On Wednesday, January 20, 2016 at 7:14:24 AM UTC-8, son_of_flubber wrote:
On Wednesday, January 20, 2016 at 2:09:34 AM UTC-5, WaltWX wrote:
I had recently equipped over the previous winter with a Mode S transponder (Trig T22), I was curious whether: 1) It was working... 2)Did the FAA use my target to call traffic.

Turned out the answer was yes! to both questions.

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.

Does your display on the radar have anything to do with the low traffic where you were flying or ATC knowledge of glider traffic in that area?

More generally, I'd like to have a better idea of when a Mode-S equipped glider is visible to ATC and when it is not visible.

The controller was misunderstanding that you have a transponder, or badly misunderstanding how radar works. The 'velocity-doppler filter" is very important and is on primary radar not SSR/transponder returns.

Transponders are the backbone of the ATC surveillance systems, aircraft with them just don't disappear because they slow down.

For gliders without transponders you damn well bet you can disappear from primary radar, especially if the radar needs the velocity-doppler filter turned up to reduce noise, that might be to help filter ou general ground scatter, cars on freeways/highways, wind generator farms, etc. Talking to your local ATC radar facility and seeing what they can and can't see on primary radar is usually interesting, and frequently an eye opener for folks in busy areas to get a damn transponder.

A Mode S or C transponder is "visible" to ATC when you are flying within SSR coverage and the transponder is turned on and correctly working.