5 ms·

For most here, this will be familiar, but what this is affecting is the radar altimeter. You've probably seen the videos where the system will do an audio coun

For most here, this will be familiar, but what this is affecting is the radar altimeter. You've probably seen the videos where the system will do an audio countdown "100, 50, 40, 30, 20, 10, retard, retard" [Airbus]. That system is critically important in zero-vis triple-autopilot landings. You don't want cell phones screwing that phase up. https://youtu.be/UV_vWtAJIow?t=142 https://youtu.be/UV_vWtAJIow?t=142

onphonenow 5y ago
You don't want to be using a radar altimeter that can't handle a 200 Mhz guard band at 20 feet. That is seriously messed up on.the plane MFG front, not cell phone front.
- BenjiWiebe 5y ago
  This is what I can't figure out. 200Mhz is a large guard band, and it isn't hard to design filters for that at all. Now, single digit khz filters at RF frequencies, that's difficult. But not 200Mhz! I guess the radio altimeter manufacturers just figured, "there's a large guard band, why play it safe and put in filters", or what?
  - meepmorp 5y ago
    The odd part is that Boeing originally proposed a 100MHz guard band. It's literally twice the headroom a major manufacturer suggested for this purpose.
    - onphonenow 5y ago
      Even weirder, the critical phase of all this is generally during airport approach and landing so at pretty low altitudes. In terms of reception if you can't design something to tell you if you are 100 / 50 / 20 feet off the ground with a 200Mhz guard band ON BOTH SIDES that is insane (600Mhz total bandwidth to tell me if I'm 20 feet off the ground??). If you are ham these bandwidth needs are just nuts. And this has been 10 years in the making. The FCC must just be rolling their eyes - why didn't FAA do something earlier. We have coverage during landing phase with a localizer and glideslope as well, plus RNAV / WAAS enhanced GPS options etc etc + the mark 1 eyeball. Somehow in the last year the US has just become helpless to solve problems. Hard to beleive folks flew across the ocean with no GPS or moving map etc etc.

onphonenow 5y ago

You don't want to be using a radar altimeter that can't handle a 200 Mhz guard band at 20 feet. That is seriously messed up on.the plane MFG front, not cell phone front.

BenjiWiebe 5y ago
This is what I can't figure out. 200Mhz is a large guard band, and it isn't hard to design filters for that at all. Now, single digit khz filters at RF frequencies, that's difficult. But not 200Mhz! I guess the radio altimeter manufacturers just figured, "there's a large guard band, why play it safe and put in filters", or what?
- meepmorp 5y ago
  The odd part is that Boeing originally proposed a 100MHz guard band. It's literally twice the headroom a major manufacturer suggested for this purpose.
  - onphonenow 5y ago
    Even weirder, the critical phase of all this is generally during airport approach and landing so at pretty low altitudes. In terms of reception if you can't design something to tell you if you are 100 / 50 / 20 feet off the ground with a 200Mhz guard band ON BOTH SIDES that is insane (600Mhz total bandwidth to tell me if I'm 20 feet off the ground??). If you are ham these bandwidth needs are just nuts. And this has been 10 years in the making. The FCC must just be rolling their eyes - why didn't FAA do something earlier. We have coverage during landing phase with a localizer and glideslope as well, plus RNAV / WAAS enhanced GPS options etc etc + the mark 1 eyeball. Somehow in the last year the US has just become helpless to solve problems. Hard to beleive folks flew across the ocean with no GPS or moving map etc etc.