On 1/20/22 13:41, Brandon Martin wrote:
From the sound of it, at least some of these altimeters were designed
around the (probably poor) assumption that there would be essentially no
RF power within half a GHz of them, and that assumption is no longer
going to be true. Was that a good design decision? Probably not, but
we need to figure out what to do about it. This is more of an FAA
problem than an FCC problem since it involves functional device
performance rather than emissions.
Indeed, it sounds like that is the case, and that's a horrible
assumption. When the spectrum was originally being allocated, if the
devices need 1200 MHz of interference-free bandwidth to function they
should have requested 1200 MHz of spectrum.
The FCC can (and should) attempt to balance the needs of existing users,
including practical performance of their equipment as deployed, with the
public good in terms of what has become spectrum that is very valuable
FCC indeed does take into account practical performance of equipment in
licensing. Early TV sets weren't very selective, so you wouldn't see
adjacent VHF channels licensed in the same market. FM broadcast licenses
need to protect existing stations up to three channels away still,
despite substantial improvements in FM receivers since the 1950s.
It sounds to me like FAA and the radar designers took a gamble by either:
A: Being capable of designing a radar that could reject out-of-band
interference but choosing to cut costs and risk safety by using a poorer
design with less selectivity.
B: Realizing that the state of the art at the time required a +/- 500
MHZ guard band but not applying for enough spectrum, ignoring the safety
concerns.
FAA puts all kinds of restrictions on what equipment is required to
perform certain maneuvers. You need a localizer, glideslope, etc. for
instrument landings. Radars are made today that can reject out-of-band
interference. If FAA simply required a certified radar that filtered
out-of-band signals during those weather conditions, the airlines would
retrofit and private pilots would also either retrofit, not fly in those
conditions, or divert to land in better weather.
It's not an FCC issue, and FAA needs to require equipment capable of
safely operating within the allocated spectrum.
--
Jay Hennigan - j...@west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV
