Autonomous Vehicles, New Mobility & the Built Environment Smart Cities Wireless

C-Band 5G’s Threat to Aviation

Note: The above explainer video from Alex Scerri was added on 12/2/20

Is the impeccable safety record of aviation threatened by the use of C-Band frequencies (specifically 3.7-3.98 GHz in the U.S.) for 5G? Yes, would be the conclusion one could draw from RTCA’s November 30th, 2020 webinar, Interference Risk to Radar Altimeters from Planned 5G Telecommunication Systems.

The RTCA is recommending that the mobile wireless and aviation industries work with their respective regulators to take appropriate steps to mitigate the risk associated with the deployment of 5G in the C-Band. The question is what impact will this interference risk have on the rollout of 5G in the C-band?

As background, the RTCA, founded in 1935 as the Radio Technical Commission for Aeronautics, is a Standards Development Organization that works closely with the FAA “to develop comprehensive, industry-vetted and endorsed standards that can be used as means of compliance with FAA regulations.” The FAA’s Technical Standard Order TSO-C87a specifies requirements for radar altimeters. In turn, this TSO references the RTCA DO-155 Minimum Operational Performance Standards (MOPS) and the EUROCAE ED-30 Minimum Performance Specification (MPS) .

The Only Above Ground Level Sensor #

The radar altimeter is an important safety feature as it is the only sensor on an aircraft that measures Above Ground Level (AGL) altitude. The widespread use of these sensors in civil aviation began in the 1970s, following several deadly Controlled Flight into Terrain (CFIT) accidents.

It works by transmitting either a pulsed or frequency-modulated, continuous-wave radio frequency carrier and listens to the reflection of that signal from the ground. Operating in the Aeronautical Radionavigation Service (ARNS) from 4.2–4.4 GHz, it is just above the soon-to-be auctioned 3.7-3.98 GHz frequencies that will be repurposed for 5G.

The last update to the aforementioned radar altimeter requirements was in 1980 when the use-cases for the 3.7 to 4.2 GHz band were Fixed-Satellite Service (e.g. distribution of cable television programming from programmer to cable headend) and Fixed Service (e.g. point-to-point terrestrial communication links).* Due to the sparsely distributed, highly-directional, and relatively low-power nature of these services, interference with radar altimeters was not a problem.

RTCA’s Study – 5G Interference Risk to Radar Altimeters #

5G Interference Risk to Radar Altimeters - Page 18 of RTCA's webinar and the SC-239 study results.
5G Interference Risk to Radar Altimeters – Image courtesy of RTCA

Enter 5G and the potential for interference with existing radar altimeters. In response to the March 2020 FCC Report and Order regarding opening up the 3.7 to 4.2 GHz for 5G use, RTCA established a 5G Task Force within a Special Committee 239 to investigate potential coexistence issues between 5G and radar altimeters. This task force consisted of representatives from industry, the Air Line Pilots Association, airlines, and the FAA. On October 7th, RTCA released their report (PDF) and submitted it to the FCC.

The core issue deals with potential interference either from fundamental emissions or spurious emissions. Fundamental emissions have to do with potential receiver overload due to lack of adequate out-of-band rejection of the 5G signals. The spurious emissions are potential frequencies emitted by 5G transmitters that would land in the 4.2-4.4 GHz range.

Three different radar altimeter use-cases were studied.

  • Commercial airplanes used for passenger travel and cargo transport
  • Business aviation, general aviation, and regional transport airplanes
  • Both transport and general aviation helicopters

Within those use-cases, they study how the lateral distance and altitude between base stations and aircraft impact interference. Their investigation also examines the impact of User Equipment (e.g. devices with 5G wireless) both on the ground and onboard the aircraft.

Real-World Examples – Houston, We Have a Problem #

What is most interesting are the real-world cases they model. One of their models looks at the precision approach path to Chicago O’Hare International’s Runway 27L. They assume 5G upgrades of the existing five LTE base stations that are near this path. For all usage categories, there is the potential for radar altimeter failure due to interference (see page 76).

“In all cases, possibility of harmful interference in this instrument approach scenario is particularly dangerous given that up to the present time, radar altimeter failures during this phase of flight have been extremely uncommon, especially on Usage Category 1 aircraft.”

To mitigate its impact, why not just be more selective about where to deploy 3.7 to 3.98 GHz?

Well, that might work well for the O’Hare use-case, but it wouldn’t work so well for the Helicopter Air Ambulance (HAA) scenario. Flight paths are lower and more broadly distributed throughout a city with HAA, as compared to the O’Hare example. The flight paths for four Houston hospitals were studied and fundamental emissions were exceeded in all cases. In one case the interference is almost 40 dB above threshold,

“the radar altimeter(s) on the HAA aircraft would be completely inoperable, limiting the capabilities of these aircraft to operate safely and dispatch quickly to those in urgent need of medical attention.”

Fundamental emissions and spurious emissions from user equipment on the ground is not anticipated to result in significant operational impacts on civil and commercial aircraft. User equipment onboard aircraft in Usage Categories 2 and 3 aircraft, however, is anticipated to “introduce a significant risk of harmful interference to the radar altimeters used on these aircraft [as much as 47 dB – see page 87].”

An Uncertain Flight Plan #

Not discussed in the webinar or report is the potential impact on the nascent urban air mobility industry, which promises to greatly multiply the approximately 180k+ aircraft in the U.S. skies today. There may be a solution for future aircraft, as RTCA SC-239 is working jointly with the EUROCAE Working Group 119 to develop new radar altimeter Minimum Operational Performance Standards (MOPS). Still, it will take years for new designs to be in place as the new MOPS isn’t expected until October 2022. The FAA would then have to approve new equipment-level designs.

RTCA points out that new designs will not fix existing radar altimeters. It is not clear what possible retrofits, such as the addition of external bandpass filters, are possible. RTCA suggests that a retrofit, if possible, would take several years to properly validate and deploy.

RTCA recommends further research into risks and mitigations and offers to help industry and regulators (page 88 & page 89 PDF)

“Therefore, it is critical that the performance of radar altimeters which are currently in service across tens of thousands of civil aircraft be understood and the risks and operational impacts due to interference be appreciated based on the characterization provided in this report. Given the planned timeline for deployment of 5G systems in the 3.7–3.98 GHz band, these radar altimeters will be exposed to such risks and operational impacts if proper mitigations are not put in place…..The SC-239 membership will work with interested parties, both regulatory authorities and industry representatives, to develop any further analysis efforts or discussion of interference mitigation approaches as needed.”

It will be interesting to see how the potential interference risks raised by RTCA impact the rollout of 5G in the C-Band.

*In 2012 the FAA created TSO-C87a, which supersedes the 1966-released TSO-C87 and references the 1980-released EUROCAE ED-30 MPS.

Disclosure: In addition to being Managing Editor of the Viodi View, Ken Pyle is an Airport Commissioner for SJC. This article is attributed solely to his role as Managing Editor of the Viodi View and is not reflective of his role as Airport Commissioner.

Author Ken Pyle, Managing Editor

By Ken Pyle, Managing Editor

Ken Pyle is Marketing Director for the Broadband Forum. The mission of this 25+-year-old non-profit “is to unlock the potential for new markets and profitable revenue growth by leveraging new technologies and standards in the home, intelligent small business, and multi-user infrastructure of the broadband network.”

He is also co-founder of Viodi, LLC and Managing Editor of the Viodi View, a publication focused on the rural broadband ecosystem, autonomous vehicles, and electric aviation. He has edited and produced numerous multimedia projects for NTCA, US Telecom and Viodi. Pyle is the producer of Viodi’s Local Content Workshop, the Video Production Crash Course at NAB, as well as ViodiTV. He has been intimately involved in Viodi’s consulting projects and has created processes for clients to use for their PPV and VOD operations, as well authored reports on the independent telco market.

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21 replies on “C-Band 5G’s Threat to Aviation”

TSO-87a is a Technical Standard Order published by the FAA that invokes an RTCA standard.. RTCA develops and publishes DO-155, Minimum Operating Performance Standards for Radar Altimeters.

The RTCA report states: “The results presented in this report reveal a major risk that 5G telecommunications systems in the 3.7–3.98 GHz band will cause harmful interference to radar altimeters on all types of civil aircraft—including commercial transport airplanes; business, regional, and general aviation airplanes; and both transport and general aviation helicopters.”

That is REALLY scary! Surprised it was not considered by the FCC prior to its C band Auction 107, which is scheduled to start December 8th. This issue did not come up during WRC ’19 which was attended by telecom regulators from all over the world. Why not?


Thanks, Alan. It is surprising that this isn’t getting more attention, as it seems to be an extremely big deal as it could be a matter of life and death.

Thanks for the reference to the Analysys Mason report. Although it is only from March, some of the information is somewhat dated, as the FCC has been busy trying to open up spectrum this year.

For instance, it opened up the 5.925 to 7.125 GHz band for various forms of licensed-by-rule on April 23rd.
They also added some spectrum for unlicensed use (5.85-5.895 GHz), taking it away from bandwidth that had been dedicated for transportation safety use.

There is also the CBRS spectrum availability both the auctioned and lightly licensed version which was made available this year, as well as the other spectrum in the 3.1 to 3.55 GHz range that is being studied.

And then there is the 3.7 to 3.98 GHz auction, which starts tomorrow.

One has to think that with all the spectrum made available this year, that the public would be better served by delaying the C-Band auction and doing more testing to confirm RTCA’s results and understand possible mitigations.

Alan Weissberger summarizes the current state of the C-Band/5G interference tussle at this link

The FCC could have been more proactive about understanding the potential ramifications of interference. For instance, as suggested above, the FCC could have coordinated a pilot program to measure the real-world impact of the 5G radios on altimeters.

Hopefully, the various players (e.g. FAA, FCC, industry) will come together to create this type of environment in the next 6 months to create a real-world pilot testing program. Six months is not a lot of time to pick sites, determine a test plan, measure, and report. They better get going.

Lastly, to put things in perspective, the FCC’s Report and Order for auctioning the C-Band was issued on March 3rd, 2020. The auction launched on December 8th, 2020. RTCA formed a special committee of industry and the FAA to investigate the potential impact of C-Band licensing for 5G shortly after the FCC’s March 2020 Report and Order and issued their report on October 7th, 2020, approximately 7-months after the FCC Report and Order.

The Federal Aviation Administration on Tuesday Dec 7th outlined flight restrictions that will take effect in January when a new 5G wireless service makes its debut, even as regulators work with telecom and aerospace companies to avoid U.S. air traffic disruptions.

The FAA order pertains to a type of 5G slated to go live Jan. 5. It would restrict pilots from operating automatic landing and other cockpit systems commonly used in poor weather, to avoid possible interference from 5G in the spectrum range known as the C-band.

The FAA and aviation industry groups have said the new 5G service could interfere with radar or radio altimeters, gauges that measure the distance between aircraft and the ground. Information from those devices feeds a number of cockpit safety systems used to land planes, avoid crashes and prevent midair collisions.

The airports that would face potential disruptions will be identified in future notices, according to the FAA order, known as an airworthiness directive. Regulators and technical experts have been working to address concerns about potential safety risks to resolve a long-running dispute between the aviation and telecom industries.

“The FAA plans to use data provided by telecommunications providers to determine which airports within the United States have or will have C-band base stations or other devices that could potentially impact airplane systems,” the agency’s order said.

Data pertaining to 5G signals’ power levels and location are expected to help air-safety regulators limit disruptions, current and former government officials have said. Aviation industry groups have warned of potentially “debilitating impacts” from such flight restrictions, saying in a Nov. 18 letter to the FCC: “Air cargo and commercial air travel will likely cease at night and in any weather where the pilot cannot see the runway.”

The FAA said it was coordinating with the Federal Communications Commission and wireless companies and has made progress “toward safely implementing the 5G expansion.”

The FCC said it looks forward to further guidance from the FAA that takes into account a recently proposed solution from telecom companies.

AT&T Inc. and Verizon Communications Inc. previously agreed to delay by a month their activation of the fifth-generation wireless service, which provides faster broadband speeds for a range of mobile devices. The service had previously been due to go live Dec. 5, but the companies agreed to hold off because of the FAA’s aviation safety concerns.

In November, the telecom companies offered to limit the power of certain 5G base stations as an additional safeguard. On Monday, a representative from the Aerospace Industries Association told the FCC in a letter the carriers’ proposed limits were “inadequate and far too narrow” to address flight safety concerns.

The U.S. telecom industry has maintained that the new 5G service doesn’t pose a safety threat to aircraft, pointing to other countries’ experience with similar wireless services. A Verizon spokesman said the company is on track to reach 100 million Americans with the new service in the first quarter of 2022 and was confident the FAA’s further analysis will find C-band service “poses no risk to air safety.”

An AT&T spokeswoman said the carrier recognizes the “paramount importance of air safety, and our use of the C-band spectrum will not undermine that imperative.”

In its order, the FAA said it determined that “no information has been presented that shows radio altimeters are not susceptible to interference” by the new 5G service. The FAA’s order said it affected an estimated 6,834 U.S.-registered airliners and other aircraft. A similar FAA order, also issued Tuesday, affects an estimated 1,828 helicopters.

The FAA also warned that interference from planned use of 5G wireless spectrum posed an air safety risk and could result in flight diversions.
One FAA directive on Tuesday said the “unsafe condition” posed by the planned use required immediate action before the Jan. 5 deployment “because radio altimeter anomalies that are undetected by the aircraft automation or pilot, particularly close to the ground … could lead to loss of continued safe flight and landing.”
The FCC said Tuesday it “continues to make progress working with the FAA and private entities to advance the safe and swift deployment of 5G networks … We look forward to updated guidance from the FAA in the coming weeks that reflects these developments.”

Thanks, Alan for the additional insight in the above two comments. Hopefully, the various parties will get together to ensure that there are no impacts to aviation safety.

It is reassuring that Verizon and AT&T do not seem concerned with potential delays. It makes sense as the amount of bandwidth gained with this spectrum is minimal compared to existing holdings.

Dec 23, 2021 Update from Bloomberg:

U.S. aviation regulators on Thursday expanded their warning about 5G service set to launch Jan. 5 on new frequencies, saying potential interference could have a broad impact on aircraft safety systems.

The Federal Aviation Administration issued a Safety Alert for Operators warning that “a wide range” of aircraft safety devices could malfunction and laid out the process it will follow in coming days to potentially issue specific restrictions on flights.

he FAA actions come as telecommunications and aviation companies agreed on Wednesday to share more data in an attempt to head off what has become a tense standoff over whether the 5G service could disrupt airlines and helicopter operations.

“The FAA is working with the aviation and wireless industries to find a solution that allows 5G C-band and aviation to safely coexist,” the agency said in a statement that accompanied the release of the alert and a Special Airworthiness Information Bulletin.

In the alert, the FAA identified 17 different safety systems and aircraft functions that could fail if 5G airwaves interfere with radar altimeters, which use radio waves to calculate an aircraft’s altitude.

As talks with the telecommunications industry are underway, the FAA is preparing what are known as Notices to Air Missions that may restrict flights in dozens of locations, it said.

The wireless industry and the Federal Communications Commission have said the new high-speed wireless service on the so-called C-band of spectrum don’t pose a threat, but the FAA and airlines are warning that there could be flight disruptions including delays and the diverting of planes to different airports. C-band airwaves are near the frequencies used by altimeters.

“We share the FAA and FCC’s confidence that we can and will have safe flights and robust and reliable 5G,” Nick Ludlum, a spokesman for the CTIA telecommunications trade group, said in an email. “We continue to work closely with the aviation industry and we look forward to joining the nearly 40 countries already operating 5G in the C-band on January 5th.”

A radar altimeter’s data is fed into numerous systems on aircraft, from basic help to pilots during landing to multiple safety devices, according to the FAA. Its data is used to prevent crashes into the ground and mid-air collisions, for example. But it also feeds aircraft automation that control throttle settings, takeoff guidance, windshear alerts and thrust reversers that help stop after landing.

“Anomalous (missing or erroneous) radio altimeter inputs could cause these other systems to operate in an unexpected way during any phase of flight – most critically during takeoff, approach, and landing phases,” the FAA said in the safety alert.

Another update: AT&T, Verizon Refuse FAA Request to Delay 5G Launch

AT&T and Verizon rebuffed a request from federal transportation officials to voluntarily delay the launch of new 5G wireless services, extending a showdown that could lead to potentially disruptive U.S. flight restrictions as soon as this week.

The cellphone carriers on Sunday offered a counterproposal that would further dim the power of their new 5G service for six months to match limits imposed by regulators in France, giving U.S. authorities more time to study more powerful signals’ effect on air traffic. The companies had planned to launch the service Wednesday in as many as 46 of the nation’s largest metropolitan areas.

“If U.S. airlines are permitted to operate flights every day in France, then the same operating conditions should allow them to do so in the United States,” the chief executives wrote in a letter reviewed by The Wall Street Journal.

The Federal Aviation Administration has been preparing to issue flight restrictions that could go into effect around the country as soon as Wednesday. The limits could restrict pilots from using certain automated systems to help land aircraft in bad weather, a move that could disrupt air travel and cargo shipments.

“U.S. aviation safety standards will guide our next actions,” the FAA said Sunday. The agency declined to comment about when it might issue the flight limits in official notices to air missions.

Telecom-industry officials have pointed to dozens of countries, including France, that have already allowed cellular service over parts or all of the frequencies in question, known as C-band. France is among the countries that have imposed wireless limits near airports while regulators study the effect the signals have on aircraft.

The letter by AT&T CEO John Stankey and Verizon CEO Hans Vestberg was in response to one sent by Transportation Secretary Pete Buttigieg and Federal Aviation Administration chief Steve Dickson late Friday. The New Year’s Eve missive asked the carriers to postpone their planned 5G launch by “no more than two weeks” while officials worked to address the wireless services’ effect on specific airports over the coming weeks.

Air-safety regulators have said the new cellular services could confuse key cockpit safety systems and have been preparing to impose potentially disruptive flight restrictions.

AT&T and Verizon, which combined serve more than half of all U.S. cellphone connections, disputed officials’ claims of air-safety risks. The companies postponed a planned December debut of the new signals to provide more time for telecom and aviation regulators to share information about the wireless infrastructure and aircraft equipment in question.

Wireless companies later pledged to limit C-band signals for another six months through early July. The letter Sunday proposed even stricter 5G limits over the same period.

But the telecom CEOs said transportation regulators’ latest delay request would be to “the detriment of millions of our consumer, business and government customers.” The CEOs added that carriers spent more than $80 billion to acquire the licenses in a Federal Communications Commission auction that closed in January 2021.

FCC authorities padded the spectrum they auctioned with a swath of buffer frequencies to prevent interference with cockpit systems. But air-safety regulators have expressed concern that more sensitive altimeters could mistake cellular transmissions for terrain.

AT&T and Verizon have spent the past year preparing to turn on new signals to provide new fifth-generation wireless technology, a faster and more capable mobile service. Wireless companies in other countries already use similar frequencies, but the spectrum wasn’t available to U.S. providers until recently.

Without a resolution to the dispute, Messrs. Buttigieg and Dickson warned the FAA’s flight limits would bring severe economic consequences.

“Failure to reach a solution by Jan. 5 will force the U.S. aviation sector to take steps to protect the safety of the traveling public, particularly during periods of low visibility or inclement weather,” they wrote in their Dec. 31 letter.

Airlines have been bracing for significant flight cancellations and diversions due to potential FAA flight restrictions because of the regulator’s aviation-safety concerns. Pilots and airlines had been awaiting details of potential FAA flight restrictions that limit the use of systems that rely on radar altimeters.

Over the past week, U.S. air travel has been snarled by a mix of winter storms and staffing challenges because of increasing ranks of airline crews calling in sick with Covid-19 as the U.S. deals with a surge by the Omicron variant. Thousands of flights have been canceled and delayed.

The competing proposals are the latest in a flurry of behind-the-scenes work by aerospace manufacturers and wireless companies, their regulators and senior White House officials. As they attempted to hammer out solutions to allow the new 5G service to go live without prompting severe flight restrictions, the trade group Airlines for America asked the FCC to consider an emergency request to delay the wireless rollout.

The airline trade group said Sunday it continued to urge the FCC and telecom industry to work with the FAA and aviation industry to reach a compromise.

The FCC is an independent agency that acts outside the Biden administration’s direct authority. The commission hasn’t shown an interest in limiting licenses that it found safe to grant in a 2020 order authorizing the 5G auction.

An FCC spokeswoman declined to comment on the airline group’s request but said the commission remained optimistic that “by working together we can both advance the wireless economy and ensure aviation safety.”

Diana Furchtgott-Roth cuts through some of the superficial in her latest Forbes article about the potential C-Band interference with radio altimeters. For instance, she points out that proposed rules in the U.S. allow for 2.5x higher power than in France. Additionally, in France the antennas are pointed downward.


And this report from the Information Technology and Innovation Foundation (ITIF) and the Eno Center for Transportation (Eno) looks at the C-Band spectrum sale/Aviation issues as a case study for identifying improvements, particularly between various government agencies, in the future allocation of spectrum.

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