Quit worrying about 5G C-band and crashing aircraft, US govt eggheads sigh

Back off FAA, I'm a scientist

Fears that 5G C-band signals could disrupt aircraft altimeters are misplaced, US government researchers claim in a report, saying that current efforts to filter any potentially dangerous frequencies are likely enough to combat problems. 

The National Telecommunications and Information Administration's (NTIA) exhaustively detailed report contains the results of tests it performed using aircraft outfitted with radio altimeters (radalt) that rely on C-band signals. The equipment was tested in real-world situations against three types of cellular towers commonly used by AT&T and Verizon that operate in the 5G C-band, the NTIA said. 

Fears over 5G C-band signals interfering with nearby radalts, particular around airports, goes back to late last year when the Federal Aviation Administration said that the wireless frequencies used by 5G C-band service, planned to roll out early in 2022, were too close to radalt C-band frequencies, and could cause interference. Radalts are used by aircraft to determine altitude during low-visibility conditions, and so incorrect information could prove fatal.

Radalts typically use the C-band frequencies spanning 4.2 to 4.4GHz. C-band 5G transmits between 3.7 and 3.98GHz, which is close but doesn't overlap, unless signal towers are incorrectly configured.

In May, the FAA called on aircraft operators to begin installing filters on aircraft equipped with a radio altimeter, which the regulator believed at the time would be effective in preventing interference. Ninety percent of commercial aircraft in the US have been cleared to fly, while some remaining craft are still under restrictions

The C-band scare: Minimally threatening, still present

The NTIA's report draws a couple of conclusions. Combined, the message seems to be that the FAA's filters are working, interference that does exist is too weak to matter, and the physics of 5G towers means most of the signal is broadcast out over the surface of the Earth, not up into the sky. 

Per the report, these 5G transmitters have high-frequency cutoffs that prevent them from surpassing 4GHz. While there is some signal spillage over 4GHz, "emissions are as much as 106 decibels lower than their on-tuned intentional-radiation power." 

Unwanted radiation from 5G towers may be even weaker, the researchers said, adding that they were limited by their equipment. "The actual unwanted emission levels may have been lower than these numbers — how much lower, we do not know," the team revealed. 

As for how 5G signals reach, "airborne radiation patterns show measurably, significantly less power than is found in 5G base station main antenna beams directed toward [devices] at ground level," the researchers noted. 

All that work and the 151-page report it generated, can for the sake of passengers and aviation professionals alike, be boiled down to a single statement: "The technical solution to such a problem might be the installation or retrofitting of more-effective RF power-rejection filters on radalt receivers for frequencies below 4200 MHz." In short, we can safely join the rest of the world in realizing 5G and radalts get along just fine, if done right. 

Early this year, British and European authorities said they didn't anticipate problems with C-band 5G and aircraft, while watchdogs in Japan said they had already solved the issue by keeping 5G masts 200 metres from aircraft approach routes. 

With the paper's release, the ball seems back in the FAA's court. According to Bloomberg, the FAA intends to keep restricting where C-band towers can be installed – eg, keeping them away from flight approaches at airports – and plans to continue with filter installation mandates, similar to what it told The Register.

"The FAA is working to ensure that radio signals from newly activated wireless telecommunications systems can coexist safely with flight operations in the United States, with input from the aviation sector and telecommunications industry," the FAA told us. In other words, it isn't changing course – NTIA report or not. ®

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