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Adventures in Engineering: 5G Expansion and Radar Altimeters

The expansion of cellular broadband in the 3.7–3.98 GHz spectrum band has crossed into the working frequency range of radar/radio altimeters (RAs). Commercial RAs are intended to operate in the frequency band of 4.2 to 4.4 GHz. However, radar altimeter receivers may be sensitive to frequencies outside this range, and this is the basis for the FAA’s current reluctance to wireless 5G wireless station deployment near airports. The FCC and the FAA are not in alignment with each other regarding 5G broadband expansion as demonstrated by their yearly finance proposals and operational goals set forth to Congress. 

Figure 1: Chris Young testing out an altimeter himself as he jumped off the wing of this plane.

What Are We Looking At?

There are two types of radar altimeters in use for commercial aviation: frequency modulated continuous wave (FMCW) modulation, and pulsed modulation. Both types operate in the 4.2 to 4.4 GHz band and are typically in use for low altitudes (less than 2500ft). The fundamental operation of the FMCW type is that the transmitter transmits a continuous wave signal that varies linearly with time. The signal bounces off the ground back to the receiver and the difference in the received frequency from the current transmitted frequency (due to the wave’s travel time to the ground and back) is directly related to altitude (Delta f -> Altitude). The pulsed modulation type transmits a pulse modulated signal and the time it takes to travel to the ground and back is directly related to altitude (delta t -> altitude). The specifications governing RA operational performance can be found in RTCA DO-155 “Minimum Performance Standards – Airborne Low-Range Radar Altimeters” as well as in EUROCAE ED30. A well-rounded description of Radar altimeter types and performance can also be found in ITU-R Recommendation M.2059-0 (02/2014) titled, “Operational and technical characteristics and protection criteria of radio altimeters utilizing the band 4 200-4 400 MHz”¹.

The current conflict between the FCC and FAA is regarding radar altimeter receiver sensitivity to out-of-band signals, specifically the adjacent 3.7–3.98 GHz spectrum band which is now occupied by commercial 5G wireless broadband. The RTCA has produced RTCA Paper No. 274-20/PMC-2073 titled, “Assessment of C-Band Mobile Telecommunications Interference Impact on Low Range Radar Altimeter Operations” ² in support of the FAA’s assertion that 5G wireless broadband deployment near airports presents a substantial safety risk to aircraft due to RF interference exceeding established safety limits.

The white paper presents a reasonable analytical approach to showing the potential for interference both theoretically and in lab tests. The white paper documents the C-Band Multi-Stakeholder Group’s technical working group (Appendix B) and public (Appendix C) comments/questions along with their resolutions and dispositions. The comments/questions cover nearly all aspects of the white paper and encompass aircraft OEM, radar altimeter OEM, FAA representatives, industry representatives, and CTIA which “represents the U.S. wireless communications industry and companies throughout the mobile ecosystem”³.

The white paper represents a collaborative analysis of potential RF interference from 5G wireless base stations to currently deployed radar altimeters. It asserts that the analysis and lab results show that 5G wireless broadband stations operating at full power near airports will produce interference levels well above established safe limits. The white paper also states, “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, and failures of these sensors can therefore lead to incidents with catastrophic results resulting in multiple fatalities.”  

In contrast, CTIA has expressed that the analysis performed by the RTCA is, in their own words, “flawed” and an electronic filing, “Ex Parte Letter, Expanding Flexible Use of the 3.7-4.2 GHz Band, GN Docket No. 18-122,” states that the analysis “cannot be relied upon.” The forementioned letter points out 5G wireless has been deployed throughout the world and no specific instances of interference from 5G base stations has been observed. The filing also points out that the center frequencies of the two spectrum bands are separated by 500MHz. The CTIA’s assertions being made are currently relying on anecdotal evidence and are not currently supported by any published detailed/structured analysis, test, or simulation.  

The FCC is and has been committed to broadband expansion in the United States as demonstrated by the agency’s history of budget estimates to Congress. Here is a basic summary of the agency’s top goals for the last five years:

• FCC 2018-2021 Budget Estimates to Congress Performance Plans identify the agency’s number one strategic goal as “Closing the Digital Divide.” The FCC strategic objectives in the agencies own words were: “Expand broadband deployment in all parts of the country,” “reduce and remove regulatory burdens and barriers to infrastructure investment,” “ensuring that effective policies utilizing basic principles of economics are in place to promote entrepreneurship and expand economic opportunity,” “create incentives for providers to connect consumers in hard-to-serve areas,” and “building awareness about the benefits of connectivity and providing consumers with education and the information necessary to drive greater broadband adoption”^4–7.
• The FCC 2022 Budget Estimates to Congress Performance Plan identifies the agency’s number one strategic goal as “Pursue a ‘100 Percent’ Broadband Policy,” which is defined as pursuing “policies to help bring affordable, reliable, high-speed broadband to 100 percent of the country.”⁸

The 3.7 to 4.2GHz band was introduced in the 2020 budget estimate to Congress (presented in March 2019) as a potential spectrum band for auction and indeed the 3.7-3.98 GHz (3.7 GHz) band was auctioned off on February 24, 2021, for $81,114,481,921 in net bids⁹. What is not mentioned in the budget estimates are any issues surrounding the FAA’s public concerns about radar altimeter susceptibility in the 3.7GHz band. 

The FAA has a primary obligation to aviation safety and is committed to the modernization of the National Airspace System (NAS). The Next Generation Air Transportation System (NextGen) is a multi-faceted program, system, and platform approach to revolutionizing civil transportation in the NAS.  Unmanned aircraft systems (UAS) and evolution of autonomous flight systems are significant components of NextGen. A cursory look at the 2018-2022 FAA President’s Budget Submission shows significant investments (over $4 billion) in NextGen and critical safety issues such as those surrounding the Boeing 737 MAX. Most significant in the FAA budget requests is there is no mention of 5G interference with currently deployed radar altimeters¹⁰.

Conclusion
The FCC’s and CTIA’s position on expansion of 5G wireless broadband in the 3.7–3.98 GHz spectrum band is economically beneficial with little risk. CTIA has published its own website regarding 5G expansion and its effect on aviation¹¹. The FAA, backed by the RTCA white paper, asserts that this expansion creates the potential for “catastrophic results” and as a result has issued numerous airworthiness directives (AD), Safety Alerts for Operators (SAFOs), and Notices of Air Missions (NOTAMs) to address these concerns. The FAA has also created a 5G and Aviation Safety website dedicated to this issue¹². The passing of bill H.R. 3684 has provided approximately $65 billion to expand broadband within the United States. This funding supporting the FCC’s initiatives has been a driving force behind the recent uptick in the conflict surrounding 5G wireless expansion as physical infrastructure is being put in place and being put online¹³. As tensions roiled in late January 2022, the FAA announced an agreement with AT&T and Verizon Wireless on how to expand 5G near airports¹⁴.  

The RTCA established SC-239¹⁵ in 2019 to formally address in band and out of band interference of RAs from known current and future transmitter sources. RA OEMs are also working on designs and workarounds to make the radar altimeters resilient to current 5G wireless transmissions in the 3.7–3.98 GHz spectrum band. While all parties involved in this conflict are taking action, it is unclear when the issue of 5G wireless expansion and radar altimeter incompatibility will be resolved.

References

1. ITU-R Recommendation M.2059-0 (02/2014) titled, “Operational and technical characteristics and protection criteria of radio altimeters utilizing the band 4 200-4 400 MHz,” International Telecommunications Union.
2. rtca.org.
3. ctia.org.
4. FCC 2018 Budget Estimates to Congress, https://docs.fcc.gov/public/attachments/DOC-344998A1.pdf.
5. FCC 2019 Budget Estimates to Congress, https://docs.fcc.gov/public/attachments/DOC-349145A1.pdf.
6. FCC 2020 Budget Estimates to Congress, https://docs.fcc.gov/public/attachments/DOC-356607A1.pdf.
7. FCC 2021 Budget Estimates to Congress, https://docs.fcc.gov/public/attachments/DOC-362381A1.pdf.
8. FCC 2022 Budget Estimates to Congress, https://docs.fcc.gov/public/attachments/DOC-372853A1.pdf.
9. Public Notice - WIRELESS TELECOMMUNICATIONS BUREAU GRANTS AUCTION 107 LICENSES, https://docs.fcc.gov/public/attachments/DA-21-839A1.pdf.
10. FAA Budgets, Federal Aviation Administration.
11. 5gandaviation.com.
12. faa.gov/5g.
13. Bill H.R. 3684 [Report No. 117–70], Govinfo.gov.
14. FAA Statements on 5G, FAA.gov, Jan. 28, 2022.
15. rtca.org/sc-239.

Chris Young is owner/lead engineer at Young Engineering Services LLC.