Re: This idea resurfaces...
> What if it's the tube that's iced and therefore unable to reflect cleanly?
I've flown on many research flights and seen icing on probes. Ice builds up on the leading edge of the probe, but not on the sides - I assume that any ice/water besides the probe is swept away in the airflow. The end result is a stick of ice at the front of the probe no wider than the probe itself. They can grow quite long.
> what if the aircraft is flying through a cloud that introduces ice crystals or other particulate matter into the beam's path?
If this is truly a Doppler-based system then the sideways motion of any interrupting particles shouldn't affect it - Doppler detection measures the along-beam motion. If there are enough particles to actually block the line-of-sight then you've flown into an iceberg.