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

Michael Loftis mloftis at wgops.com
Wed Jan 19 01:08:12 UTC 2022

On Tue, Jan 18, 2022 at 17:49 Jay Hennigan <jay at west.net> wrote:

> On 1/18/22 15:51, Brandon Martin wrote:
> > 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.
> This simply doesn't make sense. Radar receivers are usually direct
> conversion driven from the same frequency source as the transmitter,
> meaning that they are going to have rather good selectivity with regard
> to frequency.
> Furthermore, a radio altimeter used for approach and landing is going to
> have a very short time window. I'm by no means familiar with the
> internal workings of these devices, their specifications, or their
> effective range, but if the altitude to be measured is 5000 feet or less
> the device will send a pulse and then open a receive window of no more
> than about 11 microseconds to look for its return. If you're only
> concerned about being 1000 feet or less above terrain, the window is
> about 2 microseconds. The pulses are presumably sent relatively
> frequently, probably several times a second, and the results averaged.
> In addition, the radar antenna beamwidth is going to be relatively small
> and pointed more or less straight down.

GPWS, and all rescue/medevac/etc helicopter operations also use the RA, and
this is NOT just in the landing/approach of a runway. Think about landing a
helicopter at night on the  freeway or a nearby field. TAWS uses GPS to
locate in space and I don’t know where it’s altitude source is - probably
the baro altimeter until the RA starts getting a return (or thinks it is)

> Intentional broadband jamming isn't going to be very effective against
> an airplane as the jammer would need to be directly beneath a fast
> moving target and get the timing exactly right with microsecond accuracy.
> Accidental interference from a source at least 220MHz out of band with a
> beam pointed at the horizon is even more far-fetched unless, as you say,
> the radar unit's receiver is complete garbage in which case how did it
> get a TSO in the first place? Avionics equipment that is critical to a
> precision approach isn't, or at least shouldn't be, crap.

They’ve never been required to have immunity. Last spec update was AFAIK
1980s. It’s definitely a stack of problems…part of which is the FCC
auctioning the Spectrum, it puts them in conflict as both the enforcement
and beneficiary. Billions of dollars being the CTIA on one hand. On the
other RTCA, AOPA, and some other small $ fish they stand nothing to gain

Remember that the RA is sub 1W looking for reflected emissions. It’s very
possible the ground equipment for a cell base station to have spurious
harmonics…where they land requires more RF engineering chops than I’ve got,
and would obviously be very system dependent. So yes in my understanding
due to the RF voodoo of how they transmit and receive, and the .. field of
view .. those factors mitigate interference for certain…but why did the FCC
auction that chunk? Why not say ok you’ve got two years to develop a
standard, update that 1980s requirement, and 5 or 10 to implement? Instead
we’re just barely four years on and going to be seeing potentially
interesting deployments.  Interference that only can happen and only
matters in critical flight phases….

> --
> Jay Hennigan - jay at west.net
> Network Engineering - CCIE #7880
> 503 897-8550 - WB6RDV

"Genius might be described as a supreme capacity for getting its possessors
into trouble of all kinds."
-- Samuel Butler
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