Xponder question

[Matthew]

I got to wondering the other day:

How does the 'ident' work?

What does the transponder send when the ident button is pressed? Is there a general guideline on how long the button should be held?

 

[Steve]

http://www.airsport-corp.com/modec.htm

 

[RotaryWingBob]

I got to wondering the other day:

How does the 'ident' work?

What does the transponder send when the ident button is pressed? Is there a general guideline on how long the button should be held?

The ident key causes you to be highlighted on the controller's screen. AFaIK there is no need to hold it down -- just press it.

 

[Matthew]

Yeah, I knew it highlights on the controller's display, I was wondering what gets transmitted with the 'ident' function to tell the display to highlight.

From the info given by Steve, there is signal that is sent for 10 seconds along with the squawk code and altimeter.

 

[Areeda]

Took a bit of reading but it is in Steve's link. I think it's a little better explained here:

http://en.wikipedia.org/wiki/Air_traffic_control_radar_beacon_system

The oversimplified basics are:

• Transponder replies to a Mode A interrogation with your 12-bit digital squawk code
• Replies to a Mode C interrogation with a 12-bit encoding of your altitude
• Ident adds another bit delayed after the 12-bit data call SPI (Special Position Identification) which the receiver uses to identify one of multiple targets with the same squawk code.

Joe