Transponder Ident Signal Times
 

Hello all,
I'm working on an ATC system which code/callsign pairs the transponder code to the ATC flight data to give us the aircraft callsign on the radar screen. We've encountered some problems with the use of ident and want to work around that by making the radar system software wait to pair after the length of time an ident squawk lasts.
So my question is:
Is there a specified time which the transponder shall broadcast an ident signal for? Is it laid down somewhere?

Thank you.
2lo

I seem to remember it was about 15 secs but I can't recall seeing it written down anywhere.

I have in my 747 notes on ATCRBS, a mention of a "Special Position Identification (SPI) Pulse"

"The SPI pulse is initiated when the pilot pushes the IDENT pushbutton on the ATC control panel. The SPI pulse follows the framing pulse "F2" by 4.35 microseconds and is transmitted with each Mode A response occuring during the 20-second period after the IDENT switch is pushed. The SPI pulse causes a unique flashing display on the controller's radarscope, enabling him to make a positive target identification."

Unfortunately, further on in the text, it mentions a value of 18 seconds (in several places).

Hope this helps.

Regards.
Q.

Thanks QAVION and all others who've read this. 20 seconds seems to accord with observed times. However it leads to another question!

If you push the ident tit and then change the numbers for a new squawk, is the squawk locked at whatever it was prior to hitting the ident for the 20 seconds the ident pulse is sent, before the new code is transmitted on interrogation?

Think I need a specialist transponder company engineer here! Who manufactures transponders?

Bendix/King aka AlliedSignal, dont know their website do a google search. Largest producers of Transponders on tghe planet. would have to be. Just found out they merged with Honeywell, heres the URL.


www.honeywell.com


Regards
Sheep

P.S. HOPE THIS HELPS. Been out of the trade now for7 years.:(

I should think that the IDENT transmission works independently from the selected squawk.

"Think I need a specialist transponder company engineer here! Who manufactures transponders?"

Hopefully, these operational rules are set down by FAA/CAA or whoever, otherwise different manufacturers may have programmed their transponders differently (In which case, they may be reluctant to divulge this information).

Although the ident pulse is added to the chain of data pulses which contain the code, if you change your code in the middle of an ident, ATC will still probably want to see you flashing for that 20 seconds (i.e. irrespective of what code you are using). There's probably a simple timing circuit for the ident period and a summing circuit for the pulses to keep it flashing for the 20 seconds: I recall that the code in the window of our ATC control panel returns to its original value if you are a little slow punching in all four digits of a new code. The transponder might conceivably continue responding with the old code (plus ident) until it has received a completely new code entry.. at which time it simply adds the ident (what's left of the remaining 20 seconds) to the new code.

On the other hand, this sort of thing may not even be taken into consideration. Would ATC ever ask you to "ident" then change code in one radio request? Wouldn't it be more likely for them to ask the opposite of this (change code, then ident)?

Cheers.
Q.