On Friday, November 29, 2019 at 9:25:29 PM UTC-8, Charles Longley wrote:
There’s no requirement.

No requirement for a squat switch explicitly, but there is a (not very clear) requirement for some form of automatic air/ground switching for 2020 compliance and you should want to use something that uh actually works reliably. See my tenth post in this thread.

The issue is if you get down to a low ground speed in flight the transponder might go into the ground mode. I think it’s getting a little overblown on RAS. Trig recommends a squat switch to compensate for it. Some of the other transponder manufactures can compensate for it by programming. I haven’t put a squat switch in any of the power airplanes I’ve installed ADS-B transponders in.

Oh Jesus, yes but none of the aircraft you have installed ADS-B out in are gliders that fly in mountain wave. Nothing of what you have done in those power aircraft installs is relevant here.

The other transponder manufactures do not just "compensate for this by programming". I've got a pretty good understanding of GPS technology and I have no clue how this would be possible. You have no way of knowing that Garmin or other systems that use GPS detected ground speed to switch from air/ground will work in scenarios where those aircraft have near zero groundspeed without problems. A GPS in a glider or any other aircraft hovering in strong enough wave with a near zero ground speed just can't work out if it is on the ground or not. That you don't read about this in Garmin or other install documentation is likely only because those aircraft don't fly in strong enough wave/headwinds to have this problem.

Again we known for several years now this is a problem for gliders in mountain wave. Especially if your glider is going to fly in wave or strong headwinds please pay attention and get this right with a pneumatic pitot switch. Personally I'd install it that way in every glider, because you just don't know where it will end up flying.

Again if you are installing TABS just set the transponder squat switch to "None" and use manual ON/ALT switching on the transponder.

The very worse thing you can do in a glider that has any chance of being in mountain wave or a strong headwind is select GPS based air/ground switching.

If 91.227 did not imply automatic switching was required you would be better off choosing none and just relying on the ON/ALT switch on the transponder.