TT21 operation with no ICAO address

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

On Dec 9, 6:58*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

Not to derail your discussion, but why does the notion of
"broadcasting your tail number to the feds" keep coming up? It seems
to indicate either paranoia or illegal activity. Is there some other
reason to be concerned about that?

On Dec 9, 7:02*am, Westbender wrote:
It seems
to indicate either paranoia or illegal activity. Is there some other
reason to be concerned about that?

Aren't those two reasons sufficient? No others that I know of.

Andy

You're making an assumption that the Trig will actually work with no
ICAO address. It's smart enough to know when configuration is needed,
and automatically starts that process without going operational. Since
it's a Mode S transponder, and is required to have a valid ICAO
address, if I were writing the code I wouldn't let it operate at all
with a blank ICAO address. I admit I haven't tested the situation, and
I have no intention of doing so.

Further, the avionics technician will check the presence of the proper
ICAO code during the VFR transponder check (it's a Mode S transponder
so it will be checked as such), so if you subsequently blank out the
ICAO code you're making a deliberate change to the logged, tested
condition. What the legal implications are I don't know, and I'm not
going to bother looking it up since I won't be blanking my ICAO
address.

I must say I'm really sorry I started the ball rolling on Feds
tracking tail numbers in the other thread. It was not meant to be
taken seriously as an "advantage" of Mode A/C over Mode S.

-John

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

On Dec 9, 8:50*am, Andy wrote:
On Dec 9, 7:02*am, Westbender wrote:

It seems
to indicate either paranoia or illegal activity. Is there some other
reason to be concerned about that?

Aren't those two reasons sufficient? *No others that I know of.

Andy

Is this a common concern among sailplane pilots? I'm just trying to
understand the thought process behind someone wanting to fly
"anonomously" with a Mode S transponder. I don't think sailplanes
would be much good for smuggling or terrorism. The only reasonable
intent I can think of is that people don't want to be identified when
wandering into airspace they shouldn't be in. What am I missing?

On Dec 9, 8:09*am, jcarlyle wrote:
You're making an assumption that the Trig will actually work with no
ICAO address. It's smart enough to know when configuration is needed,
and automatically starts that process without going operational.

For me it's more my engineer's curiosity rather than an admission of
paranoia.

According to the TT21 installation manual the installation menu is
entered the first time it is powered up. After that an overt user
action is required to re-enter the installation menus. I don't have
one yet but if/when I do I'll check it out. (even though I'm not, I
say NOT, paranoid)

Andy

Does this mean that if the interrogator is not Mode S, the reply
will be downgraded to Mode C? From the book of Manuals, chapter 2,
verse 2.1:

The TT21 transponder responds to both legacy Mode A/C interrogations
and to Mode S interrogations from both ground radar and airborne
collision avoidance systems. In all cases, the interrogations are
received by the transponder on 1030MHz, and replies are transmitted on
1090MHz.

The idea that the unit broadcasts more information and the fact that
many of us fly at higher altitudes than the TT21 is rated for has
caused me to order a TT22. The transponders I've used in the past had
no way to tell ATC what they were, so that wasn't a problem. I don't
care about the N Number being broadcast (when you first call ATC, they
have your number to associate with your Mode C discrete squawk code)
for any other reason than it could - in future - be a problem using
equipment outside it's certified environment. Easier to pay a couple
of hundred more thingies now and have it over with.
The TT22 and certainly TT21 still use less power than, for example,
a Becker 4401-175.
Regarding the price of Mode S transponder certification, in recent
experience it's less than recertification of the pilot!
Jim

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

On Dec 9, 10:10*am, JS wrote:
* Does this mean that if the interrogator is not Mode S, the reply
will be downgraded to Mode C? From the book of Manuals, chapter 2,
verse 2.1:

The TT21 transponder responds to both legacy Mode A/C interrogations
and to Mode S interrogations from both ground radar and airborne
collision avoidance systems. In all cases, the interrogations are
received by the transponder on 1030MHz, and replies are transmitted on
1090MHz.

* The idea that the unit broadcasts more information and the fact that
many of us fly at higher altitudes than the TT21 is rated for has
caused me to order a TT22. The transponders I've used in the past had
no way to tell ATC what they were, so that wasn't a problem. I don't
care about the N Number being broadcast (when you first call ATC, they
have your number to associate with your Mode C discrete squawk code)
for any other reason than it could - in future - be a problem using
equipment outside it's certified environment. Easier to pay a couple
of hundred more thingies now and have it over with.
* The TT22 and certainly TT21 still use less power than, for example,
a Becker 4401-175.
* Regarding the price of Mode S transponder certification, in recent
experience it's less than recertification of the pilot!
Jim

Jim

Congrats on the TT22.

A Mode S transponder does not broadcast information about the device
power output. Technically the "class" describes the power output of
the transponder and that's just not transmitted AFAIK. Mode S
transponders do transmit (in the data register BDS (1,0)) detailed
information on all the many different data link things Mode S
transponders can implement (especially the high end ones used in
airliners etc.).

ATC cannot tell if you have a Trig TT21 or a Trig TT22 and they won't
care - the power output difference is really not significant in
practice. But if you want a TT22 to feel better about compliance with
regulation because you fly above 15,000' feet then go for it. The
important thing is to have and use a transponder where they are
needed.

And again to separate what _is_ really important - it is critical to
set up and use the correct Mode S/ICAO address for your aircraft
(issued to the aircraft by the FAA in its registration paperwork) on
any Mode S transponder at install time.

Darryl

On Dec 9, 11:12*am, Darryl Ramm wrote:
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.

The MOPS is actually more specific than that. It specifies that if
the transponder 24 bit discrete address is detected as all ones, or
all zeros, at start up then this error condition shall be indicated
to the pilot and the transponder shall either transition to Standby
State, Power OFF condition, or operate as a pure mode A/C transponder.
(ref RTCA DO 181D para 2.2.13.1.1. a) (1).

So reverting to Mode A/C is an allowed response to a null address
being set.

As you said, who knows what the TT21 was designed to do.

Before you jump on it again though I do see a great distinction
between entering a null address and entering an incorrect but valid
ICAO identifier. In either case it's all hypothetical since I'm NOT
paranoid.

Damn, where are my meds?

Andy