It seems like Boeing evaluated the altimeters they use and determined that with the filtering they have only a 100MHz guard band was needed.
Apparently though there exist radio altimeters with basically no filtering at all, which worked properly in practice, since sat downlinks had nowhere near the power level needed to interfere. But now we are adding more powerful transmission closer to the altimeter band than was used previously, and altimeters designed without using meaningful amounts of filtering would be affected.
That claim that 400MHz of guard band (as this initial rollout still provides) might be insufficient for some altimeters is really something. Ideally the FAA can determine which ones they are, and deem them no longer airworthy.
I'm also wondering if in fact it is just that the standards for altimeters are far looser than what all altimeters actually implement. The FAA concerns seem to be based on the signals potentially interfering with an altimeter built to just barely meet the testing specifications, but they might all actually greatly outperform those specifications.
Apparently though there exist radio altimeters with basically no filtering at all, which worked properly in practice, since sat downlinks had nowhere near the power level needed to interfere. But now we are adding more powerful transmission closer to the altimeter band than was used previously, and altimeters designed without using meaningful amounts of filtering would be affected.
That claim that 400MHz of guard band (as this initial rollout still provides) might be insufficient for some altimeters is really something. Ideally the FAA can determine which ones they are, and deem them no longer airworthy.
I'm also wondering if in fact it is just that the standards for altimeters are far looser than what all altimeters actually implement. The FAA concerns seem to be based on the signals potentially interfering with an altimeter built to just barely meet the testing specifications, but they might all actually greatly outperform those specifications.