Aircraft use transponders to communicate to ATC their position and status. Some squawk codes are reserved, such as 7700 (emergency), 7600 (communication failure), 7500 (hijacking), 1202 (glider), 1200 (VFR), etc. One of these, 7777, is apparently used for "military interception." What does this mean in the United States? Under what circumstances would it be used on a civilian / military aircraft?
In countries outside of the US, 7777 may be used by test transponders (RABMs) to check correctness of radar stations (BITE). e.g. on top of a mountain.
In the US, it seems that it is used as well on active air defense missions without ATC clearance. This would mean that the interceptor aircraft would change it's squawk to 7777 for the military/civilian air traffic controller to see it properly (if not filtered out on civil radars).
A link to the US intercept procedures is here.
According to the AIM 4-1-20(e):
- Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.
The ATC orders don't add anything useful and a lot of security procedures are classified, or at least not publicly available on faa.gov. But it seems from that information that a) 7777 is important for interception operations, and b) civilian pilots must not use 7777. That implies that 7777 is reserved for interceptor aircraft, not the aircraft being intercepted.
The FAA's interception instructions for pilots say that intercepted aircraft should squawk 7700.
See this question too.