My best friend was FDC for the Marines. He told me from the moment the first call went out for a fire mission, it took (using computers) 90 seconds to have a round impact. If adjustments were made, depending on the FO, it could take up to 30 more seconds before the Fire For Effect was called. Then all the barrells opened up.
They also practiced "cold", meaning without computers and were allowed 30 seconds longer to get the first round on target. He said most times they could usually beat that by a few seconds. Now, considering it's Twilight, you're probably not working with a lot of people who know FDC and can do the trig so quickly, so you might be looking at 2 minutes, maybe more, before the first round hits and the FO can adjust or call FFE.
"Called" is when an FO calls in a request for artillery. Spotting round is fired, corrections made (depending on accuracy of FO and FDC) and then FFE is called when "close enough". Called missions have to have the fire mission type set by the FO. If he wants WP or ICM or HE, he needs to state it.
For "Time on Target", the location is determined, the fire mission type set, and the time for the mission to take place is set. Thus the name "time on target". It's not necessarily super accurate this way, but it's good for a barrage on something like a town or other large-ish location.
And "Preplotted" is where the tubes are zeroed in on an area long before the action there begins. The location is marked. When an enemy unit enters the preplotted area, the FO calls it in, the tubes go right to the setting they need to be at (since it was all zeroed in before hand) and the barrage can begin. No need for spotting rounds for preplotted artillery. Usually the fire mission type is also preset for this, but could be changed if need be.
Hope this helps.
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