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

[Mel Beckman <mel () beckman org>]

Brandon,

Bo, it’s the radar altimeter, not the barometric altimeter. This is a radar distance measurement device for determine 
the precise height above the ground,  critical for low-visibility approaches. 

Where frequency interference is concerned, under FCC rules the existing users have priority, and are entitled to 
interference-free operation. 

-mel via cell

> On Jan 18, 2022, at 1:43 PM, Brandon Martin <lists.nanog () monmotha net> wrote:
>
> 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 techniques.
>
> 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