Transponder requirement confusion

In article qAPEg.4109$SZ3.3107@dukeread04, Ray Andraka
wrote:

That's correct. Mode C only transmits either the squawk code or the
encoded altitude, depending on the specific interrogation.

Mode 3A provides the code, 3C provides the altitude.

,
TCAS is an active system that sends an interrogation and listens for the
reply. It uses the time delay between the interrogation it sends and
the reply it recieves back to accurately measure range. Many use an
antenna array and beamforming techniques to determine azimuth as well.

TCAS II systems MUST use at least one directional antenna (the top one).
Normally both the top and bottom ones are directional.

--
Bob Noel
Looking for a sig the
lawyers will hate

In article
,
john smith wrote:

My understanding is that the Mode-S provides the communication
information.

Mode S provides a datalink so that cooperating TCAS II-equipped
aircraft can coordinate RAs. When a TCAS II-equipped aircraft
encounters the older Mode 3A/C transponder, the TCAS-II will
still provide the flying pilot an RA if necessary.

--
Bob Noel
Looking for a sig the
lawyers will hate

john smith wrote:

My understanding is that the Mode-S provides the communication
information. Mode C may contain altitude, but there is no distance or
direction information.

TCAS doesn't rely on the units communicating position information.
It does everything via the normal radar behavior. You might
be confusing it with ADS-B where units broadcast their position.

wrote:


It's not that the tube craps out in-flight... it's just that repeatedly running the transponder when you do not
"need" it (read: in B.F.E., VFR, local-flights) means that you shorten its useful life when you *DO* need it (flight
following, Class-[ABC], IFR). Any time I actually *GO* anywhere where collision avoidance might be an additional
feature, chances are it's a cross-country flight and I'll have it on for that reason.


I had the same KT78 in the plane for years and always turned it on.
It's still working (and soon should be in a friends RV). I only
pulled it because I did install a mode S transponder.

Bob Noel wrote:


Mode 3A provides the code, 3C provides the altitude.


Absolutely right. There are two different interrogations differentiated
by the spacing of pulses in the interrogation. The transponder replies
with the 12 bit squawk code when it receives a mode 3/A interrogation,
or the 12 bit altitude code from the altitude encoder for a mode C
interrogation. If you have mode C turned off on the panel or if there
is no encoder attached, the reply to a mode C interrogation has
all 12 data bits set to 0. The transpoder always replys with the squawk
code to A mode 3/A interrogation.


TCAS II systems MUST use at least one directional antenna (the top one).
Normally both the top and bottom ones are directional.

Right. TCAS II requires the azimuth. This is typically done with a
small array of antennas and the reciever processes the differences in
signal phase between antennas to determine the direction of arrival. The
advantage of a phased array is that there are no moving parts needed to
steer the antenna's look direction. I don't believe TCAS (not TCAS II)
was required to resolve azimuth.