Transponder: Mode-C or Mode-S?

On 9/13/11 9:27 AM, kd6veb wrote:
Hi Gang
I had a Trig mode S transponder installed in my new Phoenix motor
glider. It appears to work fine as a mode C transponder. After
questioning NorCal while soaring they confirmed they could detect it
with their sqwark code, it would IDENT and so on - all the mode C
stuff. However most of the US including northern California/Nevada is
not yet set up for mode S so except for the future there is no reason
to have mode S in the US. Of course there are other reasons to have
mode S, probably the most important would be if you wanted to sell a
glider in Europe where mode S is becoming mandatory. For that market
having a mode S transponder will save the buyer about $2,500 - the
cost of replacing a mode C with a mode S transponder.
Dave

I think you may be confusing Mode-S and 1090ES data-out, or certainly
risk others confusing that.

The USA is well equipped with SSR Mode-S interrogators, all these
systems are also required to interrogate legacy Mode C transponders and
any Mode S transponder is also required to behave as a Mode-C
transponder if interrogated by a Mode-C only interrogator (Mode-S
interrogators have a way of locking out all Mode-S transponders from
seeing these legacy Mode-C interrogations).

In the USA when ATC sees your transponder return/squawk
code/altitude/ident from a Trig or other Mode-S transponder they are
likely seeing all that over Mode-S not Mode-A/C.

Mode-S transponders do have some benefits over Mode-C including a unique
ICAO ID (some folks may not think that is a benefit), better altitude
reporting (depends on the transponder), do not suffer from possible
congestion/correlation problems, optional Mode-S TIS traffic uplink (not
to be confused with TIS-B) at some USA sites (the Trig transponders do
support TIS), have ground/squat status switching, etc.. Those extra
things do *not* mean that a Mode-C transponder is not a great tool for
use near high traffic areas or that glider owners should upgrade from
Mode-C to Mode-S just to get these Mode-S improvements, but OTOH buying
a new Mode-C transponder nowadays makes no sense.

Mode-S is one thing (well actually many as its fairly complex overall
standard) and the ability to do 1090ES data-out is an option on top of
the data transmitter/extended squitter capability defined in the Mode-S
specs. Some older Mode-S transponders cannot do 1090ES data-out at all.
The ground infrastructure, products, regulations/interpretations to
support ADS-B/1090ES data-out is in it's early days as I tried to
explain earlier in this thread.

Darryl