The FAA has published a Special Airworthiness Information Bulletin—AIR-21-18—that highlights potential radio altimeter interference issues by new 5G cellular networks. The 5G network deployment in the U.S. starting on December 5 is in the 3700 to 3800-MHz bands then later in the 3700 to 3980-MHz bands. Radio altimeters use the 4200 to 4400-MHz band.
In other countries, bands from 3300 to 4200 MHz are already deployed, according to the FAA, and some “have implemented temporary technical, regulatory, and operational mitigations, including temporary proximity and power restrictions.” The FAA added, “There have not yet been proven reports of harmful interference due to wireless broadband operations internationally, although this issue is continuing to be studied.”
The SAIB contains many recommendations for aircraft and avionics manufacturers and aircraft operators, ranging from reporting any problems to the FAA to documenting and reporting on the types of radio altimeters installed in aircraft.
FAA TSO-C87A, the current radio altimeter Technical Standard Order under which these devices are approved, doesn’t address compatibility with adjacent-band operations, according to the FAA. The agency said it is “conducting a risk assessment to ascertain whether further mitigation is warranted in addition to the recommended actions in this SAIB.”
Meanwhile, Gogo Business Aviation, which is deploying its own 5G network for its air-to-ground connectivity system, pointed out that its system operates in frequency bands well away from those of radio altimeters.
“Gogo has never used the frequencies under discussion, nor do we plan to use those frequencies for our 5G network,” said Sergio Aguirre, president of Gogo Business Aviation. “The spectrum bands used by Gogo, currently and following the launch of our 5G network, have been in use for decades and have never been shown to interfere with aeronautical services.”
According to Gogo, its 5G network is using “spectrum it owns in the 800-MHz band and additional unlicensed spectrum in the 2.423 GHz to 2.475 GHz range [2423 to 2475 MHz], which has sufficient spacing from the radio altimeter operating range to preclude any interference by the Gogo 5G system.”
Radio altimeter manufacturer FreeFlight Systems has already fielded a new product line that isn’t susceptible to 5G interference. Its Terrain Series RA 5500, RA 6500, and RA 7500 radio altimeters are optimized for all sizes of aircraft and have “redesigned RF circuitry built to withstand 5G interference,” according to the company.
In October 2020, the RTCA issued a report generated by a task force assigned to assess the issue, after the Federal Communications Commission voted to allocate the 3.7 to 3.98 GHz band for flexible use, including 5G applications.
According to the RTCA, “The results presented in this report reveal a major risk that 5G telecommunications systems in the 3.7 to 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. The results of the study performed clearly indicate that 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, in the absence of appropriate mitigations.”
RTCA special committee SC-239 has been formed to develop “adjacent band-compatible minimum operational performance standards for future radio altimeter designs.” The FAA is participating in SC-239 activities.