I think you are being a bit pedantic is it no the case that the reason military aircraft can not be seen on FR24 et al is they switch of their unencrypted mode S signals despite continuing to transmit an encrypted signal?
I’m not intending to be pedantic, just pointing out the limitation of what these tracking services (e.g. FR24 etc,) can provide.
The data (all data, civil and military) which hobbyist tracking services, like FR24 use, was never intended to be visible to members of the public.
It just so happens that the Mode S data can be picked up easily on receivers and used for public viewing.
BUT…. that data is only part of the raft of surveillance data used by both civil and military air traffic control, and by air defence establishments.
In other words, you are only getting a sneaky, partial look through the fence, at what goes on in the sky.
It just so happens, that with mandating the carriage of Mode S transponders, for enhanced ATC purposes and it’s primary reason (airborne traffic avoidance), that the tracking services like FR24, can receive and provide quite extensive, if not all, coverage of civil air traffic movements.
That’s more by luck, than by design.
Because of different requirements, military aircraft generally only have to use the particular Modes that FR24 can see, when operating in civil or join airspace, for the traffic avoidance function.
That’s why a large amount of military activity isn’t visible to FR24, even in normal operations.
Civil and military agencies can still see military tracks, just not the public, who were not intended to see anything.
When it comes to hostilities and active combat, that’s when military aircraft would “go dark” .
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