Time On Target (TOT) is the military co-ordination of artillery fire by many weapons so that all the munitions arrive at the target at precisely the same time. The military standard is plus or minus three seconds from the prescribed time of impact. In terms of place, the historical standard was for the impact to occur within one circular error probable (CEP) of the designated target. CEP is the area on and around the target where most of the rounds will impact and therefore cause the maximum damage. The CEP depends on the caliber of the weapon, with larger caliber munitions having greater CEPs or greater damage on the target area. With the advent of "smart" munitions and more accurate firing technology, CEP is now less of a factor in the target area.
The theory of TOT was first developed by the US Army shortly before World War II to help improve the effectiveness of artillery firepower, but the levels of communication and co-ordination required to achieve it were not reliably established until after WWII
It had been found during the World War I that most of the casualties in an artillery bombardment occur within the first few seconds. During the first few seconds however troops may be in the open and may not be prone. After that enemy troops have gone prone and/or sought cover. This dramatically lessens the casualties from shrapnel or high explosive blast. World War II Allied artillery units were often trained to fire their guns in a precise order, so that all shells would hit a target at the same time, delivering the maximum possible damage.
When multiple artillery units are firing it is important to coordinate the TOT so that all units participating have their rounds land in the target area at exactly the same time as late arriving rounds do proportionally far less damage than the initial ones. The TOT calculations thus include distances of participating units and the speed (muzzle velocity) of projectiles involved.
A shorter TOT usually means a smaller or less important target, and the deployment of less artillery or aircraft. A longer TOT means that the target is large or extremely important, so preparations for a massive bombardment or air strike will take longer.
As field artillery encompassed fire support, TOT also encompassed all delivery systems: close air support, naval gunfire, attack aviation, and direct fire ground systems. Therefore, TOT now designates the exact time and place where all weapon systems now focus firepower to destroy or neutralize the enemy target.