agency’s website said, “In addition to asking for input from the aviation community, the FAA selected the airports based on their traffic volume, the number of low-visibility days and geographic location factored into the selection.”

Whatever the underlying arguments, some major airlines are suspending flights to the US or changing the planes they use. Emirates Airlines, for one, has suspended flights to US destinations including Logan Airport. Several other major foreign carriers, including ANA, Japan Airlines, and Air India, also suspended flights to the US, even though wireless carriers AT&T and Verizon agreed this week not to switch on C-band service near some US airports.

Still, AT&T and Verizon have begun offering C-band service in other parts of the US, starting Wednesday. AT&T is being relatively low-key about the launch of its “5G+” service in Chicago, Dallas, and several other cities, but not yet Boston. Verizon is heavily advertising its “5G Ultra Wideband” system, claiming it will soon be available to 100 million people in the US, including residents of Greater Boston.

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Until now, Verizon and AT&T’s 5G service had severe limitations. They mostly used low frequencies that deliver excellent range, but data speeds not much better than 4G. The third major wireless carrier, T-Mobile, is better off; it controls a lot of mid-range frequencies that combine decent range and significantly faster speeds.

Verizon also offers a version of 5G that delivers stunning speed, but uses radio frequencies that don’t carry very far and can’t penetrate buildings.

The C-band radio frequencies are the sweet spot for 5G. A C-band signal can travel for miles, while providing speeds in the hundreds of megabits per second, more than enough for most phone users.

In 2020, the Federal Communications Commission auctioned C-band licenses to wireless phone companies for 5G services. Between them, Verizon, T-Mobile, and AT&T paid the federal government $81 billion for the frequencies.

But the frequencies are very close to the ones used by the radar altimeters on airplanes, which use a radio signal to measure distance to the ground and are vital for landing in the dark. Interference from C-band towers could make some radar altimeters dangerously inaccurate.

On Wednesday, the FAA said that the radar altimeters in 62 percent of US commercial planes are safe for low-visibility landings in areas served by C-band networks. These include the altimeters found on many large jets made by Boeing and Airbus. (On Sunday, the FAA had given an estimated figure of 45 percent.)

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In a letter sent Monday to the FCC, an airline industry trade group said that activating C-band would have forced the cancellation of 1,100 flights nationwide, disrupting travel for 100,000 people.

Many of the cancellations would involve smaller regional airlines, said Henry Harteveldt, president of Atmosphere Research Group, a travel industry research firm, because many of their aircraft use radar altimeters not yet approved by the FAA. “The altimeters on regional jets appear to be vulnerable,” Harteveldt said. “These regional jets account for approximately 40 percent of scheduled airline flights. This puts flights to small and mid-size cities at risk.”

Last year, wireless carriers said they’d cut the broadcast power of C-band cell towers located near airports. In early January, they agreed to wait an extra two weeks before launching C-band service. They also pledged to wait six months before activating C-band services near the 50 US airports considered especially vulnerable by the FAA. These include airports in New York, Chicago, and Los Angeles.

But C-band interference isn’t just a potential menace at airports. Rick Kenin, chief operating officer of Bedford-based air ambulance service Boston MedFlight, said that an unreliable radar altimeter could be a matter of life or death for someone needing an emergency flight to a hospital.

“We operate day and night from remote landing sites, the scene of an accident, for instance,” said Kenin. A nearby C-band transmitter might result in false readings from the helicopter’s radar altimeter, making it far more dangerous to land in rough terrain. “There is a potential that we would not be able to do a patient pickup,” he said.

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Kenin said more reliable altimeters are on the way, but added that “those are at least a couple of years down the road and they will be expensive.”

John Shea, director of government affairs for industry trade group Helicopter Association International, said the FAA last week issued special instructions to helicopter pilots warning them that they would need special equipment and assistance to do air ambulance flights, to avoid the danger of C-band interference. For instance, aircraft might need to install powerful searchlights to give them a clear view of the terrain below. And they’d need people on the ground with radios who could tell the pilot it was safe to land.

Shea said that if these conditions are not met, air ambulances would have to refuse requests for emergency transport. “It’s an unfortunate scenario,” he said, “but it’s not an unlikely one.”


Hiawatha Bray can be reached at hiawatha.bray@globe.com. Follow him on Twitter @GlobeTechLab.