The mobile wireless industry is preparing to deploy 5G all across the country, and 6G is already in development — but millions of people in the United States don’t even have access to household internet.
This differential in access is called the “digital divide,” and the Federal Communications Commission (FCC) is trying to bridge it.
Part of its plan is to let mobile wireless companies use broadcast spectrum in the 3.7-3.98 GHz band, commonly referred to as “C-band.” With a license to use C-band, wireless companies can provide 5G service through relatively small base stations. This would make it easier to bring 5G to rural areas, where providing internet through fiber requires vast, expensive infrastructure for relatively few customers. Verizon, AT&T, and T-Mobile spent over $80 billion at auction to get these C-band licenses.
The FCC formally issued the policy changes in March 2020. The report and order (R&O) was the result of nearly three years of consideration, during which members of the public were invited to comment on the matter. There was a wide range of interested parties, from NPR to the Mormon Church, but aviation industry groups have been among the most involved.
Aviation industry speaks up
The aviation industry’s concerns revolve around a piece of equipment called a radar altimeter (or radio altimeter). The radar altimeter is used by all kinds of aircraft to measure altitude, the distance between the aircraft and the ground. It works by transmitting a signal toward the ground, then determining altitude based on the time it takes the signal to reflect off the ground and back to the aircraft.
So, what’s the problem? Radar altimeters operate in the 4.2-4.4 GHz frequency band. The R&O would put 5G services, including devices routinely carried on board by passengers (like cell phones and tablets), on the adjacent band. In August 2021, aviation industry groups warned the FCC that if C-band services interfere with radar altimeters, we can expect “major disruptions to passenger air travel, commercial transport, and critical helicopter services.”
Aviation industry groups are keenly aware of how susceptible radar altimeters are to interference. In a 2017 letter to the FCC, the Aerospace Vehicle Systems Institute (AVSI) explained that previous plans to use the C-band for telecommunications were nixed because earlier studies found the interference to be so unpredictable.
“The objection of the aviation industry is understandable,” Techsponential analyst Avi Greengart said in an email. “Since the government has already allocated C-band frequencies to 5G, if there are problems, particularly with older or out-of-spec radar systems, it will impose costs on the aviation industry to improve performance.”
The R&O established power and emission limits for 5G base stations to prevent interference. The 220-megahertz buffer between the 3.7-3.98 GHz band and the 4.2-4.4 GHz band used by radar altimeters is double the buffer mentioned in a 2018 letter from Boeing. Even with these precautions, the R&O agreed with AVSI that further study was needed. Aviation and mobile industry groups were encouraged to set up a multi-stakeholder group to figure out how to proceed safely.
As it turned out, more study would only complicate the matter.
The RTCA Report
Updating radar altimeters was already a priority for the aviation industry. In December 2019, a U.S.-based nonprofit organization, RTCA, formed Special Committee 239 (SC-239) to study the matter. RTCA develops standards and technical guidance for government regulators. RTCA members hail from government and private organizations from all around the world and have expertise in the aviation industry.
In response to the FCC’s call to form multi-stakeholder groups, SC-239 became the SC-239 5G Task Force. Anyone with relevant expertise could contribute, including representatives from the wireless industry. Its goal was to study the potential interference of 5G telecommunication signals and to update radar altimeter standards to reflect the risk.
Mixed signals
Before looking at the results, it’s worth understanding the basics of the problem. Radar altimeters operate at a low power level, and they receive relatively weak signals. At cruising altitude, the signal will have traveled at least 30,000 feet to the ground and back.
In regard to 5G base stations and mobile devices, they will usually emit signals in the 3.7-3.98 GHz band. These are called “fundamental emissions,” and are outside the normal bandwidth for radio altimeters, so they can be filtered out. But even with a filter, it’s possible for a strong signal to overwhelm the radar altimeter receiver, which is called “blocking interference.”
Think of these strong signals like spicy food. When you eat something spicy, your taste buds start to go numb. You’re not going to taste the next bite. This is what blocking interference does to radar altimeters. The weaker signal gets washed out by the stronger one.
These 5G s can also create “spurious emissions.” These are unwanted signals in the 4.2-4.4 GHz band. Since these signals are within the same bandwidth radar altimeters are supposed to receive, they can’t be filtered out. The radar altimeter has no way to tell them apart from the returning signal, so it may determine altitude incorrectly.
The weaker signal gets washed out by the stronger one.
A false altitude report is a serious error that can cause several other systems to respond inappropriately. Radar altimeters operate through the entire flight, and the data isn’t just displayed to the pilot. 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. In October 2020, the RTCA Report shed some light on the danger of false altitude reports during landing.
Houston, we have a problem
The RTCA Report used two scenarios to model their evaluation. These scenarios use real flight paths to see how interference from nearby LTE base stations may affect aircraft during landing if those stations were upgraded to 5G. One scenario models helicopters flying into the Texas Medical Center in Houston, and the other models the approach to runway 27L at Chicago’s O’Hare International Airport.
For simplicity’s sake, let’s start with helicopters. The Texas Medical Center in Houston is packed. There are t21hospitals in a two-square-mile area, and many of them have rooftop heliports. The medical complex also has mobile base stations throughout the area.
Hypothetical base stations caused harmful interference in every approach at every heliport. The interference was enough to make radar altimeters inoperable in some cases. Aside from base stations, user equipment like cell phones caused “a significant risk of harmful interference” to the radar altimeters on helicopters. In short, 5G deployment could seriously impact the ability of helicopters to navigate cities, where they have to move carefully near a variety of obstacles including other aircraft.
The other scenario involves planes approaching O’Hare’s runway 27L. Interference from the bases was above the safe threshold for small planes (Category 2) during much of the approach but lessened as the planes dropped in altitude. This could cause several problems during landing, but it’s actually not the most concerning part of the report.
Potential for catastrophe
Larger planes in Category 1, like commercial or passenger planes, have a higher safe threshold for interference. In the chart below, the solid line is the threshold, and the safety margin is in red. One type of 5G base station caused enough interference to cross the threshold, and only in certain situations — but those rare cases are particularly dangerous.
Note the large spike of interference at around 275 feet. Most passenger planes have two radar altimeters, and interference above the threshold could cause them both to malfunction. Since they may not malfunction in the same way, the report outlines four outcomes:
- Both radar altimeters stop operating;
- One stops operating, and the other reports altitude inaccurately;
- They both provide inaccurate altitude readings, but the readings are different;
- They both provide inaccurate altitude readings that are the same.
In the first case, the flight crew has to decide if it’s safe to land the plane. The spike of interference happened at around 275 feet, leaving the hypothetical flight crew with about 20 seconds until touchdown. If visibility is low, the pilot may not be able to see anything near the runway that can help them estimate their actual height above the ground. This situation is risky whether they land or not — and it’s actually the best-case scenario.
The second case is trickier. Having two radar altimeters isn’t just important in case one breaks. It’s also helpful for determining if one is broken. If the autopilot systems and flight crew receive two different readings, it’s clear that at least one of them isn’t correct. In some of these planes, “this situation may not result in the pilot being alerted to abort the landing.” With an incorrect altitude, the crew will probably configure for landing either too early or too late, resulting in a hard landing or in “a catastrophic impact with the ground.”
Now for the third case. When the radar altimeters report two different altitudes, the autopilot systems will identify the mismatch. This is basically the same as the first case, where the pilot has to decide if they can safely land the plane without a radar altimeter. However, the report points out that on some aircraft, the autopilot system will continue to use incorrect data. If the pilot doesn’t realize this in time, the results are likely to be catastrophic. This is what caused the Turkish Airlines flight 1951 crash in 2009.
The fourth case is the most dangerous by far. When both radar altimeters provide the same altitude readings, the autopilot system and flight crew will have no way to know they’re incorrect. This will result in “the autolanding system executing the flare maneuver and autothrottle retard at the incorrect time.” If the plane is too low, it will crash directly into the ground. If the plane is actually higher than expected, it’s still going to crash into the ground, but it’s going to stall first.
This is what caused the Turkish Airlines flight 1951 crash in 2009.
The report emphasizes that while this situation may not be very likely to occur, it’s particularly dangerous because radar altimeters rarely fail during landing. Now, 5G deployment may introduce one more risk to the stressful process of landing during low-visibility conditions. The interference that impacted passenger planes was caused by fundamental emissions, which can be blocked by filters. Installing band bypass filters on every aircraft would take years, but they can’t be the only solution. Filters can’t block spurious emissions, which were above the safe limit for Category 2 planes and helicopters. According to RTCA, the aviation industry and mobile wireless industry need to work together for solutions.
Mobile industry group disputes the findings
CTIA is a trade association for members of the wireless communications industry. Shortly after the RTCA report was filed, CTIA disputed many of the technical aspects of the report, including the 5G power levels used in the model, the safety margin, and the worst-case landing scenario. CTIA also argues that the “wireless industry did not have insight into the development of the RTCA Report,” leaving them “without the ability to review and understand the data.”
The purpose of the RTCA Report was to provide the FCC with the “aviation industry technical position,” but wireless industry representatives did provide some data. The report includes an information exchange with Technical Working Group 3 (TWG-3). RTCA and CTIA were both represented in TWG-3, which was formed by the C-Band Multi-Stakeholder Group. Their information exchange is made up of questions and answers dated June 12, 2020, through August 16, 2020, and it is available in its entirety in Appendix B. TWG-3 disbanded in November 2020 because members were unable to reach a consensus.
Appendix C (above) includes all comments made during a public commenting process before the report’s release. Some of CTIA’s 30 comments were incorporated into the report, but not all. SC-239 provided a specific reason whenever it rejected a recommendation, whether it was from CTIA or another party.
CTIA also disagrees with the RTCA Report’s findings for what it deems a common-sense reason: 5G is being deployed all around the world with no apparent interference problems. In its March 4, 2021, letter, CTIA suggests that Japan’s 90,000 5G base stations with operations up to 4.1 GHz are evidence against RTCA’s conclusions.
Others agree with this point. “It is not clear that there will be a widespread problem,” says Greengart. “The U.S. military has operated near these frequencies for decades without incident, and other countries have been operating 5G networks in nearby frequency bands already, again without clear interference problems to avionics.”
“We have no idea whether these problems are real or not,” says Sascha Segan, the lead analyst for PCMag. “One side says they are, one side says they aren’t. But I’ll note that the carriers have been testing C-band for a few months now, and no actual helicopters have fallen out of the sky.”
Segan remains optimistic about the mobile industry’s plans. “If there are problems, they can be fixed as part of the network builds — putting exclusion zones around airports and aiming antenna panels down, for instance. The only way I could see this slowing 5G down involves using fewer large macro-sites (the big cell towers) and more small sites on buildings.”
In April, CTIA and representatives from AT&T, T-Mobile, U.S. Cellular, and Verizon reiterated that “the commission correctly determined that C-Band 5G can operate without causing interference, let alone harmful interference, to neighboring services in nearby bands.” They “urged the commission to disregard the RTCA Report.”
Aviation industry stands by RTCA Report
In May, 20 aviation groups filed a response to CTIA’s attempt to discredit the RTCA Report. These groups (“Organizations Supporting Aviation Safety, or OSAS”) include trade associations, the largest pilots’ union, companies like Garmin and Honeywell that manufacture equipment for aviation, and others. The organizations argue that CTIA’s claims “display a lack of understanding concerning aviation and aerospace design, certification, manufacturing, and operations, including the fundamentals of aviation safety analysis.”
After reading some of the corrections, it’s hard to disagree with that. As just one example, CTIA stated that the results are being driven down by a radar altimeter that “could not have been certified by the commission in the past 40 years.” OSAS pointed out that the radar altimeter in question was manufactured in 2020, and that the model is widely used. Apparently, CTIA mistook the model’s authorization date for that unit’s age.
Putting aside the technical aspects of CTIA’s critique, OSAS made two points that are worth noting. The first is that CTIA doesn’t dispute the findings regarding 5G spurious emissions, which caused the most significant interference for Category 2 planes and helicopters. Second, CTIA’s argument that “the lack of reports of widespread altimeter interference” disproves the RTCA Report is, by definition, an argument without evidence. A lack of reports is not proof that there will not be interference.
Isn’t everyone using C-band?
In the U.S., the C-band isn’t being used for 5G yet. More base stations will provide more real-world test cases and more potential for dangerous interference. And a look at which spectrum bands are being used for 5G around the world reveals another flaw in CTIA’s argument. A presentation from Qualcomm dated December 2020 provides an overview of several countries’ allocations in the C-band.
This is consistent with the information provided by the FCC. Licenses in most of Europe follow guidance from the Radio Spectrum Policy Group of the European Commission, which mandates that 3.4-3.8 GHz will be the first primary band for 5G. While Australia is investigating the possibility of using the 3.7-4.2 GHz spectrum for 5G, it hasn’t issued licenses within that band yet. Apparently, even South Korea and Taiwan haven’t issued licenses in the upper portion of that spectrum, and they’re neck and neck for the fastest 5G speeds in the world.
The chart above is from a report prepared for the CTIA. If the U.S. goes ahead with its plan to issue licenses above the upper limits of other countries, then the spectrum allocation of other countries can’t be used to argue that there is no risk of interference. In either case, warnings from countries like France and the United Arab Emirates suggest that confidence based on lack of interference problems so far may be unwarranted.
Less interference, more cooperation
CTIA says “a single, flawed report” isn’t enough to prove that C-band 5G will cause harmful interference to nearby bands. That may be true, but so is the aviation industry’s point that the RTCA Report is “the only thorough safety study supported by aviation subject matter expertise.” With the industries pulling in two different directions, the only thing that’s clear is the need for more information.
When aviation industry groups spoke with the FCC in August, they repeated their warning: Radar altimeters provide many critical services, and interference can have cascading consequences. There are measures the aviation industry can take on its own, like installing band bypass filters, but finishing that project before 5G service in the C-band begins in December is “a practical impossibility.” In other words, the wireless industry also needs to take some precautions to close the “mitigation gap.”
What would closing the “mitigation gap” look like? We reached out to Mike Dano, the editorial director of 5G and Mobile Strategies for Light Reading, a publication for telecommunications industry professionals. “If it’s deemed necessary, the 5G industry might be required to “turn down” their broadcasts in the spectrum at issue, in order to prevent interference. It would be good to avoid airplane crashes, but if that happens, [it] would also represent a failure of the government regulatory agencies to foresee that kind of problem,” he says. “The job of the [Federal Aviation Administration], FCC and [National Telecommunications and Information Administration] is to figure this stuff out first, before auctioning the spectrum for 5G.”
Dano says that another solution would be replacing radar altimeters with models that aren’t as susceptible to interference. “That obviously would be time-consuming and expensive. But the C-band spectrum auction raised $81 billion in winning bids. And I suspect the airline industry is very aware of that amount.”
Aviation industry representatives insist they share the goal of advancing 5G.
The aviation industry asked the FCC to join with the FAA and work toward implementing solutions. Aviation industry representatives insist they share the goal of advancing 5G. They use wireless services, too, after all.
But PCMag’s Segan thinks this is an attempt to shift the blame. “The FCC has already changed the C-band plan to accommodate aviation industry concerns; the problem is that in the aviation industry’s opinion, it wasn’t changed enough. The whole “work with the FAA” line is just an attempt to shift the discussion into what they perceive is a more favorable court for them.” He