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Aviation altimeters make it difficult for 5G to take off in the US, for fear of interference


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The US operators AT&T and Verizon activated their 5G networks last Wednesday with the licenses obtained in the auction at the end of 2020, after a joint payment of 95,000 million dollars. However, 5G networks will have to operate at lower power than expected and their radio stations at least two miles (3.2 kilometers) away from the runways of US airports for fear of interference with signals coming from altimeters, and for at least six months. The problem, it seems, is not in the 5G specifications or in the proximity of the C-band frequencies to those used by aircraft and airport altimeters, but rather that the transport authority, the FAA, does not know the status of the altimeters and if they incorporate the mandatory signal filters that prevent the invasion of foreign frequencies, less than 4 GHz.


In the traditional economic and technological forecasts for 2022 made by the American media at the end of the year, there was a lot of talk about the future possibilities of the metaverse, autonomous driving and quantum computing, although they are still far away in their full magnitude. Instead, a reality that will be more important this year, especially in the United States, was almost completely ignored: the deployment of 5G networks.


Nor was excessive importance given when on December 5 the Federal Aviation Administration (FAA), the government body that regulates aviation in the United States, published, a few hours before the 5G networks were scheduled to be switched on by AT&T, a security notice in which he informed that the pilots of the planes should be very attentive to the reading of their altimeters because, although he cryptically assured that he did not have absolute evidence, he warned that the 5G networks could cause interference in the measurements of the altimeters.

The problem of the possible interference of the signals of the altimeters does not come, apparently, from the 5G signals but, in any case, from the fact that the altimeters emit radioelectric signals beyond the permitted frequencies and interfere with those of the 5G networks, leading to reading errors

The hours after this warning of a lack of security for the approach and landing maneuvers of aircraft at airports in the United States due to possible interference with the signals of the 5G networks were of absolute bewilderment on the part of the Biden Administration and the Federal Communications Commission (FCC), the government body that regulates all telecommunications systems in the United States. The FCC had granted the licenses for the use of 5G networks with the so-called C band, a spectrum located between 3.7 and 3.98 GHz, through an auction that began in December 2020, with Trump in the presidency, and completed in January 2021, already under the Biden mandate.

The bidders bid for the 280 MHz of spectrum auctioned a whopping 81,000 million dollars, plus 14,000 million to compensate and relocate the satellite broadcasting companies, including Intelsat, which occupied the C band thanks to a license granted for life decades ago. Verizon acquired 161 MHz for 53,364 million dollars and AT&T 80 MHz for 27,695 million, an exorbitant amount but necessary to have spectrum in the middle band, the most suitable for 5G. T-Mobile, the third national operator, paid 10,666 million for 27 MHz, because it already had other licenses for 5G thanks to the merger with Sprint.


Apart from paying 95,000 million dollars to the coffers of the United States, it was specified from the outset that the licenses would not be available until December 5, when the frequencies for 5G use were scheduled to be released. Verizon and AT&T worked throughout the past year to be ready as soon as they could access the licenses. AT&T had planned to turn on multiple 5G networks on December 5 with the new licenses and had prepared a major advertising campaign, while Verizon had postponed it for a month later, in early January 2022.


When AT&T was ready for the 5G switch-on, the FAA’s lack of security notice arrived. The FCC quickly intervenes and the White House mediates in the conflict. In the end, as is well known, AT&T and Verizon reluctantly agree to postpone the 5G switch-on for a month, because the FAA is committed to reviewing the security systems of aircraft altimeters and runways during these 30 days. airports to check for possible interference. But January 5 arrives and the FAA requests a new postponement of another two weeks, promising not to delay the issue any longer. However, last Wednesday, January 19, the FAA continues to insist that the planes will not be safe if the 5G networks are launched due to possible interference, even acknowledging that they have no evidence that this may be the case.

Verizon and AT&T turn on 5G networks, despite everything

In the last breath, and when some airlines, such as JAL or Emirates, had canceled their flights to the United States on January 18 due to the imminence of the 5G networks being turned on and the alleged lack of security, the operators, the Administration Biden, the FCC and the FAA agree on a compromise solution: operators agree to limit the power of their 5G radio stations and place their antennas at least 3.2 kilometers away from the runways of United States airports, during the next six months (leaving in the air what would happen next).


Thus, last Wednesday, January 19, after two delays, Verizon and AT&T turn on their 5G networks. Initially, AT&T does so in eight cities and Verizon improves its fixed wireless Internet access (FWA) service with the combination of the C band and its millimeter wave licenses, at very high frequency. Verizon has not hesitated to welcome the initiative and published in the Wall Street Journal last Saturday, and on Sunday in the New York Times and the Washington Post at least the following double-page ad.

Neither short nor lazy, however, the FAA published a day after turning on 5G, on January 20, a notice in which it estimates that only 78% of the United States commercial aircraft fleet will be able to make landings in conditions low visibility at airports where 5G networks have been deployed, including some regional aircraft.


The FAA adds in the statement that “it is working diligently to determine altimeters that are reliable and accurate where 5G has been deployed in the United States” but “anticipates that some altimeters will be too susceptible to interference [with signals] 5G”. And it warns that planes with these altimeters will be prohibited from landing in places where 5G networks have been deployed, precisely because the altimeters could provide erroneous information.

Public opinion baffled

The most prestigious media, such as The Wall Street Journal, The New York Times and The Washington Post, which until then had promptly reported on the subject with some astonishment in the face of such a surreal situation, in recent days have exploited and published editorials and articles of opinion asking for explanations of how this situation has come about.


Last Wednesday, a Wall Street Journal editorial headlined “How to Mess Up a 5G Rollout” and ended by saying that the Biden Administration “needs fewer political spins and more competent governance.” A Washington Post columnist, David von Drehle, headlined that “The FAA’s 5G freakout raises a big red flag.” And two days later Peter Coy, a columnist for the opinion pages of the New York Times, simply titled that “The 5G Mess Was Avoidable”. Few tech topics have elicited as many negative reactions and disbelief in such a short time.


Deciphering how this situation has been reached and closing it so falsely is not an easy process. On January 14, in addition, the FCC closed another auction, Auction 110, for the use of “flexible” 5G licenses between 3.45 and 3.55 GHz, for which 22.5 billion dollars were paid. AT&T, Dish and T-Mobile US bid heavily for these waves that were in the hands of the military and have given up part of their use; not so Verizon, because it had already acquired enough licenses in the January 2021 auction, the one that is now so problematic.

The reasons that have caused the FAA not to act years ago to solve the reading of the altimeters, if it was aware of the possibility of interference, are unknown, and both the Trump and Biden Administrations acted as if nothing had happened

In principle, these waves that have just been auctioned may also be susceptible to creating the same interference because, although they are a little further away from those used in altimeters, the FAA does not link the possible interference to a certain proximity between radio waves but rather to that he is unaware of the actual status of operating altimeters on aircraft and airports in the United States.


The causes that have led to this situation are now beginning to be clear, although they are by no means justifiable. Last December, just before asking for an extension to turn on 5G networks with C-band, the FAA argued that the C-band and operating waves were too close together and that the signal strength of radio stations 5G was very high, 1,585 watts, when in France it was 631 watts and there was also more space between the 5G signals and the runways, as he explained in a very explanatory graphic and in a special bulletin on December 23.

The FAA’s main line of reasoning in this alert was that there was insufficient space between the waves approved for 5G use in the United States and those used by altimeters, about 200 MHz; this seemed to be one of the reasons why there is no conflict with 5G in Europe or in other countries. However, everything now indicates that the main cause is that some altimeters may not have the appropriate signal filters that prevent the invasion of signal frequencies far removed from those used by 5G, either directly or with the emission of spurious or multiple signals. harmonics, thus causing interference with altimeter measurements.


But it is still an assumption, because the FAA makes it very clear that it does not know the real status of the operation of the aircraft and airport altimeters; that is to say, it does not know for sure how many altimeters are equipped with the mandatory filter that prevents them from invading foreign frequencies and if there are some that do not have it. In the January 20 statement, the FAA is only certain that 78% of aircraft are safe and, as noted above, “anticipates that some altimeters will be too susceptible to 5G interference” and will be prohibited from landing. these planes when there are 5G antennas. The FAA apparently has no evidence that interference with 5G can occur, but it seems clear that if there are no guarantees that there are reliable signal filters in the altimeters, interference is very possible.

Indications of a decision more political than technical

In its print edition last Saturday, the Wall Street Journal lists a series of government actions carried out in the last three years that suggest that in the approval of the controversial auction of band C, and the subsequent granting of licenses for the signals 5G, political criteria and the need to release spectrum for market reasons and to face the technological threat from China weighed more than doubts about aviation security. In short, the FCC won the game over the doubts expressed by the FAA.


Since 2015, the FAA has noted that decades-old equipment used by aircraft can be disrupted by new 5G mobile phone signals. But the telecommunications industry and its regulator, the FCC, have always ignored them. In 2019, concern about the issue grew within the FAA when an aerospace research group warned of potential interference and a year later the RTCA, a non-profit organization that establishes the FAA’s technical regulations together with the industry aeronautics, published laboratory results that suggested the vulnerability of the altimeters, according to information from the Wall Street Journal.


The United States Government has the National Telecommunications and Information Administration (NTIA), a federal office that is above the FCC and the FAA and responsible for mediating conflicts between them. But, coincidence or not, in the last three years there have been different interim managers at NTIA, none of them permanent. It was not until the week that the position of head of the NTIA and with authority to decide was finally filled, says the Wall Street Journal.


According to the sources of the financial newspaper, during the Trump Administration, investments were promoted for the development of 5G services and Republican and Democratic legislators warned that without more 5G licenses, the US industry would be behind China. The need to accelerate the deployment of 5G increased, to the point that the FCC promoted the so-called 5G FAST plan. And lawmakers submitted a draft titled “Beat CHINA for 5G Act,” the newspaper says.

The FAA, looking at the landscape, tried to delay the auction of C-band for 5G, which finally started in December 2020 and ended in January 2021. In a letter sent to NTIA in late 2020, Steve Dickson, administrator of the FAA, and Steven Bradbury, then head of the US Department of Transportation, warned of possible interference and requested a transition period to find solutions. But Larry Kudlow, director of the National Economic Council in the Trump Administration, and experts from the White House Office of Science and Technology rejected the FAA’s demands.


Previously, in March 2020, the FCC approved a 258-page plan detailing the entire process of relocating C-band occupied by satellite companies and its use by 5G operators, including a security band between the working frequency of the altimeters and the 5G signals, much greater than in other countries. But the FAA never agreed with the FCC’s approach and in December 2021 it released the now famous special bulletin in which it put aircraft safety on alert.

Centennial altimeter technology

The technology that makes altimeters work was developed and patented by Lloyd Espenschied almost a century ago, and was imposed on airplanes after World War II, because it allowed knowing their relative position with respect to the ground by sending different radio signals and thus fly even in conditions of reduced visibility. It is clear that if interference occurs the altimeter reading can be wrong, but many experts are not convinced that this interference can occur.


With modern digital technology, it is very easy to put in signal processors and filters that prevent altimeter signals from exceeding the 4.2 to 4.4 GHz band of operation. But altimeters can be decades old on some planes, especially those that fly regional routes and even those that work with analog technology, with more imprecise filters. This ignorance of the altimeter park in the United States is, apparently, the origin of the problem.


In any case, what some are now asking is why the FAA did not act diligently three years ago, when it was foreseeable that the C band would be auctioned for 5G signals, and was concerned to know the situation of the altimeters and, in their case, force them to function within their limits. Or, at the very least, that it would have acted from January 2021, when licenses were granted for 5G networks to work from December 5, 2021 and not wait until then to recommend that planes not fly because they were not safe. , holding the aviation industry and passengers hostage.


If the situation is so complex, it was clear that the 30-day delay of the 5G networks would not fix it, nor would it be fixed with an additional fifteen days, as requested by the FAA. Now, it is safe to trust that with the additional half year of margin and the distancing of the 5G antennas from the airports, a large part of the problem can be solved, although there are some (few) planes that cannot fly even after the summer. In any case, in one of the potentially most interesting markets for 5G, inside airports, there may not be 5G for at least half a year.