". A true future use would be "I'm going to have to get up very early tomorrow morning". In your example, though, it is doubtful that the speaker truly means it as future tense.
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Mike2015a police officer is interrogating two suspectsIs the person being addressed very agitated? It seems that would be the situation in which the 'remain calm' sentence would be used.
Mike2015The police officer is quite angry.OK, but I asked about the other person. How is the person who is being questioned by the officer? Very nervous? Calm? Screaming? Quiet?
Mike2015The person who is being questioned is very loud and talkative.(No comma.)
Mike2015So, (I'm going to have to...etc) is an emphatic and alarming order?It's the preface to such an order. It's what comes next that's the real order, of course.