Battle of the Bands

The potential use of some or all of the Upper C-band spectrum raises the risks of D-spectrum interference on radio altimeter systems, and US Congress is proposing changes to mitigate this threat.

5G is the fifth generation of cellular technology, designed not just for delivering enhanced digital experiences for the consumer, but improving the performance of business applications for enterprises, industries, and public infrastructures.

Speaking in November 2020, Houlin Zhao, ITU Secretary-General lauded the introduction of 5G specifications, as the “backbone of tomorrow’s digital economy, transforming lives and leading industry and society into the automated and intelligent world.”

With faster connectivity speeds, 5G technology has a theoretical peak speed of 20 Gbps, while the peak speed of 4G is only 1 Gbps, ultra-low latency and greater bandwidth, 5G technology introduces advances throughout network architecture, covering frequency bands in the mid- and high spectrum range not used in 4G.

Mid-band 5G spectrum (1GHz to 6GHz) is regarded as 5G’s ‘backbone’, combining a favourable combination of good propagation characteristics (coverage) and wider bandwidths (capacity). This includes existing 3G/4G bands, as well as new spectrum licensed for mobile services.

Full-scale commercial deployment of 5G networks began taking off in 2022 following ITU’s publication of a key radiocommunication technology recommendation. According to the GSM Association (GSMA) the advocacy and lobbying organisation for the mobile communications industry, live 5G networks in over 90 countries are using spectrum in 3.3-4.2 GHz (the 3.5 GHz range). More than 60% of 5G network launches have used this band so far, adding up to around 260 networks and further auction plans are expected in the years to come.

However, as William Raillant-Clarke, the International Civil Aviation Organization’s (ICAO’s) Communications Officer draws attention to, whilst new 5G mobile networks offer fast wireless connections to millions, some now transmit in frequency bands immediately next to those used by aircraft radio altimeters (also known as Radar Altimeters) which are crucial to onboard safety systems and the safety of approaches and landings.

Aircraft rely on radio altimeters (RA) systems to measure height above terrain and obstacles in all phases of flight. The RA provides this information to the pilot and to the aircraft’s interconnected navigation and safety systems to support functions such as low-visibility approaches and landings, terrain awareness and alerting, wind shear detection and recovery, aircraft collision avoidance, automated rotorcraft systems, and other flight control systems.

Lower-band issues
Strong emissions from nearby 5G transmitters can overwhelm an altimeter’s receiver, a phenomenon known as “receiver desensitisation,” because many older altimeters lacked sufficient filtering to reject the adjacent-band signals.

As the Joint Aviation Community, a coalition of US aviation stakeholders note, any degradation or failure of these altimeters can disrupt numerous automated functions, delay terrain avoidance alerts, and lead to potentially dangerous misinformed flight crew decisions which can significantly increase safety and operational risks, especially for aircraft when operating close to terrain.

According to the Community, all large commercial aircraft are fitted with at least two radio altimeters, while many general aviation helicopters and light twin-engine piston airplanes are equipped with at least one radio altimeter. In the business aviation community, larger and more advanced turbojet and turboprop aircraft are generally equipped with at least one radio altimeter. As estimated by the Federal Aviation Administration (FAA) currently, this totals over 40,000 aircraft that operate in the US airspace with nearly 60,000 separate radio altimeter units across nearly 14,000 owners and operators.

As the FAA explains, RA systems work by emitting and then detecting low-powered signals returning from the ground or other obstacle. The 4.2-4.4 GHz frequency band (RA band) is allocated for RA operational use globally. Before 2020, satellite operators and other low-powered sources used the neighbouring frequency bands, and their low-power signals in-band and out-of-band did not interfere with RAs.

However, the dynamic changed when the Lower C-band was reallocated in February 2020 to permit high-powered commercial wireless services (with the 3.7-3.98 GHz band allocated for wireless services in the contiguous US) and existing satellite operations repacked into the upper 200 megahertz of the band (4.0-4.2 GHz).

According to the Joint Aviation Community, this agreement eventually removed or greatly reduced the operational restrictions on aircraft that had been impacted by Airworthiness Directives (ADs) issued as a result of the Lower C-band deployment. These directives required operators to revise their aircraft flight manuals to include operational limitations and alternative procedures, such as legally prohibiting certain operations for aircraft equipped with susceptible radio altimeters when 5G C-Band interference was present.

The ADs established a framework for identifying specific locations where the restrictions applied through Notices to Air Missions (NOTAMs). The FAA later proposed a mandatory AD requiring all transport-category airplanes to be equipped with 5G C-Band tolerant altimeters or equivalent radio frequency (RF) filters.

This mandatory directive established a firm deadline for equipment upgrades. As part of that process, existing commercial airline radio altimeters were fitted with filters to improve signal isolation in the 3.7-3.98 GHz band. Several ADs currently restrict operations to resolve the unsafe conditions caused by wireless services in the Lower C-band.

This aviation equipment retrofit was combined with the 5G carriers implementing several mitigations: The most significant in achieving compatibility were a national elevation mask for 5G antennas, and a reduction of their conducted spurious emissions in the 4.2-4.4 GHz band from -13 dBm/MHz to -48 dBm/MHz.21 Additional operational mitigations related to power and antenna directionality for wireless carriers were also put in place near airports and other sensitive airspace to mitigate potential interference scenarios.

Scope of the issue
According to Raillant-Clarke, in some states, the rollout of 5G services in the adjacent band below the radio altimeter band has prompted aviation experts and industry to conduct detailed technical studies.

“These studies show that different altimeter designs respond in various ways to their surrounding radio environment. Some models, especially on helicopters and smaller aircraft, are more sensitive to potential interference, while commercial jets generally use altimeters with greater resilience. This information has helped those States to introduce temporary technical, regulatory, and operational measures to protect radio altimeters from 5G operations in the adjacent band below the radio altimeter band,” he says.

On 19 January 2022, the FAA began tracking and analysing reports of potential interference affecting RAs and integrated safety systems. As of 19 August 2025, the FAA has received 659 reports of potential C-band interference, and 493 of these reports were associated with RAs or related systems. The FAA says it has completed analysis of 625 of these reports and identified 118 events where all other potential sources were eliminated as likely causes and were potentially caused by C-band interference. “Most of these 118 events consist of RA display errors, including erroneous altitude data, and/or nuisance alerts from integrated safety systems dependent on RA data to function properly. The quantity and details of reports received to date reflect the current spectrum environment defined by the wireless voluntary commitments and mitigations imposed by ADs to address the highest-risk operations. These reports demonstrate that wireless signals disrupt radar altimeters as predicted,” the organisation says.

However, a robust rebuttal has been made by the GSMA. In its “5G and Radio Altimeters Coexistence in 3.3-4.2 GHz” paper, published in October 2025, the Association writes that, “As a result, the aviation industry wished to claim extreme protection from 5G networks for a handful of poorly designed and obsolete altimeter models built on outdated standards from the 1970s. Those models are not designed to adequately reject signals emanating from outside of the altimeter frequencies authorized for aeronautical radionavigation. Altimeter equipment designed against such an outdated standard should not be in use in today’s radio-frequency environment.”

What is not disputed is that since the introduction of 5G, the aviation and wireless industries have worked to enhance their collective understanding of the varied interests in this and nearby frequency ranges, providing lessons learned for improved outcomes moving forward.

A new need
This is particularly important as a new safety concern is presented by the potential allocation of the 4.4–4.8 GHz band to 5G, directly above the radio altimeter frequency band at 4.2-4.4 GHz.

To ensure safe, efficient, and reliable aviation operations in the presence of wireless signals in the Upper C-band, the FAA is proposing new regulations (Notice of Proposed Rulemaking (NPRM)) to address the concerns around higher power signals.

These issues have been previously mitigated with wireless companies voluntarily agreeing to limit base station power level and out-of-band emissions in the Lower C-band and operators making changes to their RA units to improve interference tolerance. However, with the voluntary agreements expiring in 2028, and the mandate for FCC to auction off the Upper C-band spectrum (3.98 to 4.2 GHz) adjacent to the RA band (at least 100 MHz of spectrum by 4 July 2027, these measures will no longer be adequate to prevent RA interference and associated catastrophic risk to air operations.

These new radio altimeters must withstand interference from wireless signals in neighbouring spectrum bands and continue to provide accurate altitude readings to both pilots and integrated aircraft safety systems. According to the FAA, the minimum interference tolerance requirements proposed in this rule reflect the best achievable interference rejection using current technology without compromising radio altimeter system performance.

As Sharon Pinkerton, SVP Legislative & Regulatory Policy at Airlines for America (A4A) writes in public comments submitted to the FCC on this issue in late January, “While A4A did not take a position on the merits of repurposing the Upper C-Band in response to the Notice of Inquiry, Congress subsequently passed the One Big Beautiful Bill Act, requiring an auction of at least 100 MHz in these frequencies. A4A therefore stands ready to support FCC efforts to satisfy this statutory requirement and create a regulatory regime that advances both aviation safety and the smooth and timely deployment of new services in the Upper C-Band. The record in response to the Notice of Inquiry reflects broad agreement that both wireless carriers and airline carriers incurred unanticipated and avoidable costs, delays, and interruptions in service during the introduction of 5G in the Lower C-Band. This record also reflects optimism that the partnerships forged since that time, and the enhanced collective understanding of the varied interests in this frequency range, can improve outcomes for all parties during the introduction of new services in the Upper C-Band.”

Technology update
It’s agreed that today’s existing altimeter stations were not designed to withstand full-power operations in the adjacent Upper C-Band.

To address this, the Radio Technical Commission for Aeronautics (RTCA) and EUROCAE joint committee, have been developing an industry consensus standard for next-generation RA systems since 2019. These next-generation RA systems will be responsible spectrum users, with an up-to-date design to provide the best currently achievable performance to tolerate and reject potential interference.

RTCA SC-239 was established in 2019 and tasked with revising RTCA/DO-155. RTCA SC-239 is working on these MOPS jointly with EUROCAE WG-119, which will also be releasing an update to ED-30. The joint committee has completed a draft standard that is undergoing validation, which involves testing and analysis with prototype new designs to ensure that the requirements are both achievable and sufficient to meet the industry’s needs. RTCA plans to publish a final new standard in March 2027.

The FAA is proposing two compliance dates for RA retrofits: The initial RA performance deadline (between 2029 and 2032), would apply to all aircraft equipped with an RA operating under Part 121 and aircraft equipped with an RA operating under Part 129 with 30 or more passenger seats or a payload capacity of more than 7,500lbs. The FAA would require an earlier compliance date for part 121 and 129 operations which covers major domestic and international airlines. Any other aircraft operating in the airspace of the 48 contiguous States and the District of Columbia equipped with an RA would have two additional years from the first compliance date to retrofit with an RA system that meets the proposed performance requirement.

According to Pinkerton, with the recent publication of the FAA’s Radio Altimeter NPRM, the scale of the required upgrade for Part 121 aircraft is coming into focus for the commercial airline industry.

“A4A understands that operators will be able to bring most existing Part 121 aircraft into compliance with the forthcoming FAA regulations through the addition of new filters on radio altimeter systems. However, some portion of the existing Part 121 fleet will require the addition of entirely new transceivers to comply with the proposed rule.

A4A’s members are actively engaged with radio altimeter manufacturers to understand which radio altimeter stations installed in their fleets can be upgraded through a filtering solution, and which require an entirely new transceiver. Based on initial conversations, A4A estimates that as much as 30% of the existing Part 121 fleet will require entirely new transceivers to comply with the FAA’s proposed rule.”

The FAA estimates the total undiscounted cost to retrofit with interference-tolerant RA units at $4.49 billion (£3.29bn), or $424 million (£311m) annualised at a 7% discount rate over 20 years, which is estimated to be the average remaining service life for current fleet aircraft.

As Pinkerton explains, the primary costs for Part 121 operators to implement the altimeter upgrade include new equipment, labour, and potential lost revenues from taking aircraft out of service. For those aircraft that require a filtering solution, Part 121 operators potentially will be able to perform upgrades during the normal course of aircraft maintenance and repair, minimising the need to remove such aircraft from service. However, installing entirely new transceivers could require a multi-day installation and certification process. The lost revenue from taking aircraft out of service can be significant when spread over a third or more of all Part 121 aircraft.

The cost of new compliant transceivers is one of the more challenging variables to predict, she states. “While A4A has requested estimated pricing estimates from the largest radio altimeter manufacturers, many of those manufacturers are still digesting the FAA NPRM. In those instances, in which a manufacturer has provided an estimate, the manufacturer has heavily caveated the estimate, making the estimate difficult for A4A’s members to use for business planning purposes.”

In addition to equipment, labour, and revenue costs, Part 121 operators will incur significant in-house costs managing the logistics of the upgrade. For many Part 121 operators, the upgrade will involve a logistically intensive process managed by in-house employees or subcontractors. These teams will be responsible for procuring new equipment, managing maintenance and repair operations, as well as coordinating upgrade schedules of aircraft.

“Disruptions to the upgrade schedule, whether caused by supply chain or equipment delays, labour shortages, severe weather, or flight delays and diversions, can add significant cost and delay to the upgrade process. Many Part 121 operators will thus need to dedicate significant in-house or subcontracting resources to ensure a timely and efficient upgrade process,” she states.

Without the availability of new interference-tolerant RAs, either due to failure to certify the new product in time, uncertainty regarding supply within the compliance timeframe, or not issuing the proposed regulations on RA performance, alongside the expiration of the wireless agreements in 2028 and expansion into frequencies closer to the RA band from the upcoming FCC auction, the FAA says it would likely prohibit specific operations and ground aircraft that cannot operate safely without interference-resistant RAs. “These ADs would not be applicable to non-US registered aircraft, so other methods would be required to ensure safety, such as issuing NOTAMs and amending the U.S. AIP to address changes in the spectrum environment,” it notes.

As the Joint Aviation Community concludes in its written response to Congress, “The aviation industry is taking decisive action to rapidly accelerate equipment development efforts in direct support of the Commission’s statutory obligations and public interest objectives. Retrofitting or replacing every radio altimeter system operating within the Contiguous United States is a complex, industry wide endeavour—one that can only be achieved with the coordination of all affected federal agencies and support and collaboration off affected industry stakeholders.”

As Raillant-Clarke advises, the upcoming International Telecommunication Union (ITU) World Radiocommunication Conference (WRC-27) may change international radio regulations and create a new challenge. Taking place in 2027, WRC-27 will address several spectrum allocation decisions, including the possible introduction of 5G services in the upper adjacent band to the band used by the radio altimeters.

“ICAO is working closely with partners such as civil aviation authorities and regulators to ensure that aviation’s safety requirements are fully considered in those spectrum policy decisions.”

He adds that, “Ongoing work at ICAO and among industry partners is also supporting the creation of new, modernised standards for radio altimeters, designed to ensure long-term compatibility and resilience as the radio environment changes. ICAO is also supporting Member States by helping align aviation and telecom priorities, facilitating knowledge-sharing, and advancing harmonised solutions worldwide.

“This international collaboration follows the goals set out in ICAO Assembly Resolution A42-7: Support of the ICAO policy on radio frequency spectrum matters, which was adopted at its 42nd session in October 2025. As a result of these efforts, the safe, reliable, and efficient use of global flight infrastructure will continue to offer peace of mind for travellers.”

By Alex Preston