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Will the 5G FAST Plan Lead to Spectrum Issues for Aircraft?

By Kelsey Reichmann | January 18, 2021
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The demand for 5G technologies that boast improved speed and reduced latency is quickly growing. In response to this demand, the Federal Communications Commission (FCC) is pursuing the Facilitate America’s Superiority in 5G Technology (the 5G FAST Plan) as their strategy for implementation. The 5G FAST Plan includes opening up what is known as the C-Band, or the 3.7-4.2 GHz band, for public auction. However, aviation groups claim that this will interfere with aircraft radar altimeters, also referred to as radio altimeters, and could lead to catastrophic results.

Aviation groups are asking the FCC to halt the auction of the C-Band spectrum until more research on the effects of 5G operations in the C-Band can be understood and aviation groups can improve the resilience of future radar altimeter designs.

“The deployment of 5G in the C-band could lead to possible harmful radio frequency interaction with radar altimeters,” David Silver, AIA Vice President for Civil Aviation, said in an emailed statement to Aviation Today. “Protecting the frequency bands used by these sensors, which provide direct measurements of an aircraft’s clearance height over terrain or other obstacles, is imperative to the safe operations of thousands of civil aircraft and the well-being of the flying public.”

Spectrum illustration showing 5G fundamental and spurious emissions. (RTCA)

Fifteen aviation associations sent a letter of opposition to the FCC in December urging them to reconsider the C-Band auction. The associations included Aerospace Industries Association, Airborne Public Safety Association, Air Line Pilots Association, Aircraft Electronics Association, Aircraft Owners and Pilots Association, Airline for America, Cargo Airline Association, General Aviation Manufacturers Association, Helicopter Association International, National Air Carrier Association, National Business Aviation Association, RTCA, and Regional Airline Association.

“Radar altimeters are deployed on tens of thousands of civil aircraft in the United States and worldwide to support several critical safety-of-life aircraft functions throughout multiple phases of flight,” a group of 15 aviation associations wrote in a letter to the FCC. “Radar altimeters are the only sensor onboard a civil aircraft which provides a direct measurement of the clearance height of the aircraft over the terrain or other obstacles.”

The 4.2-4.4 GHz band is where aeronautical radio navigation services operate, according to the FCC. There is a 220-megahertz guard band between the band that aircraft operate in and the band the FCC is auctioning.

“In the C-Band Order, the Commission concluded that our rules would protect radio altimeters used by aircraft,” an FCC spokesperson told Aviation Today. “And we continue to have no reason to believe that 5G operations in the C-Band will cause harmful interference to radio altimeters. Among other things, these altimeters operate with more than 200 megahertz of separation from the C-band spectrum currently being auctioned, more protection than is afforded in some other countries.”

However, aviation groups like RTCA say their research has shown the separation is not sufficient. In October 2020, RTCA published a report in which it claims that 5G base stations and equipment onboard aircraft will affect radar altimeters. The report does not show harmful interference from 5G user equipment on the ground.

The report separates aircraft into categories to assess the level of impact from interference. It found that regional air transport, business, and general aviation airplanes would not be able to escape the interference.

“For Usage Category 2, which covers commercial airplanes used for regional air transport as well as business and general aviation airplanes, the impact of 5G interference from base stations is inescapable,” RTCA's report states. “Every base station configuration produces harmful interference both from fundamental emissions in the 3.7–3.98 GHz band and spurious emissions in the 4.2–4.4 GHz band, across virtually all operational scenarios and relative geometries between the aircraft and base station. In the worst case, the safe interference limit for the fundamental emissions is exceeded by over 47 dB, and the safe interference limit for the spurious emissions is exceeded by over 27 dB.”

The report showed that interference for transport and general aviation helicopters was almost as broad as for category 2 aircraft. The impact to commercial airplanes and cargo transport was limited to specific scenarios, the report states.

The FCC used a report from the Aerospace Vehicle Systems Institute (AVSI) to justify the coexistence of 5G in the C-Band and radio altimeters, according to a March FCC Report and Order on C-Band. The AVSI report modeled a worst-case scenario with in-band interference from base stations in the C-Band.

“The preliminary results show that there may be a large variation in radio altimeter receiver performance between different manufacturers,” the FCC report states. “The measured PSD [power spectral density] levels at which errors occurred ranged from -21 to - 51 dBm/MHz for the various types of altimeters that were tested."

AVSI concluded that “most of the altimeters reported broadly consistent susceptibility to OoBI [out-of-band interference] PSD levels until more than approximately 200 to 250 MHz of OoBI was introduced.” The organization also determined that as the amount of active spectrum increased above 3.9 GHz, the acceptable levels of PSD began to decrease.

After a subsequent report from Alion and commissioned by T-Mobile, which refuted the findings from AVSI, the FCC concluded that the limits for the 3.7 GHz service were sufficient to protect the 4.2-4.4 GHz band, according to the FCC’s report.

“We agree with T-Mobile and Alion that the AVSI study does not demonstrate that harmful interference would likely result under reasonable scenarios (or even reasonably “foreseeable” scenarios to use the parlance of AVSI),” the FCC report states. “We find the limits we set for the 3.7 GHz Service are sufficient to protect aeronautical services in the 4.2-4.4 GHz band.”

However, the FCC concedes that more analysis is needed to explore why there is a potential for interference, the FCC’s report states. Aviation groups agree with the FCC that more research is needed, they just want the FCC to halt the auction while the research is completed.

Aviation groups are currently working to improve the resilience of radar altimeter designs and performance standards but 5G rollout timelines are on track to deploy before they are able to be completed.

“This risk is widespread and has the potential for broad impacts to aviation operations in the United States, including the possibility of Catastrophic failures leading to multiple fatalities,” RTCA’s report states “Further, this risk cannot be adequately mitigated by the aviation industry acting alone.”

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