son_of_flubber wrote on 1/20/2016 7:14 AM:
I've a Trig T21. An ATC controller told me that I might be
filtered/deleted from his display when circling or flying in wave due
to my low ground speed.

I know primary radar signals might be filtered based on speed to remove
ground clutter, but never heard that would be done to a transponder
equipped aircraft. Gliders in wave aren't the only thing that can move
slowly - consider helicopters and balloons.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf