On Dec 9, 4:58*am, Andy wrote:
I'm moving this to a new thread since it has nothing to do with the
requirement for altimeter TSO.

I asked what happened if a mode S transponder was installed without an
ICAO address. *That was a sort of tongue in cheek reaction to the idea
that someone would not want their tail number broadcast to the Feds.

Darryl responded:

A Mode S transponder absolutely has to transmit the aircraft ICAO
address, a correctly configured ICAO address is required for the
transponder to actually work--bad things might happen if two aircraft
had the same default ICAO address were being interrogated at the same
time.

I like to continue that discussion.

Isn't it true that a mode S capable transponder cannot respond to a
mode S interrogation if it has no valid ICAO address?

Isn't it also true that a mode S transponder is also required to
respond to both mode A and mode C interrogations.

If both are true then doesn't it follow that a mode S transponder such
as the Trig TT21, if installed with no ICAO address, will not respond
to mode S interrogations but will respond to all mode A and mode C
interrogations.

If that is indeed the case then any new transponder purchaser who
wanted mode S capability in the future, but was paranoid about
broadcasting the tail number, could disable mode S responses by
leaving the ICAO address entry blank and use it just like a mode C
transponder.

Andy

I really wish this was not started.

There are different types of Mode A/C interrogations. A old style real
Mode A/C interrogator does very simple interrogations, the difference
is just the timing between two interrogator pulses. A modern Mode S
interrogator will interrogate in all of Mode A, Mode C or Mode S
(although TCAS does not actually need to interrogate in Mode A). The
Mode A or C interrogations a modern interrogator sends out look to old
Mode A/C transponders like standard interrogations but have a third
pulse hidden behind the first two. A Mode S transponder recognizes
that third pulse and then ignores the whole interrogation--it will
instead respond to the separate Mode S interrogations, either Mode S
all-calls or targeted Mode S interrogations at just that transponder.
A Mode S transponder being interrogated by a old style Mode A/C
interrogation without that "ignore" pulse will respond just like a
Mode A/C transponder. An example of a pure Mode A/C interrogator is
the lower end TAS/TCAD systems in GA aircraft. Most ground based
interrogators and all TCAS interrogators are Mode S capable and will
transmit Mode A/C interrogations with ignore pulses.

Some folks will remember the old TRT 250 Mode C transponders and
mandatory ADs on them. One of the issues there was they did not handle
that hidden third pulse properly and might not reply to the
interrogation. Although Terra argued that the actual behavior was not
defined in the RTCA specs and so the FAA was assuming an arbitrary
behavior that they should not have been required to meet--they lost
with that argument.

Some Mode S transponders are designed to revert to behaving as a pure
old style Mode A/C transponder if they lose the ICAO address. I
suspect this was more designed for complex installs with multiple
transponders and separate ICAO address programmers. I am not going to
speculate how a TT21 works in all corner cases. It is just absolutely
the wrong thing to do to not correctly set up the ICAO address in the
transponder.

The real danger with ICAO addresses is some clown decides they will
enter some dummy number. Then that clown decides they'll enter the
same dummy number in another Mode S transponder in the same area. The
ICAO address is not just some thing advertising your ID, it is a
fundamental part of the radio communication protocol used by the
interrogator to talk to the transponders, a collision of addresses
could cause serious problems. The aircraft registration or flight ID
is a completely different thing, its just data in the packet and not
fundamental to the data communications, but should be set to your N-
number.

Setting the wrong ICAO address for your aircraft may eventually get
noticed by ATC and lead to follow up from the FAA. And any A&P or
pilot doing this deliberately would deserve the ensuring pain.

Darryl