I used FPN-47 at Mountain Home. We used SIF exclusively. It did recognize IFF too. It just worked very poorly. I hated seeing the C-47s come in, cuz it was a beyotch trying to ID them.
If you don’t have the daily code it won’t work.
daily code was top secret.
I screwed up one day and the entire US military had to reset the code.
I had to write a letter to the NSA that owns these machines
to explain what happened.
Scary for a USAF E2.
BTW I made E3 6 months later.
I left the USAF as E5.