On Dec 8, 4:18*pm, Dave Nadler wrote:
On Dec 9, 7:18*am, Darryl Ramm wrote:



On Dec 8, 12:03*pm, jcarlyle wrote:

For two reasons: *(1) it costs less to perform the biannual
inspection, and (2) you aren't broadcasting your tail number to the
Feds. Of course, you'll only get a 10 year life out of it...

-John

On Dec 8, 2:54 pm, Andy wrote:

Why would anyone buy a mode C transponder when for only a little more
one can buy a mode S unit?

Andy

Actually the 10 year life is not completely accurate, even if I agree
a bit with probably the sentiment behind it. I just don't want any
glider pilots with Mode C transponders to think their transponders
have an abslute hard limit where they stop being useful in 10 years.

Mode C transponders are usable it the USA well beyond 10 years - but
to meet the 2020 ADS-B data-out carriage requirement (a requirement
for power aircraft in similar airspace as transponders are required
now) you would need to add a UAT transmitter or replace the
transponder with a Mode S with 1090ES data-out. Right now its
impractical from a cost, STC paperwork hassle (on non-experimental)
and final -B rev compliance requirements on many products, to add ADS-
B data-out to most light aircraft or gliders but you can install a
Trig TT21 and in future update the firmware and add the ADS-B data-out
GPS.

How current aircraft owners manage the transition to the 2020 ADS-B
data-out mandate will be interesting to see - i.e. whether Mode C
equipped aircraft add UAT devices or swap out their transponders to
get new Mode S units with 1090ES data-out. I expect many will take the
transponder upgrade route since it is a chance to refresh older
transponders which they still need to carry (in may owner aircraft)
and older transponders can become a maintenance liability, so why not
refresh both in one box? For newer Mode C transponders there exists
more of an argument to add a UAT device. In the glider community in
the USA with PowerFLARM looking like important and popular technology
Mode S 1090ES data-out is a better technology to consider than UAT
data-out.

Darryl

Don't forget, mode S will use a lot less power
under non-trivial interrogation...
From soggy Oz,
]See ya, Dave

Dave is correct that Mode S transponders will consume less power in
some environments where they are being interrogated as a Mode S
transponder since they do not see the extra Mode A/C interrogations
(at least not those from a Mode S capable interrogator) and so need to
transmit fewer replies than a Mode C transponder. However that does
*not* get close to explaining the apparent high power consumption
(1.1 Amp!!) of this Mode C transponder vs. the Trig Mode S
transponder.

The exact details of power consumption in different situations for SSR
radar depends on details of the ATCRBS ground interrogator. The
transponder will still likely get hit by multiple interrogations per
sweep and the individual transmission from the Mode S transponder may
well take more power than that a Mode C transmission. The power
required for a Mode A/C transmission depends on the squawk code
(==pulse pattern) but the power required for the Mode S data-packet
does not change like this.

In complex TCAS I and II environments the Mode S transponder will see
fewer interrogations but even a Mode C transponder will only reply to
a small fraction of the realtively high interrogation rate of a TCAS
system since TCAS uses the "whisper-shout" trick to cause Mode C
transponders within a certain distance range to reply. Its all that
TCAS Mode C interrogation and replies from some of the Mode C
transponders that is a bandwidth hog (and why goign to Mode S only in
the USA might have been a good idea to support ADS-B 1090ES). Mode S
is much more elegant. TCAS discovers the Mode S transponder by its
acquisition squitter (advertising it is there without needing to be
interrogated) and the TCAS interrogator can interrogate Mode S
transponders uniquely without using the whisper-shout hack.

If being interrogated by a Mode C only interrogator (e.g. by GA
aircraftTCAD/TAS systems (which have traditionally not supported Mode
S interrogations) then the Mode S transponder will transmit Mode A/C
replies without any of the Mode S stuff.

Darryl