Baseball Hall of Famer Yogi Berra was notable for his skill on the field and was equally notable for many of his quotes. “Yogi-isms” were things he often said that were both amusing and insightful.
Referring to a popular restaurant, he said, “Nobody goes there anymore; it’s too crowded.” Sounds like a contradiction, but it’s easy to follow his meaning. Customers chose not to go there because it’s difficult to get through the crowds.
Technology can be like that, too — sometimes with troubling consequences.
The introduction of 5G wireless brought new features to our online wonderland: higher speeds, lower latency, greater capacity, and wider bandwidth. And more became possible in 5G-equipped networks.
Technology does not stand still, and it certainly has not in wireless telecom applications. Near airports, new 5G wireless sites have been unwelcome neighbors due to the potential for electromagnetic interference (EMI) to aircraft radar altimeters (RAs).
Radar altimeters used in aircraft are safety-critical for planes making their final approach. A safe landing depends on an experienced pilot knowing exactly the altitude in the seconds before the wheels meet the runway.
New 5G devices operate within the C-Band between 3.7 and 4.2 GHz, a frequency range close to the altimeters, which have operated between 4.2 and 4.4 GHz for many years. Wireless services shoulder-to-shoulder in frequency create an interference risk.
Though those spectrum segments don’t directly share frequencies, their proximity can be a danger. Any transmitted signal, no matter how clean or precise, generates harmonics that appear as multiples of the fundamental frequency beyond the original.
The RA is especially vulnerable, since it relies on minute radar signals processed critically as the aircraft is making its final approach. Corruption of that signal can make the difference between a graceful landing and a tragic crash.
Aircraft navigation safety clearly is a paramount concern. In December 2023, the Federal Aviation Administration (FAA) updated its guidance on the effects of 5G C-Band interference on RAs.
The agency recommended that aircraft operators install bandpass filters to minimize the interference from 5G sites operating in the C-Band between 3.7 and 4.2 GHz.
Electromagnetic compatibility (EMC) engineers have at their disposal an array of tools to minimize or eliminate EMI. Bandpass filters are among those on the tool kit’s top shelf.
Filters capable of blocking or diverting unwanted signals are effective solutions but are added after the fact to devices that are interference victims.
Radar altimeters most often see use when a plane makes its approach in limited visibility. Landing in a fog requires precise altitude readings before the wheels hit the tarmac.
Aircraft are capable of operating without RAs most of the time in clear weather using barometric altimeters, but not having the use of the RA restricts some approaches.
In the US, there are airports that rely on RA readings for safe approaches because of geography or other factors. Pilots are given lists of those landing sites that require the use of the RA as the aircraft approaches the field.
A fundamental part of good EMC practice is to prevent an interference problem from happening, whether through design choices or application restrictions.
The door swings both ways in EMC. Devices and systems can be built to minimize emissions and the interference those emissions might cause. Those devices can also be designed to be immune from reasonable levels of external radio frequency (RF) energy.
One of the choices to be made in system planning is spectrum management, where frequencies are chosen to minimize both the risk of causing interference and the risk of being victimized by it. A product would not be expected to function properly if it’s known that there will be disruption of signals at certain frequencies.
But regarding 5G’s altimeter interference potential, the network carriers, the aviation industry, the FAA, and the Federal Communications Commission (FCC) did not coordinate their plans. Concerns were raised in recent years as plans for new 5G deployment near airports proceeded.
Initial 5G services launched in the 3.5 GHz band for most networks but more spectrum slices will be needed as 5G usage increases. Low- and mid-band (<1 GHz – 2.6 GHz), and high band (24 GHz – 40 GHz) spectrum are also being deployed to support the full range of 5G uses.
The 5G mid-band range (3.7 GHz – 4.2 GHz) remains problematic for the network carriers as they work to resolve potential altimeter interference.
On October 12, 2023, the FAA published SAFO (Safety Alert for Operators) 21007, which provided guidance “regarding the risk of potential adverse effects on RAs when operating in the presence of 5G C-Band wireless broadband signals.”
It explained that telecommunication companies agreed to coordinate their transmitter information to the FAA around specific airports, but there remain other safety systems relying on RA data that could be affected by 5G signals.
The FAA is especially concerned about missing or erroneous RA input signals that could disrupt systems in other flight phases. Systems that integrate RA input as part of their function include Traffic Alert and Collision Avoidance (TCAS), Terrain Awareness Warning Systems (TAWS-A and HTAWS), windshear detection, auto-flight systems, and more.
The FAA warned that 5G C-Band systems are being deployed throughout the contiguous U.S. and that all operators and pilots should assume that transmitters are anywhere in a flight path. Aircraft not equipped with a 5G C-Band-tolerant RA will be subject to harmful interference.
The FAA concludes SAFO 21007 by encouraging all operators to equip their aircraft with 5G C-Band-tolerant RAs and reminds the industry to remain aware of RA-reliant system degradation.
Higher speed technology operating in an increasingly crowded frequency spectrum brings EMC design into sharp relief. Bandpass filters and other technical fixes work after the fact, but managing the spectrum to reduce the initial risk of adjacent-signal interference is preferred.
Yogi’s observation holds true for radio-spectrum space: if it’s too crowded, nobody will go there. That’s not a solution, either for aviation or for the telecommunications industry.