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