On May 28, 9:12*am, jsbrake wrote:
Aside from asking, "Who really wants to soar in Class A and B
airspace?" (wave windows aside), I would be interested in knowing if
this means that transponders are being phased out and being replaced
by ADS-B? *So... anyone with a transponder is about to own a $2000
doorstop?

Sorry for the long-winded reply, but I'll try to cover a lot of
relevant stuff here.

No. Transponders are not replaced by ADS-B. It would be great to
reduce the box cound and cost in many cockpits but unfortunately
that's just not how things are going to work out.

First remember this ADS-B data-out mandate does not apply to gliders,
and is something that kicks in for other aircraft in 2020 so there is
lots of time for new devices to come out and prices to reduce. For now
we are in an early adopter market, with corresponding higher prices
and more complexity/confusion about things for a while.

Transponders provide a backup for the ADS-B based surveillance, and
with any fairly radical introduction of new technology having a back-
up is a nice thing. And today the ultimate anti-collision safety
mechanism is TCAS, often TCAS-II as carried effectively by all large
airliners and many fast jets, military transports etc. TCAS systems
rely on interrogating a transponder, TCAS II is the only system that
can issue a RA (resolution advisory) that a pilot is required to
follow (overriding ATC instructions) to avoid a collision. ADS-B does
not replace TCAS II and does not provide any RA mechanism.

And what's the chance of ATC and/or pilots making a mistake and having
a situation where TCAS II could save an airline full of passengers
from a mid air?... Allegheny 853, Pacific Southwest 182 and Aeroméxico
498 were all fatal USA airline mid-air collisions with light aircraft
that TCAS II and transponders, had those aircraft been equipped,
should have provided critical RA avoidance instructions. The
Aeromexico 498 mid-air was the trigger that cased the NTSB to
basically force the Mode C mandate and TCAS II adoption. Closer to our
community, had the ASG-29 near Minden had its Mode C transponder
turned on the TCAS II in the Hawker 800 would have commanded an RA
that should have avoided the mid-air collision. Had the Glider had its
transponder on or had ADS-B in a working ADS-B environments (Reno
still ADS-B coverage is coming) then hopefully the controllers would
have provided traffic separation, however having the ultimate fallback
of TCAS II is a good thing. Over time the airliners and fast jets will
add ADS-B CDTI traffic displays, and CDTI adds benefits of longer
range situation awareness to TCAS, but again without TCAS II is not
capable of taking that last safety step of issuing an RA.

For gliders flying near airline, fast jet and military traffic it is
important to continue to use transponders and plan on continue to use
them along with ADS-B in future. If you are not flying near those
areas then I don't see any reason for transponder usage in gliders and
although maybe a confusing mess at the moment, ADS-B will offer some
interesting benefits over time for the gliding community. ADS-B has
lots of potential benefits, including better ATC coverage outside of
SSR coverage, in-cockpit traffic awareness (including in gliders), FIS-
B weather data (for UAT receivers), high precisions tracking for SAR
and hopefully eventually for XC soaring and contests etc. And gliders
are not even covered by the current mandate -- so it is dissapointing
to see the alarmist misinformation in the Tom Knauff's newsletter.

For gliders in those high density airline, fast jet and military
traffic areas who also want to do ADS-B data-out they keep their
current Mode C transponder will be able to add a UAT device or look at
a Mode S transponder like the Trig TT21 that does ADS-B data-out over
1090ES.

----

For early adopter folks --- it is important to understand that ADS-B
receivers whether UAT or 1090ES (like the interesting recently
announced PowerFLARM) require a local ADS-B data-out transmitter in
the aircraft to work properly. You cannot receive reliable traffic
data just by having an ADS-B receiver. The way ADS-B works in the USA
with the dual-link layers is that aircraft transmit it ADS-B position
on either UAT or 1090ES and the ground based transceivers (GBT)
retransmit that on the other link layer -- but those GBT only do so if
they know the other traffic is within a "threat cylinder" (my name --
its' actually a pretty large cylinder) around your aircraft and that
your aircraft is equipped to receive the ADS-B transmission. The GBT
also retransmit SSR derived radar data -- that's called TIS-B. So if
you are flying around with say a UAT receive only device (there are
several on the market already) but no ADS-B transmitter you will see
other local UAT devices transmitting but you won't say see the
airliners transmitting their ADS-B position on 1090ES because the GBT
do not know you are there. And using a 1090ES receiver without an ADS-
B transmitter is equally as bad. Not only must the GBT know where you
are but it must know that you are equipped to receive either or both
of UAT or 1090ES data-in. That requires your ADS-B transmitter "UAT"
or "1090ES" "capability code" bits to be set correctly.

Not all currently available ADS-B transmitters support setting these
"capability code" bits properly and some higher-end systems may not
allow a user to configure these properties to support portable
receiver devices (several vendors are working to add these features to
comply with the latest standards) -- and a 1090ES or UAT device should
be able to set either bit in it's transmission, You can send data-
outon one link layer and receive data-in for traffic information on
another etc. So bottom line, there is absolutely no way that a receive
only ADS-B traffic device will work properly if you do not have -

1. A local ADS-B data-out transmitter (Mode S 1090ES tranponder or
UAT), and
2. That device is properly transmitting its position, and
3. The aircraft data-in "capability code" is configured properly in
the transponder/UAT.

There are ADS-B receiver devices (e.g. those made by NavWorx) where
the vendors do a poor job explaining that any of this an issue/
requirement. The PowerFLARM device will do all its' FLARM and PCAS
magic, but again if you want to ultimately use it as a 1090ES ADS-B
traffic receiver you will need an properly configured ADS-B
transmitter (which in my case since I do fly in those high density jet/
airline/military areas would be a Trig TT21).

Darryl