I have long suspected that DoD units operating outside CONUS may in some cases be assigned Mode-S addresses from a pool of addresses not specific or regularly assigned to that aircraft. I may have finally found official proof of that in a USAF in Europe instruction. And I quote:
"Mode S Address. Prior to each flight, ensure the Mode S address is entered correctly. MAJCOMs will manage assigned dynamic addresses to ensure no two aircraft are airborne with the same address."
The way I read it, it would imply that there is a pool of Mode-S addresses they can call upon for certain operational missions/situations. That now makes some of my Mode-S hex address holes and blocks even more interesting to watch.
I have found a second reference regarding Mode-S aircraft addresses. Here are the highlights of the DoD Mode-S guidelines:
• No two aircraft should have the same AA!
• “Default” address assigned to specific tail numbers
• “Dynamic” address assigned only to military aircraft
• Block of addresses to aid in day-to-day OPSEC
• Distributed by DoD AIMS/PO to MAJCOMs
• Mode S address are unclassified, and considered FOUO information.
Each aircraft uses a unique 7-character “Aircraft ID” (a.k.a. Flight ID). Flight Plan Callsign assigned to mission (i.e. EAGLE11). What ATC controllers see on their screens. Allows for flight plan correlation to Mode S response.
MODE S ADDRESS - Also known as: Default Code, Default Address, Aircraft address , 24-BIT Address, 8 digit Mode S address
• DEFAULT – 24-Bit Address permanently assigned to a particular aircraft that correlates with its BUNO or Tail Number.
• DYNAMIC – 24-Bit Address that is temporarily issued to an aircraft and is for specific mission use (CONOPS required)
• TEMPORARY – 24-Bit Address that is issued to an aircraft and is used for platform development and integration tests.
Now if I could only find a copy of the DoD Mode-S handbook, that would make me a happy camper.