Transmission antennae near dozens of airports have not been turned on due to fears of interference with certain models of radio altimeters.
The stories, however, rarely point out that this is a problem that has been nearly four years in the making.
It begins in April 2018 with the proposed large-scale deployment of 5G—which essentially offers a much faster service for mobile phone users—and the US Federal Communications Commission (FCC) asking the public to comment on the more intensive use of the 3.7GHz to 4.2 GHz frequency band.
Even at this stage, both the Federal Aviation Administration (FAA) and many aviation stakeholders, including IATA and Airlines for America, raised concerns to the FCC. By October 2020, the Radio Technical Commission for Aeronautics (RTCA) had completed an assessment of 5G interference with radio altimeter performance, finding a “major risk that 5G telecommunications systems in the 3.7GHz to 3.9GHz band will cause harmful interference to [radio] altimeters on all types of civil aircraft.”
Despite the technical input, the FCC auctioned off C-band spectrum in the 3.7 to 3.98 GHz range in February 2021, uncomfortably close to the 4.2 to 4.4 GHz range used by radio altimeters.
Because the US C-band 5G is also allowed to transmit at a lot of more power than elsewhere, the fear is that 5G “shouting” near an airport could interfere with radio altimeters that not only tell an aircraft its height from the ground but also feed into other safety-critical systems.
Why the FCC allowed the auction to go ahead is something of a mystery although the FCC was under no obligation to accept aviation’s objections or to listen to the FAA.
In any case, as Noppadol Pringvanich, IATA’s Head ATM Engineering & Aviation Radio Spectrum, points out, that has become a moot point. Now that the auction is completed and billions of dollars have changed hands to use the spectrum, the telecoms companies, principally AT&T and Verizon, are naturally desperate to get their money’s worth.
“But the FAA must put safety first and so had to make airlines aware of the risk of using certain radio altimeters at certain airports,” says Pringvanich.
In a letter to the White House, Transportation Department, FCC and FAA, signed by most airlines in the United States, it was pointed out that “more than 1,100 flights and 100,000 passengers would be subjected to cancellations, diversions or delays" at key hubs.
For now, AT&T and Verizon have agreed to delay 5G implementation around 50 priority airports, so the majority of flights are unaffected.
5G is being rolled out worldwide but few other countries are having this problem. The reason why varies from lower 5G power outputs, different parts of the spectrum being used, locations of the 5G antenna, and the angle of antennas on transmission masts. In short, when US 5G advocates contrast successful rollouts elsewhere with the mess in the United States, it is comparing apples and oranges.
But that doesn’t mean the US situation is insoluble. “Currently, there is good engagement between the telecom and aviation industries,” says Pringvanich. “Importantly, when engineer can openly talk to engineer, the technical solutions are much easier to find.”
While this work is ongoing, temporary solutions beyond a delay in 5G implementation may be possible.
Under the current agreed delay, the power output of US transmission antennae has been turned down, for example. Additionally, vicinity around airports could be declared “no-go zones”. This mimics regulation seen in Europe and Canada that have much larger buffer areas around airports.
There are also Alternative Means of Compliance (AMOC), which allow flights to continue as long as it can be assured that there will be no harmful interference with radio altimeters around specific airports or runways. AMOCs have an important role in the short term but they are very specific to aircraft type, radio altimeter models, and even particular airports or runways. Moreover, each AMOC approval will last no longer than three months. Forever relying on AMOCs and its recurring process would result in unsustainable complexity.
“Longer term, the industry continues to improve altimeters and we expect to see the new design becoming available by 2023,” says Pringvanich. “But for the new design to be sufficient into the future, we need a stable and plannable spectrum environment. Also, we must consider whether it is fair for an airline to have to pay to retrofit equipment to fly safely? The airlines are blameless in this so why should they suffer the consequences?”
Once a solution is agreed on it will need to be put into regulation. A permanent fix is essential given the political sensitivities surrounding the issue.
“We could keep improving radio altimeters but if 5G carries on cranking up the power or getting closer to the altimeter’s spectrum, then we would continue to have problems,” concludes Pringvanich. “We need telecoms and aviation to continue their discussions and we need governments to heed the lessons being learned and to codify the necessary technical conditions so that aviation and 5G can safely co-exist. IATA stands ready to help all stakeholders so that airlines can respond to the demand for their services without restrictions.”
5G in other countries
In Japan, the macro cell power levels are only 4% of that permitted in the United States and the small cell power levels are less than 1% of that permitted in the United States.
In Europe, the spectrum is in the 3.4GHz to 3.8GHz range, a significant difference. The power levels permitted in most of Europe are 23% less than those permitted in the United States.
Australia’s 5G operates even farther away from the radio frequency band used by radio altimeters. The power levels permitted in Australia are 76% lower than those allowed in the United States.
Canada introduced exclusion zones on an interim basis. 5G C-band antennas also have a national down-tilt requirement.