Transponder: Mode-C or Mode-S?

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

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

Darryl Ramm wrote:
The TT-21 as a Transponder utterly blows away any other transponder
available in the USA for use in gliders.

Just be aware that the TT21 is a class 2 tansponder, i.e. not certified
for use above 15,000 feet, and therefore not suited for gliders. Buy the
TT22 instead which is class 1.

Using the same logic, don't buy anything other than V rated tires for
your car, as the common S rating is only good for 112 mph (180 kph).

-John

On Sep 12, 7:55 am, John Smith wrote:
Just be aware that the TT21 is a class 2 tansponder, i.e. not certified
for use above 15,000 feet, and therefore not suited for gliders. Buy the
TT22 instead which is class 1.

Am 12.09.11 14:57, schrieb jcarlyle:
Using the same logic, don't buy anything other than V rated tires for
your car, as the common S rating is only good for 112 mph (180 kph).

If I wanted to drive with 112 mph (180 kph), then you'd be perfectly
correct. Glider pilots on the other hand tend to want to climb as high
as they can. In fact, the possibility to get a clearance to climb has
been the main reason for me to buy a transponder. So it would have been
pretty pointless to buy a class 2 transponder.

As always YMMV, but at least one should be aware of this limitation
before spending money on a class 2 transponder.

On Sep 12, 7:07*am, Stefan wrote:
Am 12.09.11 14:57, schrieb jcarlyle:

Using the same logic, don't buy anything other than V rated tires for
your car, as the common S rating is only good for 112 mph (180 kph).

If I wanted to drive with 112 mph (180 kph), then you'd be perfectly
correct. Glider pilots on the other hand tend to want to climb as high
as they can. In fact, the possibility to get a clearance to climb has
been the main reason for me to buy a transponder. So it would have been
pretty pointless to buy a class 2 transponder.

As always YMMV, but at least one should be aware of this limitation
before spending money on a class 2 transponder.

Except for the fact that the folks on the ground that grant the
clearance don't know or care whether you've a Class 1 or Class 2
transmitter as long as they are receiving your signal. They also
don't care about you when you are more than 40 miles out, and if 135
watts won't get you 40 miles, something's wrong with your
installation. A properly installed* Class 2 with a dipole antenna in
a fiberglass (not carbon) fuselage glider will get more radiated power
out than most GA with metal fuselages and the 1/4 wave monopole
antenna they are forced to use. If you've got a carbon fuselage,
you're stuck with the 1/4 wave.
* a major contributor to lost outpout power is the cable from the
transceiver to the antenna. Don't use RG-58. It's unsuitable for L-
band.

John Smith wrote:
Darryl Ramm wrote:
The TT-21 as a Transponder utterly blows away any other transponder
available in the USA for use in gliders.

Just be aware that the TT21 is a class 2 tansponder, i.e. not
certified for use above 15,000 feet, and therefore not suited for
gliders. Buy the TT22 instead which is class 1.


This has been discussed here a lot before. the power difference will
make no practical difference. By all means pay slightly more and install
the TT22 but the important thing is for folks that fly in busy airspace
near airliners etc. to have a transponder (and even an older Mode C
transponder works fine fir that). Most transponders installed in gliders
in the USA appear to be class 2.

Darryl

On Sep 12, 6:57*am, Darryl Ramm wrote:
John Smith wrote:
Darryl Ramm wrote:
The TT-21 as a Transponder utterly blows away any other transponder
available in the USA for use in gliders.

Just be aware that the TT21 is a class 2 tansponder, i.e. not
certified for use above 15,000 feet, and therefore not suited for
gliders. Buy the TT22 instead which is class 1.

This has been discussed here a lot before. the *power difference will
make no practical difference. By all means pay slightly more and install
the TT22 but the important thing is for folks that fly in busy airspace
near airliners etc. to have a transponder (and even an older Mode C
transponder works fine fir that). Most transponders installed in gliders
in the USA appear to be class 2.

Darryl

The TT22 draws about 20 percent more power when transmitting than the
TT21. It's not a lot, but with PowerFlarm going in to my panel as well
I will end up with a 1.5 amp total current requirement, so I figure
every milliamp is worth saving. It seems odd to me that Trig didn't
design with the Class A floor as the break point between design
specs. Then again...

9B

When the panel became a power hog, Rex Mayes installed a 2-panel
Strobl Solar system.
On blue days it produces almost half the required current, a decent
battery extender.
But not very efficient under clouds.
Jim

On Sep 13, 1:24*am, Andy wrote:

The TT22 draws about 20 percent more power when transmitting than the
TT21. It's not a lot, but with PowerFlarm going in to my panel as well
I will end up with a 1.5 amp total current requirement, so I figure
every milliamp is worth saving. It seems odd to me that Trig didn't
design with the Class A floor as the break point between design
specs. *Then again...

9B

Andy wrote:

The TT22 draws about 20 percent more power when transmitting than the
TT21. It's not a lot, but with PowerFlarm going in to my panel as well
I will end up with a 1.5 amp total current requirement, so I figure
every milliamp is worth saving. It seems odd to me that Trig didn't
design with the Class A floor as the break point between design
specs. Then again...

9B


The 15,000 feet "limit" comes from wording in the relevant FAA TSO and
ultimately the RTCA standards. That the level was not set to FL180 is
kind of unfortunate. And I expect Trig folks would say the same thing.
But then this all goes back to days of traveling wave tubes and much
different technology and reliability/power output/cooling issues/cost
factors.

Darryl