I heard this line in Top-Gun (1986):

Voodoo 1: Mustang, when you get the chance, can you give me a mode-four check?

What does "mode-four check" mean?


1 Answer 1


Mode 4 is one of several modes of the IFF, (Interrogator, Friend of Foe) system used in military aircraft.

Think of it like a radar transponder, or ADSB-out: A discrete code, known to other cooperating entities, is entered into the IFF control head. Other aircraft including AWACS, ship based weapons systems, and fighter aircraft have the ability to transmit an interrogation signal. If the targeted aircraft is properly coded it returns a signal that identifies them as part of the friendly force.

Once airborne and outbound on a mission it is standard practice to ensure that the system is working before leaving the vicinity of the ship lest you be unable to be identified upon your return.

  • 5
    $\begingroup$ To add to this, mode 4 is protected by crypto. The interrogator encodes the pulses with special timing, and a transponder that doesn't have matching crypto cannot send response pulses at the proper times - these pulses (at least in the past) would show up as a "fringe" behind the dot on the radar scope, but if the transponder has the right crypto, this "fringe" becomes a nice solid arc behind the contact. It's handled differently these days I'm sure, but the basic idea is the same - it's all about that pulse timing. $\endgroup$
    – Steve
    Nov 15, 2022 at 17:12
  • $\begingroup$ @Steve, Thanks for that additional info. I didn't think the OP was looking to go too deep into details of its operation, and I am only familiar with how it's used in the field. $\endgroup$ Nov 15, 2022 at 17:44
  • 2
    $\begingroup$ One (trivial) addition, is that the crypto code is "injected" into each aircraft's systems (both the radar - for request transmissions, and the IFF transponder for responses to those requests), on a coordinated regular schedule. When I was on active duty, that was once every 24 hours. $\endgroup$ Nov 16, 2022 at 14:44
  • 1
    $\begingroup$ You can learn more about Mode 4 by reading about IFF Mark XII development. $\endgroup$
    – JBH
    Nov 16, 2022 at 16:47
  • 1
    $\begingroup$ I can sense when someone asks a question because they are an aspiring avionics technician or electrical engineer, vs when they are a movie fan who just wants to know what something means. My money for this question on the latter. I appreciate the feedback, but don't plan to modify my answer to add further detail on the inner workings of the black box. $\endgroup$ Nov 16, 2022 at 21:28

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .