4 ms·

I was also curious about this, so I looked at the Airworthiness Directive[1] referenced in the example NOTAM. Basically, the argument is that the radio altimet

I was also curious about this, so I looked at the Airworthiness Directive[1] referenced in the example NOTAM. Basically, the argument is that the radio altimeter receivers "must detect faint signals reflected off the ground to measure altitude" and [C-band 5G signals] "could significantly degrade RA functions [...] if the altimeter is unable to sufficiently reject those signals". So it seems it is basically a question of how wide is the RF input filter bandwidth of RAs on commercial aircraft. [1] https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/625b2b44ac41e39a862587a600628feb/$FILE/2021-23-12.pdf https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...

p_l 5y ago
I'm not good with analog circuits, but someone mentioned in previous thread that designing a 200 MHz wide filter that would have "brick wall" response to frequencies outside it is non-trivial, and technically the 200MHz is just the transmit range, not receive range...
- aDfbrtVt 5y ago
  That's why virtually every RF frontend oversamples sufficiently to do the sharp filtering in DSP.
  - pgorczak 5y ago
    Although when receiving a weak signal with strong out of band interference, you may still need a lot of dynamic range to successfully filter it out digitally.
    - VLM 5y ago
      3rd order IMD performance can become an issue. 3rd ord IMD for non-EE could be described as low power mixers like mixing low power signals and usually the low power signal comes from your intentionally designed radio oscillator, but, whack it with enough incoming RF power and it will go non-linear as its designed to do and start mixing with that incoming signal, mostly making massive levels of interference. You can always improve 3rd order IMD by simply running the oscillator and mixer at higher power levels (which all turns to heat), usually not the best strategy for battery powered gear.
- guerby 5y ago
  This is all ridiculous, there's still a 200 MHz band guard between the FAA band and the 5G proposed band. Here is what a $1 ESP wifi dongle has to follow: https://en.wikipedia.org/wiki/IEEE_802.11 https://en.wikipedia.org/wiki/IEEE_802.11 "The mask requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the center frequency" So 2 dB/MHz filter. I let you do the math. FAA is just ridiculous here if they let old junk radio hardware handle safety landings for airplanes, but well after 737 max what do you expect...
  - redis_mlc 5y ago
  - p_l 5y ago
    The super wide bandwidth is on purpose - not accidental. It's also not "old junk", it was serving perfectly fine till some people decided they wanted to change the rules, and even then, work on avoiding interference been ongoing for decades due to different sources of interference. n.b. Wifi (and BT and others) got to use 2.4GHz band because of interference from aircraft kitchens causing a push to make 2.4GHz into junk spectrum with less regulation.

p_l 5y ago

I'm not good with analog circuits, but someone mentioned in previous thread that designing a 200 MHz wide filter that would have "brick wall" response to frequencies outside it is non-trivial, and technically the 200MHz is just the transmit range, not receive range...

aDfbrtVt 5y ago
That's why virtually every RF frontend oversamples sufficiently to do the sharp filtering in DSP.
- pgorczak 5y ago
  Although when receiving a weak signal with strong out of band interference, you may still need a lot of dynamic range to successfully filter it out digitally.
  - VLM 5y ago
    3rd order IMD performance can become an issue. 3rd ord IMD for non-EE could be described as low power mixers like mixing low power signals and usually the low power signal comes from your intentionally designed radio oscillator, but, whack it with enough incoming RF power and it will go non-linear as its designed to do and start mixing with that incoming signal, mostly making massive levels of interference. You can always improve 3rd order IMD by simply running the oscillator and mixer at higher power levels (which all turns to heat), usually not the best strategy for battery powered gear.
guerby 5y ago
This is all ridiculous, there's still a 200 MHz band guard between the FAA band and the 5G proposed band. Here is what a $1 ESP wifi dongle has to follow: https://en.wikipedia.org/wiki/IEEE_802.11 https://en.wikipedia.org/wiki/IEEE_802.11 "The mask requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the center frequency" So 2 dB/MHz filter. I let you do the math. FAA is just ridiculous here if they let old junk radio hardware handle safety landings for airplanes, but well after 737 max what do you expect...
- redis_mlc 5y ago
- p_l 5y ago
  The super wide bandwidth is on purpose - not accidental. It's also not "old junk", it was serving perfectly fine till some people decided they wanted to change the rules, and even then, work on avoiding interference been ongoing for decades due to different sources of interference. n.b. Wifi (and BT and others) got to use 2.4GHz band because of interference from aircraft kitchens causing a push to make 2.4GHz into junk spectrum with less regulation.