On 1/20/12 9:28 AM, Marc wrote:
On Jan 20, 8:42 am, JJ wrote:
[snip]

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

Marc has it pretty nailed here.

Some extra comments...

In classic Mode A/C interrogations the ident (SPI) bit is set in Mode A
transmissions only. All the "Mode C" transponders out there reply to
both Mode A (any transponder receiving this interrogation reply with
your squawk code) or Mode C (any transponder receiving this
interrogation reply with your altitude) interrogations. Ground based SSR
systems will interrogate with an interleaved pattern of Mode A and Mode
C interrogations, with typically several (few to tens) of interrogations
of the transponder as the rotating beam sweeps past the aircraft (all
this happens very fast, you only see one blink of the interrogation
light). Other interrogators like TCAS and TCAD systems only issue Mode C
interrogations so never see your squawk ID and never know if you are
pressing Ident etc. (and would not know/care what to do with it anyhow).

Just to confuse things technically that extra ident pulse in Mode A
replies is called the SPI for "Special Position indicator" but it adds
no position information, its just telling the interrogator that there is
something "special" about this reply and ground based SSR systems just
highlight that corresponding aircraft symbol on the controller's display.

Mode S (e.g. in the case of the Trig transponder that started this) is a
bit more complex but the idea is basically the same. A Mode S
transponder will reply to a Mode S interrogator with a digital data
packet, not a simple pulse train that Mode A and Mode C signals use,
and within that digital packet reply is a single SPI bit that has
exactly the same meaning as the SPI pulse in a Mode A reply. Mode S
transponders will reply using conventional Mode A/C signals to non-Mode
S capable interrogators, and will transmit a SPI pulse in their Mode A
replies.

The reason for singling out Mode S is that it invites some extra
confusion. All Mode S transponders transmit a 24 bit ICAO aircraft
identifier that uniquely identifies your aircraft (a unique 24 bit
pattern tied to your callsign/registration, but not directly encoding
your callsign). This ICAO ID is totally separate from the squawk code
and SPI bits that are also transmitted within the digital packets (I'm
simplifying here as there are multiple different Mode S messages types).
I've seen some confusion where people think pressing ident with a Mode S
transponder causes the transponder to send, or to reply to
interrogations with either one or more of your ICAO ID, callsign etc.
All it does is set that SPI bit. ATC gets to see your aircraft's ICAO ID
at all times you are within Mode S SSR radar coverage (basically all ATC
radar coverage).

Mode S transponders are also different from "Mode C" transponders in
that they are capable of transmitting in cases when they are not
interrogated. The common examples being acquisition squitters that are
short messages to advertise their presence to TCAS equipped aircraft--to
help cutdown in unnecessary TCAS interrogations. And the extended
quitter used by ADS-B 1090ES data-out (if the extra ADS-B data-out
equipment is installed/configured/turned on). The extended squitter will
tranmsit ADS-B GPS derived position data etc. about once per second
within a Mode S digital packet. Pressing the ident button on a Mode S
transponder has nothing to do with acquisition or extended squitters and
does *not* cause ADS-B or any other data packet to be transmitted (its
dumb as nails and just sets that SPI bit/pulse in Mode S or Mode A
replies for the next 18 seconds).

Please don't go pressing the ident whenever you feel like it. Only press
ident when instructed to by ATC, you may well **** off a controller
and/or have them think you are in trouble/signalling a problem. The
especially annoying thing that happens is when a controller asks one
aircraft to ident and somebody else does it by mistake. If you are not
sure the controller is asking you to ident ask them. Another point of
confusion I have heard is pilots thinking they should press ident on
first contact with ATC, or you should press it after changing squawk
code. Please don't do either automatically--press ident only if you are
asked to (and you may not be) - e.g. only press it if you hear a request
to your callsign to "squawk ident".

Can't wait for winter be over...

Cheers


Darryl