Second, a recollection: it's my recollection that ATC radar is
designed to filter out slow-moving targets.

That's primary radar, because of ground clutter problems. It would be a
most unusual situation to eliminate a transponder return. They certainly
don't do it out here in the Pacific Northwest - I've asked.


I've asked to, not quite as northwest as you are. The answer I got is
it depends on how the controller sets up there radar. At most Class C
airports there probably is no filter turned on. At larger airports
they might turn it on to eliminate the returns from aircraft taxing on
the ground. I was also told the can select the speed at which it will
filter the return but typically they might set it up for 25kts.
Typically they have the primary radar turned off unless we tell them
there are gliders or non-transponder equipped aircraft in the area.

Brian