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FAA SB: Risk of Potential Adverse Effects on Radio Altimeters

SAIB: AIR-21-18

Date: November 2, 2021

This document contains information and recommended action. The contents of this document do not have the force and effect of law and are not meant to bind the public in any way. This document is intended only to provide clarity to the public regarding existing requirements under the law or agency policies.


This Special Airworthiness Information Bulletin (SAIB) informs aircraft manufacturers, radio altimeter manufacturers, operators, and pilots of the planned deployment of wireless broadband networks in the 3700-3980 MHz bands, which is scheduled to begin on December 5, 2021 in the 3700-3800 MHz bands. This SAIB recommends that radio altimeter manufacturers, aircraft manufacturers, and operators voluntarily provide to federal authorities specific information related to

altimeter design and functionality, specifics on deployment and usage of radio altimeters in aircraft, and that they test and assess their equipment in conjunction with federal authorities. Results from that testing and assessment should be reported to the appropriate civil aviation authorities (CAAs) and

spectrum regulators. The FAA is currently collaborating with the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA) to assess the need for mitigation beyond the recommended action in this SAIB.


For the past 15 years, the 3 GHz band globally has been the subject of harmonization activity for mobile broadband, both at the International Telecommunication Union and within regional groups. A number of countries already have allocated and assigned spectrum for wireless deployments in the 3 GHz range and additional countries are following suit. Many countries around the world are already

deploying wireless networks in the bands from 3300-4200 MHz; some countries have implemented temporary technical, regulatory and operational mitigations, including temporary proximity and power restrictions, on wireless broadband networks operating in bands ranging from 3700-4200 MHz. There have not yet been proven reports of harmful interference due to wireless broadband operations internationally, although this issue is continuing to be studied. In the United States, there has been wireless broadband deployment in the 3.65-3.7 GHz band since 2007. The FCC started a proceeding to authorize mobile broadband service in the 3.55-3.7 GHz band in December 2012 and adopted final rules in April 2015 and October 2018. Commercial deployment started in September 2019, with no known issues for altimeters to date.

With respect to 3.7-4.2 GHz, the FCC first sought comment on introducing mobile broadband into the band in 2017. In March 2018, the MOBILE NOW Act directed the FCC to evaluate the feasibility of commercial wireless deployments in the 3.7-4.2 GHz range. The FCC started a proceeding in May 2018 and adopted the C-band Report and Order authorizing flexible use of the 3.7-3.98 GHz band in March 2020. In February 2021, the FCC completed an auction of the 3.7–3.98 GHz frequency band and subsequently issued licenses to several wireless network providers subject to license conditions about deployment timing and parameters. Under the FCC’s rules, actual wireless broadband deployment will occur in phases with operations beginning only in the lower 100 megahertz of the band (3700-3800 MHz) and only in 46 markets as of December 5, 2021. (See 47 CFR § 27.1412(b)(1), which specifies the 46 market areas as Partial Economic Areas (PEAs) 1-4, 6-10, 12- 19, 21-41, and 43-50. A list of PEAs is available at


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