What do you think about this airline vs 5G brouhaha?

Brandon Martin lists.nanog at monmotha.net
Tue Jan 18 21:41:06 UTC 2022

On 01/18/2022 15:29, Michael Thomas wrote:
> I really don't know anything about it. It seems really late to be having 
> this fight now, right?

The issue seems to be old aviation equipment that has poor receiver 
selectivity on its radio (not radar) altimeter.  This is, apparently, a 
secondary, but still very important, instrument for instrument 
approaches upon landing.

This older equipment can be subject to meaningful interference by 
signals as much as 500MHz outside the actual assigned radio altimeter 
band limit.  Note that the radio altimeter band is only about 500MHz 
wide itself, so even a naive single-conversion receiver could/should 
have better selectivity that this.  The reason for this poor selectivity 
seems to simply be that, at the time, there was nothing else using the 
RF spectrum nearby, so they could get away with it, and it made the 
receiver somewhat simpler.

The system apparently also responds poorly to both narrowband and 
wideband jammers i.e. it does not employ what we'd consider robust, 
modern error-correction or coding systems or even digital error checking 

Both of these are basically issues with how old the system is and how 
old a large amount of deployed equipment using it is.  The former is 
probably hard to fix in a backwards compatible way, but the latter is 
mostly a matter of upgrading your instruments more than once every 25 
years which, for planes that are actually routinely making use of this 
system (largely commercial and charter operators), doesn't really seem 
like that big of an ask.

I think the issue is that the FCC did some rulemaking assuming that 
existing service users were being reasonable with their equipment 
design, then a giant game of chicken got started, and nobody blinked in 
time for anything to get done until a collision was imminent.

The C-band spectrum at issue here has become very valuable, both 
economically and from a public usage perspective, for mid- and 
short-range wireless communications.  The FCC allocated some of it based 
on "reasonable" expectations of existing users and provided an ample 
(arguably rather large) guard band between services.

In the end, I'd say that aviation folks are in the wrong, here, but they 
also have a lot of history to contend with and a large install base of 
gear that, whether it "should" or not, apparently does need to be 
upgraded to prevent detrimental interference to an important flight 
safety and operations facility.  A pause in deployment seems reasonable 
in that light, though it would have been nice if folks could have gotten 
this resolved sooner.

Brandon Martin

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