SKYKING messages are special EAM, Emergency Action Messages, presumably the nuclear go/no-go codes. Urgent ones are repeated three or four times, instead of the usual two. SKYKING is a group callsign for units in the Single Integrated Operation Plan (SIOP)
for strategic operations." "Do not answer" is the standard procedure for self-authenticating broadcasts, meaning that recipients don't need to give away their positions or existence by challenging the traffic.