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Why 5G is upsetting airlines

19 January 2022

We were told that 5G would revolutionise the communications game.

Greater bandwidth, lower costs, ubiquitous signal -- these would be the hallmarks of a move to 5G technology. Just as 2G was gazzumped by 3G which later was left in the dust by 4G, 5G promises to deliver the future to our phones, computers and other devices.

However, the airline industry is currently more than a little unhappy with what 5G will deliver.

They're claiming that it represents a safety hazard and in a growing number of places around the world, the roll-out of 5G tech around airports has been stalled or delayed while a key problem is addressed.

The problem revolves around a rather old and simple technology being used by commercial aircraft to help them land in low (or zero) visibility conditions.

The issue is the way the radio altimeters work in many of these aircraft.

Such devices were originally designed and built at a time when the C-band of the RF spectrum was relatively unused. In fact, most C-band activity was satellite-based which meant that signal levels down near the ground were very low and actually required a high-gain dish antenna to be heard at all.

Having the luxury of a "clean" band to work in, the designers of radio altimeters designed their systems accordingly -- not worrying too much (if at all) about the potential for strong signals to exist on nearby frequencies.

In fact these things are so simple in concept that there are even DIY projects online that enable anyone to build their own radio altimeter.

The problem with the rather basic designs currently in use is that they're the equivalent of a crystal set -- and I'm sure at least some of those reading this column will recall what a crystal set is.

If you ever spent any time playing with a crystal set you'll remember that they worked amazingly well when picking up strong signals that were widely spaced but were very poor at separating more closely spaced radio stations. More often than not, if there were two strong signals close on the dial then you'd end up hearing both at the same time, no matter how hard you tried to tune to just one of them.

Well this is the basic problem that these radar altimeters in aircraft are suffering from. They just listen to too much of the radio spectrum. They're not selective enough to distinguish between the intended signal and a legally licensed 5G signal a few hundred megahertz away. This has worked fine to date because prior to the licensing of the spectrum to 5G operators, there were no strong signals near to the altimeter's operating frequency.

Now however, things have changed markedly and almost any terrestrial signal on the licensed 5G band is going to be enough to potentially disrupt the readings of these radio-altimeters.

Whose fault is this?

Well it's not the 5G operators' fault. They're operating in accordance with the license granted to them by the spectrum management authority. The power levels and frequencies are quite legal and technically well outside the allocation given to radio altimeter use.

It's not the spectrum management authoritie's fault either. They've allocated quite distinctly separate bits of spectrum to each of these uses.

So I guess that leaves the manufacturers and certifiers of the radio altimeters to blame.

Technically speaking, the radio altimeters are not violating the license given them by the spectrum management authority because their own broadcasts remain well inside the allocated frequencies. Likewise the 5G operators are only broadcasting on those frequencies they're authorised to use.

However, the radio altimeters are effectively *listening* to frequencies that 5G is using and that's where the problem arises.

"Hey, we can save a few bucks by not using any significant filtering on the input to our radio altimeters" may have been the conversation back in the day when these devices were first designed. "There's nothing nearby to filter out so why waste money?".

Of course this leaves authorities with a huge problem right now.

5G operators have paid for spectrum and they need a return on that investment so it's likely unreasonable to demand that they don't use something they're paying for (those C-band frequencies).

However, aviation safety is a *huge* thing and there's no way we can allow people's lives to be put at risk by flying aircraft that can no longer rely on a key piece of navigational equipment. After the losses inflicted by CV19 it seems to be a very heavy penalty to effectively ground large numbers of craft at a time when things were just starting to come right.

So ultimately, shouldn't the radio altimeter companies carry the can for this fiasco... given that it's their gear which has caused this fiasco?

The answer is probably "yes" -- but there's no way they can afford to cover the losses that either the 5G carriers or the airlines would be experiencing. Odds are they'd simply go bust and then you have no support for this gear and no chance of it being put right.

In a perfect world, the aviation regulators would withdraw the certification for any radio altimeter that was affected by legal 5G broadcasts -- but that's not going to happen either because $$$$. Remember - profits first, safety second.

The reality is that radio altimeters are not complex bits of kit. The best resolution would be for manufacturers to produce a unit that does have decent input filtering and retrofit all those airliners using older gear ASAP. Where the cost for that is to be placed is something that should be worked out between all the parties.

Is that how this situation will play out?

Given the ridiculous levels of mismanagement seen to date (ie: everyone involved has known about this looming problem for a very long time and done nothing about it), I doubt it.

The more I see of the way governments regulate stuff, the more I worry about the competence of such regulatory bodies.

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