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

Tue Jan 18 23:51:29 UTC 2022

On 01/18/2022 16:57, Mel Beckman wrote:
> 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.

Hmm, I'm seeing that "radar altimeter" and "radio altimeter" can indeed 
refer to the same class of instrument, so perhaps there's confusion 
(perhaps including by myself).

Nonetheless, while indeed existing users are granted some reprieve from 
interference by new users of other services, this is mostly in the 
planning stage of things and not the actual operations.  The time to get 
this addressed would have been back when this portion of the band was 
re-allocated to wireless systems (from space-to-ground satellite 
systems) several years ago.

Further, it seems that good engineering practice was not used in the 
design of these vulnerable systems and that they are subject to 
interference from broad-spectrum "jammers" (i.e. signals that, in terms 
of modulation and timing, don't necessarily correspond to what they're 
expecting to receive) transmitting well outside their allocated band (by 
separation comparable to the entire band in which they operate) let 
alone outside the expected, tuned frequency of signal reception.  All of 
these are typically very high on the list of consideration when 
designing an RF receiver and seem to have been either ignored entirely 
or at least discounted in the design of these instruments from what I'm 
hearing.

That is, I have yet to see any source (even from the aviation industry) 
claiming that there is in-band interference issues from the new wireless 
systems or that these radio altimeter systems somehow need such extreme 
receiver sensitivity that a several hundred-MHz guard band between 
services (with an existing service in between, albeit one with the 
transmitter usually in the other direction) is not sufficient to ensure 
proper receiver isolation from unwanted signals.
--
Brandon Martin