So I noticed every Mode-S parser I’ve ever examined processes DF 17 messages while completely ignoring 18 and 19. The thing is, upon further research I discovered that 17, 18 and 19 all share exactly the same data format in the rest of the frame. Only those two bits are different: 17 indicates a transponder that can respond to interrogations, 18 indicates a non-transponder that cannot respond to interrogations such as TIS-B, and 19 indicates a military aircraft transponder.
So i modified my copy of dump1090 to treat all three downlink format codes exactly the same a few days ago, and then yesterday I actually noticed Air Force 2 presumably trying to get as far away as possible from the UN General Assembly Debate. It was broadcasting an altitude but no position; normally a transponder like that would get triangulated by MLAT so it occurred to me that it may very well have been broadcasting DF 19 and if so I might have been about the only person seeing its mode-S messages at all which would also explain why MLAT never picked it up. And check for AF2 on flightaware, there is no sign of it even though it was transmitting its tail number 99-0004 which IS on FA and is clearly the Veep Express.
Well even Janet airlines isn’t so secretive, presumably because it flies to and from a civilian airfield. So the whole point of this post is to ask if this is simply an oversight and should I submit a PR on GitHub or are we intentionally not tracking DF19 messages because they tend to be transmitted by… Errr… sensitive aircraft?