I think you are missing one point : primary radars, UTM, ATC or Airports authorities are not going to prevent this from happening .
Let me explain briefly : There are basically 3 was to operate civil Drones ; those operated commercially , those by hobbyists and those used for criminal intent.
The commercially operated ones are asking for regulation and UTM as they do not want to lose their vehicles and what they carry ( cameras, detection or cargo , e.g amazon ) in a collision It is to them that we are developing UTM .
The hobbyists, are currently being contained by both education and regulation ( e.g. 400 ft/ 1 km away ) 99.9% will stay away from airports as they do not want to lose their equipment either.
The last group is the one we are talking about here . For those regulations or detection / UTM will not refrain them ( even if detected, if launched from near a airport fence, it can reach their goal/target in a few dozen seconds ). It is not the task of Airports authorities or ATC to take care of criminal activity , this is the task of the police and/or the armed forces..
In other words, both detection and correction of drone activity inside airports areas is a State security forces issue , not an ATC or an Airport operator issue.
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