While telecommunications and aviation interests await the result of consultations between the FAA and the Federal Communications Commission over how to safely roll out 5G telecommunications networks around airports, the prospect of those systems’ interference with radar altimeters in airplanes remains a point of contention. Although pressure from the FAA, airlines, and aerospace groups resulted in an agreement by U.S. telecom giants Verizon and AT&T to delay the introduction of the networks until January 19, standards for the frequency band fidelity of radar altimeters haven’t changed since the 1970s, leaving the likely prospect of the need to limit the strength of 5G signals around major airports or risk flight disruptions and deviations.
The issue at hand centers on the close proximity of the frequency bands in which 5G and the altimeters operate. Commercial aviation radar altimeters operate in the 4.2 to 4.4 GHz range, while 5G signals fall within the 3.7 to 3.98 GHz range, part of the so-called C-band. The problem arises in potential cases of radar altimeters’ “bleed over” into 5G networks’ part of the spectrum.
Recognizing the potential conflict, the RTCA—a private nonprofit corporation that works with government and industry experts to develop technical performance standards for regulatory compliance—in the fall of 2019 established a committee to set new standards for radar altimeters to ensure tighter frequency tolerances, thereby mitigating the possibility their signals stray into adjacent ranges. The committee included representatives from the FAA, Airlines for America, and, initially, the telecommunications lobbying group CTIA, among several others.
On Oct. 8, 2020, the RTCA submitted an ex parte filing in the FCC docket including a report that assessed the C-band interference on altimeter operations. The report—informed by what the RTCA called detailed information made available by the commercial wireless communications and aviation industries—found “serious threats of harmful interference to today’s installed radar altimeters.” Still, the RTCA acknowledged that the report should serve as a basis for ongoing analysis to ensure that altimeters work as intended. Until now, no “authoritative” studies have contradicted the findings, according to RTCA president Terry McVenes.
“Unfortunately, nobody's ever given us any different data…which has been kind of frustrating,” McVenes told AIN on Thursday. “Because I tried earlier last year to bring the CTIA and some of the telecommunications folks into the room with some of the radar altimeter engineers. So, [we wanted] a good, honest exchange of data and analysis to work it together. Because the only way we're going to solve this is by working it together. But we never could get any collective effort together with them. They just weren't all that interested.”
McVenes equated arguments by the CTIA that 5G hasn’t interfered with altimeter function in the other countries to comparing “apples to oranges” because of differences in power levels and the fact that they operate at the low end of the 3.7 to 3.98 GHz frequency range.
“So there's a larger gap between where radar altimeters operate and where those 5G signals are operating,” he explained. “There's a difference in power levels that are being produced by some of these in other countries. There are other mitigations that these other countries have implemented, like keeping them away from airports and things like that. So, yeah, it's true they've been implemented in 40 other countries, but it's not the same implementation.”
McVenes characterized the two-week implementation delay in the U.S. as a matter of “buying a little more time” for negotiation between the FAA and FCC over power levels, for example. But, he said, all the stakeholders need to communicate more to address the issue over the long term. In fact, it could take as much as 10 years before acceptance of a new standard for radar altimeters gains approval and takes effect. In the meantime, the same issue could arise not only in altimeters, but with new avionics technology that use similar frequency bands.
“When you look at new technologies that are coming, there's going be more competition for the spectrum bands,” predicted McVenes. “I think we're going to see other uses where it's going to butt up against those frequencies that traditionally aviation has been using.”
The RTCA established a new committee last month to take a broader look at frequency spectrum conflicts, analyze all the current standards, and identify potential conflicts. “Because we’ve never looked at it that way before,” said McVenes. “Let’s look at what we have right now and see what the potential is for future issues like this and try to be out of front of it so we don’t have to go through what we’re going through today. It’s going to come up again.”