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
