Morteza Ansari wrote:
I recently bought a new bird and need to put a transponder in it. The
choice was obvious (until I took a closer look). Trig TT21 seems like
a great choice: small footprint, built-in encoder, 1090ES ADS-B out,
and descent price. However a bit more looking revealed more questions
than answers. It seems it requires an ISO GPS and one could not use
typical GPS sources we have in gliders, it is not clear if the ADS-B
out work with PowerFLARM (at least from what I have heard), and the
biannual testing cost 2-3x as much as mode-C.

Any thoughts/suggestions from more knowledgable folks?


Cheers,
Morteza


The TT-21 as a Transponder utterly blows away any other transponder
available in the USA for use in gliders. it's power consumption, split
box install options, built in encoder, etc. are totally compelling by
themselves.

You should think of this as a Mode-S transponder today. That does not
use/need any GPS source. The 1090ES data-out capability is really there
for future use. And exactly what features/compliance you are aiming for
there is a longer discussion. You should think of ADS-B as a future
thing, not something here today. Many ADS-B devices require upgrades to
meet the 2020 mandate specs, and to meet this full spec (remember
gliders are exempt) you must use aviation (not consumer) style GPS
sources.

You cannot currently install any ADS-B data-out in any certified
aircraft including certified gliders without a STC approval, and since
the FAA is unlikely to grant an STC for an install with a non
Aviation/WAAS GPS that would today mean spending several $k to include
that GPS-and finding somebody to develop the STC. Those GPS prices will
fall in future and the STC requirement will go away. But if the FCC ever
allows a field approval install for a non-aviation GPS as a data-source
on a certified glider who knows (but don't hold your breath). This while
area is a mess at the moment and just not worth most glider pilots
losing sleep over.

Technically sophisticated folks who understand this stuff and who fly
experimental gliders today have other options including using
non-certified GPS sources. Setting this up is described in the Trig
manuals. If you are having to ask questions about this my inclination is
you should just install the TT-21 as a Mode-S transponder today.

I do not understand your specific concern about compatibility with
PowerFLARM. You need to clue us in to "what you have heard" if you want
cogent comment on that. PowerFLARM today will see/display other 1090ES
data-out equipped aircraft but will not provide an alarm for any threats
with those aircraft. That is coming in a future update. None of that is
specific to using a TT-21.

There are _very_ few 1090ES data-in equipped aircraft around right now
so this is not a big issue and other PowerFLARM equipped gliders will
see you via FLARM-FLARM anyhow. The basic Mode-S (or Mode-C) transponder
by contrast is seen by ATC, by TCAS which is widely used in airliners
and private jets, military transports, many tactical military aircraft
etc. and TCAD in some GA aircraft and by PCAS systems used widely in
lower end GA aircraft.

It is the 21st century buying a Mode C transponder makes _no_ sense. You
might be able to find a different test provider - not all uplift so
much for a Mode-S. There is no valid reason for such an inflated cost.
Some will do great deals to come to the gliderport/ramp and test a whole
fleet of gliders in one day.

Darryl