Let's hope we never find out.
Last week, a number of U. S. air carriers filed an emergency request with the FCC to delay the turn-on of 5G mobile-phone towers near major airports. AT&T and Verizon were planning to turn on the towers as part of their 5G rollout, which uses a part of the electromagnetic spectrum called C-band to improve transmission rates while maintaining good geographic coverage.
The reason the airlines were worried is that for decades, devices called radar altimeters have provided pilots with absolute altitude readings as they land. Altimeters are used below about 2500 feet (800 meters) altitude in both fair and inclement weather as a landing aid. In recent years, they have been incorporated into autopilot and terrain-awareness warning systems, and a malfunction of the radar altimeter could cause false warnings or more serious problems such as a crash.
The altimeters currently in use employ the 4.2-4.4 GHz range of frequencies with a technology called FMCW, standing for Frequency Modulation Continuous Wave. As the transmitter's frequency slides up and down linearly, the time it takes the signal to go from the plane, bounce back, and return to the altimeter is translated into a low-frequency tone which is proportional to the plane's absolute height. It's a simple, reliable system, which is why it is used for many safety-related purposes on modern aircraft.
Last year, AT&T and Verizon paid $80 billion for rights to a chunk of spectrum just below the radar altimeter band, from 3.7 to 3.98 GHz. Their new 5G systems use this band instead of an older lower-frequency band for improved data rates and nearly the same geographic coverage ability. Other parts of the world have made this switch already, but with certain restrictions around airports for the same reason that has upset U. S. airlines: the potential that mobile-phone tower transmissions will cause problems to radar altimeters.
Although the two frequency bands are separated by what is known as a "guard band" (a kind of frequency no-man's-land intended to provide adequate separation between allocations), whether a given altimeter will be messed up by a given mobile-phone tower is not that easy to figure out.
Radio devices of all kinds discriminate against interference with a combination of digital filtering (in software, essentially) and analog filtering, which consists of pieces of hardware called band-pass filters. Unfortunately for the software people, a strong enough interfering signal outside of a device's intended band can overwhelm the "front end" (the part that turns an analog radio signal into a form usable by digital systems) and disable it. To prevent this, the system needs to have better analog filtering, which consists of mainly passive frequency-selective circuits that cost money and space.
Before 5G came along, I'm sure the radar-altimeter people put just enough filtering into their systems to prevent the worst existing adjacent-channel offenders from messing up their radars. That might have been good enough a few years ago, but now that the phone companies are putting transmitters in the 3.7 to 3.98-GHz band, it may or may not be good enough.
These kinds of issues are typically straightened out with the assistance of field tests, but it appears that the Federal Aviation Administration (FAA) has not surveyed radar altimeters with a view toward their interference-rejecting ability until the last few weeks, according to one report.
As things stand, AT&T and Verizon have agreed to delay for another six months, but not cancel, their plans to roll out 5G near airports. It looks like a big game of chicken has been going on between the wireless carriers and the Federal Communications Commission (FCC) on one side, and the airlines and the FAA on the other side, with each side pulling rank and blaming the other for causing the problem. The only thing that seems to have stopped the rollout this time was intervention by the White House, which extracted a promise from the phone companies that the rollout will be delayed another six months. But AT&T and Verizon say that's the last delay they're going to put up with, as we're falling further behind other countries already with our 5G rollout.
It's a sad thing when you see not only businesses, but federal agencies, duking it out in public and making grandstanding plays to force resolution over what should be a strictly technical issue. But that's an engineering-nerd point of view, the viewpoint that ignores money and politics and sees only the technical aspects of the problem.
Compared to places like France, the FCC allocation allows higher power and closer frequency spacing to the altimeter band, and so the airlines may have a valid concern that turning on the 5G towers could mess up some altimeters. Because the altimeters are so critical to aircraft safety systems, that is a problem that you want to stay away from by a wide margin.
Evidently, more recently designed altimeters use improved band-pass filters that are more likely to reject 5G interference, but the airlines were unwilling to retrofit their planes with all-new altimeters. And the FAA seems to be complicit in this, dragging its feet about even doing tests or inspections until recently.
The airlines and the 5G system operators now have six more months in which to work out a solution that is both technically sound, keeping us far away from having radar altimeters jammed by Aunt Suzy's phone call (or movie, more likely), and financially agreeable. AT&T and Verizon are the big spenders in this picture, laying out billions for the spectrum and more billions for their upgraded systems, which they now want to start profiting from, understandably. On the other hand, the airlines have not been having a good couple of years lately, in case you hadn't noticed, and I suppose they thought they could avoid yet another expense by jawboning and threats to shut down the entire air-transport system. It worked once, but it's unlikely to work again.
I'm glad to be writing about a bureaucratic tempest in a teapot rather than about a clear-air crash that is discovered to be caused by a 5G tower interfering with a radar altimeter. But this whole thing could have been handled better, and now the parties involved have six months to do it right.
Sources: I referred to an article carried by the Austin American-Statesman on Jan. 19 from AP by David Koenig entitled "AT&T, Verizon to delay new 5G" and a piece by Jake Hertz on the allaboutcircuits.com site at https://www.allaboutcircuits.com/news/how-did-the-5g-c-band-threaten-to-ground-thousands-of-flights/. I also referred to the Wikipedia article on radar altimeters.
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