On Jan 20, 8:42*am, JJ Sinclair wrote:
On Jan 20, 7:43*am, "Dan Marotta" wrote:









Having installed a Trig TT22 in my LAK-17a, I've noted that, when I press
the IDENT button, the reply indicator turns on for some finite time which is
longer than a normal reply to an interrogation. *Since I know absolutely
nothing about transponder protocol, I sent a message to Trig's tech support
and got a very detailed reply about how it works.

Not wanting to step on any toes (unusual for me, huh?), I replied to Andy at
Trig thanking him for the information and requesting permission to post his
reply. *If he says, "Yes", I'll post it here.

In the mean time, Steve's info sounds correct to me.

"Steve Leonard" wrote in message

....
On Jan 20, 3:15 am, GC wrote:

I'd be interested to know how pressing IDENT causes a flag to be set on
the controller's display 100 miles away without any transmission from
the transponder. Telepathy?

GC

As my education continues to rise, allow me to pass along what I think
I have learned. *When you press the IDENT button, you tell your
transponder "If I get interrogated in the next X seconds, reply with
this extra bit of information as well." *And X seconds is the amount
of time your "reply" light is on after you push the IDENT button.
This additional information that is sent will be received by the
ground station and set a flag to highlight you on the display for the
controller.

Have not personally verified this, but have gotten the same info from
what I consider to be a couple of VERY good sources. *So, it is not
done via telepathy, but anticipation. *You are asked to IDENT, you do,
and you prepare your transponder for the next interrogation.

See how much we learned this week? *:-)

From my Becker 4401-3 manual.
Pressing Ident button (will)............. In mode A and Mode A+C this
triggers the transmission of an identification impulse additional to
the Mode A reply code for approx 18 seconds. During this time "idt"
appears in the bottom line of the LC display.
It says, "triggers the tranmission"
I'm just say'n,
JJ

JJ and Dan,

My vague understanding of how SSR (Secondary Surveillance Radar) works
will probably help make sense of the Ident function. The classic SSR
setups use a highly directional transmit/receive antenna that rotates
at either 6 or 15 RPM (if I remember correctly). The transmitter
emits inquiry pulses as it rotates, when a reply is received, the
rotational position is used to determine azimuth, the delay between
the last transmission and the reply determines distance. Pushing
Ident simply sets a flag within the transponder that results in a
specific bit being set (in addition to the selected code) in the reply
when the next inquiry is received. The flag remains set for 18
seconds to ensure that at least one Ident reply is transmitted. Note,
though, that if the antenna isn't pointing at you, it can't see the
reply, so (at least for Mode C transponders) when you press Ident,
nothing is transmitted unless an SSR inquiry is received within the
next 18 seconds...

Marc