Airlines Warn 5G on C-Band May Cause Major Disruptions

The mobile wireless industry is working to deploy 5G across the country. 6G is in development, but millions of Americans don't have internet access at home.
This access gap is known as the digital divide and the Federal Communications Commission (FCC), is working to close it.

Its plan includes allowing mobile wireless companies to use broadcast spectrum in the frequency band 3.7-3.98GHz, also known as C-band. This license allows wireless companies to provide 5G service via small base stations. This would allow 5G to be brought to rural areas that have limited access to fiber internet. These C-band licenses were purchased by Verizon, AT&T and T-Mobile for more than $80 billion.

In March 2020, the FCC officially issued the policy changes. After nearly three years of discussion, the report and order (R&O), was finally completed. During that time, members of the public were invited for their comments. NPR and the Mormon Church were among the many interested parties. However, the aviation industry was the most involved.

Aviation industry speaks up

A piece of equipment known as a radio altimeter or radar altimeter is the center of aviation industry concerns. All types of aircraft use the radar altimeter to determine altitude and distance from ground. It transmits a signal towards the ground, and then determines altitude based upon the time taken for the signal to reflect off of the ground.

What's the problem? Radar altimeters work in the 4.2-4.4 GHz frequency range. The R&O would place 5G services on the adjacent frequency band, which includes devices that passengers routinely bring onboard (like tablets and cell phones). Aviation industry groups warned FCC in August 2021 that C-band services could cause interference with radar altimeters. This could lead to major disruptions in passenger air travel, commercial transport and critical helicopter services.

Aviation industry groups are acutely aware of the vulnerability radar altimeters to interference. Aerospace Vehicle Systems Institute (AVSI), in a 2017 letter to FCC, explained that plans to use C-band for telecommunications had been canceled due to earlier studies showing the interference was unpredictable.

Avi Greengart, a Techsponential analyst, said that the objections from the aviation industry are understandable. C-band frequencies have been allocated to 5G by the government. If there are any problems, especially with older or non-standard radar systems, the government will impose costs on aviation to improve performance.

To prevent interference, the R&O set power and emission limits for 5G bases stations. Boeing's 2018 letter mentions a 220-megahertz buffer that exists between the 3.7-3.98 GHz and the 4.2-4.44 GHz bands used by radar altimeters. Despite these precautions, AVSI agreed that more research was necessary. To ensure safety, aviation and mobile industry groups were encouraged by the R&O to form a multi-stakeholder team.

It turned out that more research would only make matters worse.

The RTCA Report

The aviation industry had made it a priority to update radar altimeters. To study the issue, a U.S.-based non-profit organization, RTCA, created Special Committee 239 (SC-239) in December 2019. RTCA creates standards and technical guidance to assist government regulators. RTCA members are representatives of both government and private organisations around the globe and have extensive knowledge in the aviation industry.

SC-239 was created in response to the FCC's request to form multi-stakeholder organizations. Anybody with relevant expertise could participate, even representatives from the wireless sector. The group was formed to examine the interference potential of 5G telecommunications signals and to adjust radar altimeter standards in order to address the risk.

Mixed signals

It is worth learning the basics of the problem before you can look at the results. The radar altimeters work at a low power level and receive weak signals. The signal will travel at least 30,000 feet from the ground to cruising altitude.

Mobile devices and 5G base stations will typically emit signals in the frequency band of 3.7-3.98GHz. These signals are known as fundamental emissions and they are not within the normal bandwidth of radio altimeters so can be filtered out. However, even with filters, it is possible for strong signals to overwhelm radar altimeter receivers, which is known as blocking interference.

These strong signals can be compared to spicy food. Your taste buds become numb when you eat spicy food. You won't be able to taste the next one. This is the effect of blocking interference on radar altimeters. The stronger signal drowns out the weaker one.

These 5G sources may also produce spurious emissions. These signals can't be filtered because they are in the same frequency range radar altimeters are expected to receive. They are not distinguishable by the radar altimeter, which may cause it to determine altitude incorrectly.

The stronger signal drowns out the weaker one.

False altitude reports can lead to other systems responding incorrectly. The data from radar altimeters is available throughout the flight. It's not just for the pilot to see. Altitude data feeds into important systems, like the Traffic Collision Avoidance System and Automatic Dependent Surveillance-Broadcast System, which monitors the airspace to prevent midair collisions. The RTCA Report, published in October 2020, shed light on the dangers of landing with false altitude reports.

Houston, there's a problem

Two scenarios were used to model the RTCA Report's evaluation. These scenarios simulate real flight paths and show how interference from LTE base stations nearby could affect aircraft landings if they were upgraded to 5G. The first scenario shows helicopters landing at the Texas Medical Center in Houston. The second model the approach to Chicago's runway 27L.

Let's start with helicopters, for simplicity's sake. Houston's Texas Medical Center is crowded. In a 2-square-mile area there are 21 t21hospitals. Many of these have rooftop heliports. Mobile base stations are also available throughout the medical complex.

The interference caused by hypothetical base stations was harmful to every approach at every helicopterport. In some cases, radar altimeters were rendered inoperable by the interference. Other than base stations, cell phones and other user equipment created a serious risk of interference to radar altimeters on helicopters. The 5G deployment could have a serious impact on helicopters' ability to navigate cities. They must be careful around obstacles and other aircraft.

Another scenario is when planes approach OHares runway27L. The interference from the bases was high during the approach, but decreased as the planes fell in altitude. Although this could lead to problems at landing, it is not the most alarming part of the report.

There is the potential for disaster

The safe threshold for interference is higher for larger planes in Category 1 (commercial or passenger) The safety margin is shown in red. A single 5G base station type caused enough interference to exceed the threshold. This is only true in rare situations, but it can be dangerous.

Notice the interference spike at approximately 275 feet. Two radar altimeters are standard on passenger planes. Interference above this threshold could lead to both of them malfunctioning. The report lists four possible outcomes, even though they might not all malfunction in the exact same way.

Both radar altimeters cease to operate; one stops operating and the other reports altitude incorrectly. They provide different altitude readings but they provide the same inaccurate readings.

The flight crew must decide whether it is safe to land the plane in the first instance. The interference spike occurred at approximately 275 feet. This left the flight crew with only 20 seconds to reach touchdown. Low visibility may make it difficult for the pilot to see the runway from which they can calculate their height. This is a risky situation, regardless of whether the pilot lands or not. It's actually the best-case scenario.

This second case is more difficult. It is important to have two radar altimeters in case one fails. It is also useful in determining whether one of the radar altimeters has failed. It is clear that at most one of the two readings from the flight crew and autopilot system are incorrect. This may not be the case for all planes.

Let's now look at the third. The autopilot systems will detect the mismatch if the radar altimeters report two differing altitudes. The pilot must decide if the plane can be safely landed without a radar altimeter. The report does point out that the autopilot system may continue to use incorrect data on certain aircraft. The consequences could be disastrous if the pilot does not realize this. This was the reason for the Turkish Airlines Flight 1951 crash in 2009.

This fourth scenario is by far the most dangerous. The flight crew and the autopilot system will not be able to tell the difference if both radar altimeters give the same altitude readings. The autolanding system will execute the flare maneuver and the autothrottle retard incorrectly if the plane is too low. The plane will crash into the ground if it is higher than anticipated, but it will stall first.

This is the reason for the Turkish Airlines Flight 1951 crash in 2009.

This report stresses that although it is unlikely, it can be dangerous as radar altimeters are very reliable during landing. The stress of landing in low-visibility conditions may be increased by 5G deployment. Fundamental emissions can block interference from passenger planes. It would take many years to install band bypass filters on all aircraft, but that is not the only solution. These spurious emissions are beyond the safe limit for helicopters and Category 2 planes. Filters cannot block them. According to RTCA the mobile wireless and aviation industries must work together to find solutions.

The findings are disputed by the mobile industry group

CTIA is a trade organization for members of wireless communications industry. CTIA disputed many technical aspects of the RTCA Report, including the 5G power level, safety margin and worst-case landing scenario. CTIA also claims that the wireless industry didn't have any insight into the development and execution of the RTCA Report. They were unable to analyze the data or understand it.

Although the RTCA Report was intended to inform the FCC about the technical position of the aviation industry, wireless industry representatives provided some data. The report also includes information exchange with Technical Working Group 3 (3 (TWG-3). CTIA and RTCA were represented in TWG-3. It was created by the C-Band Multi-Stakeholder Group. Their information exchange consists of questions and answers dating from June 12, 2020 through August 16, 2020. It is available in full in Appendix A. TWG-3 was disbanded in November 2020 due to inability to reach consensus among members.

Appendix C (above), includes all comments made in a public commenting period before the report's release. Some of the 30 comments from CTIA were included in the report. SC-239 gave a reason for rejecting a recommendation from CTIA or any other party.

CTIA disagrees with RTCA Reports findings because it considers 5G to be common-sense. It is being deployed around the globe with no interference issues. CTIA suggests in its March 4, 2021 letter that Japan's 90,000 5G bases stations operating up to 4.1 GHz is evidence against RTCAs conclusions.

This point is supported by others. Greengart says it isn't clear that this will cause widespread interference. The U.S. military has been operating in these frequency bands for decades without any problems. Other countries have also been operating 5G networks within the same frequency band, again without interference to avionics.

According to Sascha Segan (PCMag's lead analyst), we don't know if these problems exist. They are either true or false, according to one side. However, I will note that C-band has been tested by carriers for several months and no helicopters have actually fallen from the sky.

Segan is optimistic about the future plans of the mobile industry. There are many ways to fix problems. For example, you can put exclusion zones around airports or aim antenna panels down. This would slow down 5G in my opinion by using smaller sites on buildings and fewer macro-sites (the large cell towers).

CTIA, representatives from AT&T and T-Mobile, U.S. Cellular and Verizon reiterated in April that the commission had correctly concluded that C-Band 5G could operate without causing interference to neighboring services in nearby band bands, let alone harmful interference.

The aviation industry supports the RTCA report

CTIA attempted to discredit the RTCA Report in May. Twenty aviation groups responded. These organizations (Organizations Supporting Aviation Safety or OSAS) are trade associations, the largest pilots' union, and companies such as Honeywell and Garmin that make equipment for aviation. CTIA's claims, according to the organizations, show a lack in understanding about aviation and aerospace design, manufacturing, operations, and safety analysis.

It is difficult to disagree with the following corrections after reading them. CTIA claimed that the results were being driven down because a radar altimeter was not certified by the commission within the last 40 years. OSAS pointed this out, pointing out that the model in question was manufactured in 2020. CTIA apparently misunderstood the authorization date of the model for its age.

OSAS has made two important points, despite the criticisms of CTIA on technical issues. First, CTIA does not dispute the findings about 5G spurious emission which caused the greatest interference for Category 2 aircraft and helicopters. CTIA's second argument that there are no reports of widespread interference from altimeters disproves RTCA Report is by definition an argument without evidence. The absence of reports does not prove that interference will not occur.

Isnt everyone using C-band?

The C-band in the U.S. is not being used yet for 5G. There will be more base stations to provide real-world testing and more interference. CTIA's argument about 5G in the spectrum band is flawed. Qualcomm has presented a December 2020 presentation that gives an overview of the allocations to various countries in the C-band.

This information is consistent with that provided by FCC. Most licenses in Europe follow the Radio Spectrum Policy Group guidelines of the European Commission. This group mandates that 3.4 to 3.8 GHz be 5G's primary band. Australia is currently investigating the possibility that it could use the spectrum between 3.7 and 4.2 GHz for 5G. However, licenses have not been issued within this band. Even South Korea and Taiwan aren't issuing licenses in that upper spectrum. They are tied for fastest 5G speeds.

The CTIA prepared the chart shown above. If the U.S. decides to issue licenses that exceed the upper limits of other nations, the spectrum allocations of other countries cannot be used to claim there is no interference risk. Warnings from countries such as France and the United Arab Emirates indicate that trust based on the absence of interference problems may not be warranted.

More cooperation, less interference

CTIA claims that a flawed report alone is not enough to prove C-band 5G will be harmful to adjacent bands. While that may be true, it is also the case for the aviation industry, which claims that a single, flawed report is not enough to prove that C-band 5G will cause interference to nearby bands. The only thing that is certain is that more information is needed.

In August, aviation industry groups met with the FCC to reiterate their warning that radar altimeters provide critical services and interference can have devastating consequences. While there are some measures that the aviation industry can take, such as installing band bypass filters in their aircraft, it is impossible to complete the project in time for 5G service in C-band in December.

How would closing the mitigation gap look? Mike Dano is the editorial director at 5G and Mobile Strategies for Light Reading. This publication serves professionals in the telecommunications industry. The 5G industry may be required to reduce their broadcasts in the affected spectrum if necessary to avoid interference. While it would be nice to avoid plane crashes, he states that if one does occur, it would also indicate a failure by the government regulatory agencies, which would fail to anticipate that problem. This is the job of [Federal Aviation Administration, FCC and [National Telecommunications and Information Administration] to first figure out this stuff before bidding for spectrum for 5G.

Dano suggests that another option is to replace radar altimeters by models that aren't as susceptible. This would obviously be costly and time-consuming. The winning bids for the C-band spectrum auction brought in $81 billion. This amount is probably well-known by the airline industry.

Representatives from the aviation industry insist that they support 5G.

The FCC was asked by the aviation industry to work with the FAA in implementing solutions. Representatives from the aviation industry insist that they share the goal to advance 5G. They also use wireless services.

PCMags Segan believes this is an attempt at shifting the blame. The FCC already modified the C-band plan in order to address concerns from the aviation industry. But the problem is that the aviation industry doesn't think it was sufficiently changed. The whole effort with the FAA is an attempt to shift discussion to what they perceive to be a more favorable court.

What does this leave us with?

RTCA creates standards and guidance that are the foundation of FAA regulations. They make sure every flight is as safe as possible. You can be confident that the plane will deliver you safely when you board a flight. Dano states that 5G could have an impact on airplanes. This is a question that will be debated hardcore RF engineers. This debate will be governed by the NTIA, FCC, and FAA. As a traveler, 5G should not cause planes to crash. It would be terrible.

Already, the debate is underway. The wireless industry claims that there are significant risks. However, the aviation industry believes there are enough safeguards. We should at the very minimum expect FAA and FCC consensus.

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