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

Jay Hennigan jay at west.net
Wed Jan 19 00:48:53 UTC 2022

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.

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.

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

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