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.na...@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
