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C-band and Altimeters: As Expected, Filters were the Solution

C-band and Altimeters: As Expected, Filters were the Solution
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by Sam Benzacar, President, Anatech Electronics

Nearly three years after the FCC proposed auctioning C-band spectrum to wireless carriers, the “Great C-band Fiasco” seems about to come to an end. Last month, the National Telecommunications and Information Administration issued a 151-page report (NTIA Report 22-562) that described the results of tests conducted to determine whether 5G C-band base stations present an interference hazard to aircraft radar altimeters on approach and landing.

It’s a comprehensive 151-page document that goes into extraordinary detail about the tests and their results and provides some recommendations. You’d think this would slam the gavel down on this issue because it concludes that while interference is certainly possible, it’s highly unlikely if radar altimeters have adequate filtering. Remember those last two words, because they are the crux of the matter.

Readers of this magazine must surely have been scratching their heads when this “crisis” arose because while the C-band frequencies allocated for carriers are from 3.7 to 3.98 GHz and radio altimeters operate between 4.2 and 4.4 GHz, there remains 200 MHz between the high end where carriers can operate and the low end where altimeters operate. Alas, while you might assume that radar altimeters should have bandpass filters with sharp skirts, many of them do not because until recently there was little nearby activity, and thus no real interference threat. Therein lies the rub.

To refresh your memory, the FCC in 2021 auctioned a portion of the C-band spectrum in Auction 107, bidding began in December 2020, and by July of that year most of the 5,684 licenses had been granted. Only the last 20 MHz between 3.98 and 4 GHz could not be used, as it was reserved as a guard band. The incumbent users, satellite operators, had by this time cleared the band, so the plan was for carriers to begin using the band by early 2022. But the fun was just getting started.

It’s important to note that while three hundred megahertz might not seem like much when 5G channel bandwidths can exceed 100 MHz, there are few unused frequencies available below about 7 GHz where signal propagation is favorable. So, it’s a bonanza for carriers, especially Verizon, which placed their bets on the potential of millimeter-wave frequencies whose time has not yet come, and whose “mid-band” allocations are, or were, until the C-band release, lacking.

In December 2021, just before carriers were authorized to begin C-band operation, Boeing and Airbus asked the U.S. government to put a hold on the rollout. They were concerned that C-band-enabled base stations close to airports could interfere with radio altimeters on aircraft that are landing. They were later joined by the entire commercial airline industry, pitting themselves against wireless carriers, with equipment manufacturers stuck in the middle, and the FAA tasked with figuring out a solution.

If you’re not particularly familiar with altimeters, there are essentially three types, using pressure, a laser, or electromagnetic energy in the form of radar, the latter being extremely accurate and largely unaffected by weather conditions. This makes them very well suited for aircraft approaches and landings in inclement weather and when visibility is low. They are effective up to an altitude of about 2000 feet.

Like all radars, signals are sent from the altimeter using an antenna, in this case mounted under the body of the aircraft. The reflections from the signal aimed at the ground return to the antenna and the height of the aircraft is determined. This data is fed to the pilot and auto-throttle on the aircraft that is part of the flight computer. Radar altimeters are essential systems that fall within the category of ground proximity warning systems, and many aircraft have multiple radar altimeters so there is always a backup and the results from each one can be compared with the others and the pilot can be alerted to discrepancies.

While the wireless industry has extraordinarily comprehensive specifications regarding system performance for 5G, the same can’t be said for radio altimeters, for which there is no internationally recognized standard. As a result, a study conducted by the Aerospace Vehicle Systems Institute (AVSI) showed that there could be a wide variation in radio altimeter performance among manufacturers. What this means is that altimeters may or may not have adequate receiver filtering.

However, the FCC concluded that the AVSI study “does not demonstrate that harmful interference would likely result under reasonable scenarios” or even “reasonably foreseeable scenarios.” To support this conclusion, T-Mobile cited a study conducted by the engineering firm Alion that concluded that the assumptions in the AVSI study were extreme and resulted in extreme conclusions.

Wireless broadband networks have been operating at 3.7 to 4.2 GHz in the U.S. and other countries for years and authorities in Britain and Europe stated they had not seen any problems between C-band 5G and aircraft, and those in Japan had already solved any potential problem by keeping 5G towers 200 m from aircraft approach routes. But as always seems to be the case, the U.S. had to go its own way, which resulted in the fiasco.

The FAA first proposed a solution in a Special Airworthiness Information Bulletin in which they called for radio altimeter manufacturers, aircraft manufacturers, and the airlines themselves to voluntarily provide information to federal authorities related to altimeter design and functionality, and that they evaluate and assess their equipment.

What’s more, the FAA decided that “sensitive” equipment in these and other aircraft should be replaced, which would cost the industry enormous amounts of money and, according to the airline industry, would require at least two years. With thousands of commercial and general aviation aircraft in service throughout the world all using radar altimeters, the ramifications of this situation cannot be overstated.

A letter from Boeing and Airbus noted that analysis by the trade group Airlines for America (A4A) found that if the FAA 5G directive had been in effect in 2019, about 345,000 passenger flights and 5,400 cargo flights would have faced delays, diversions, or cancellations. And United Airlines stated that the FCC directives would effectively bar the use of radio altimeters at about 40 of the country’s largest airports.

As a stopgap measure, AT&T and Verizon delayed their launch of C-band wireless service and began implementing measures to limit interference, but other industry groups said that wasn’t sufficient. The CTIA said there was no demonstrable proof that 5G was unsafe and accused the aviation industry of fearmongering and presenting misleading facts.

Other issues concerned the lack of information about precisely where 5G C-band base stations would be located and how their beams would be steered. As Andrew Roy, director of engineering services for Aviation Spectrum Resources said in a National Business Aviation Association webinar, “They can do what’s called beam steering and steer the energy in certain directions electronically to make sure they get the best coverage possible. It’s a very clever system, but it’s very difficult for aviation then to say, well, where are you pointing the beam? This sort of level of detail is what we’ve really been trying to get hold of to make an accurate decision.”

After continuous back-and-forth deliberations and heated exchanges between the airline industry, the FCC, the FAA, Congress, the CTIA, and wireless carriers, Verizon and AT&T announced that until July 5, 2022, they would not turn on their C-band equipment at 600 transmission towers near the runways at 87 airports and would reduce radiated power at others in response to the concerns.

When the NTIA released the results of a comprehensive study based on tests of three models of 5G base stations being deployed by AT&T and Verizon, it noted that all three demonstrated effective RF bandpass filtering with a high-end roll-off at 4 GHz. Spectral emissions above 4 GHz were sometimes a remarkable -106 dB below the power radiated in the 3.7 to 3.98 GHz band. In addition, the beams from the C-band antennas have well-defined, relatively narrow beams and don’t send much energy skyward.

That is, “the airborne radiation patterns show significantly less power than found in 5G base station main antenna beams directed toward devices at ground level.” As for the radar altimeters, the authors recommended that “the technical solution to any potential interference problem might be installation or retrofitting of more effective RF power-rejection filters on radalt (radar altimeter) receivers for frequencies below 4200 MHz.”

The NTIA did note that 4G MIMO arrays have “distinct nulls” relative to the main antenna beam power levels above the arrays. “These radiated-power nulls will reduce vertical height separations between 5G towers and aircraft where any given power level will be encountered by radalt receivers passing through the sky above the 5G MIMO arrays. This observation and the data we have collected show this effect should be addressed for…radalt receivers whose flight paths carry them directly above 5G base station transmitter arrays.”

In short, the NTIA’s report showed that as 5G antennas limit interference, the technology should safely coexist with aviation. However, as of this writing, the FAA intends to continue restricting where 5G base stations can be deployed, essentially some distance from flight approaches, and has no plans to end the retrofit of existing radar altimeters with external bandpass filters.

Of course, as a manufacturer of RF and microwave filters, the use of more attenuative bandpass filters was not a surprise, and I suspect many of you immediately came to the same conclusion—that filters remain the go-to solution for eliminating or at least significantly reducing the potential for interference. What remains confounding is why it took so long and so much work to reach the current stage, why Boeing and Airbus issued their letter only once wireless carriers were about to deploy their systems, and why all this could not have been wrapped up a lot faster and without the chaos.

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