Originally Posted by
Squawk7700
I had a controller say recently “I can’t see you on ADSB because I can see you on mode C” which I found interesting.
Would be good to hear from an ATC’er as to what actually appears on the screen.
When we're receiving both SSR and ADS-B returns we see an SSR symbol on the screen (a circle) plus we get a small letter "b" appended to one line of label data indicating ADS-B is also being received, so we know we can continue using surveillance standards when the aircraft enters ADS-B only coverage. If only SSR is being received (either no ADS-B coverage or aircraft not ADS-B equipped) we don't get the "b". For TCU the "b" isn't displayed at all.
In ADS-B only coverage we see a four bladed "propeller" symbol and the "b" disappears. For TCU they still see a circle symbol regardless of whether it's SSR or ADS-B.
We can toggle the display to show what you're squawking but SSR takes priority if both are being received, i.e. we can't see your ADS-B code while you're in SSR coverage.