In fire without an FDC, the forward observer makes corrections differently than when operating with a fire direction center. He makes all deviation connections with respect to the gun-target line rather than with respect to the observer-target line. All deviation corrections are sent to the mortar in mils or turns of the traversing handwheel.

Observer Within 100 Meters of the Mortar Position. The best location for the forward observer for rapid-fire adjustment is at the mortar position where his deviation spotting and deflection correction in roils, to be placed on the mortar sight, are the same. The tactical employment of the mortar usually makes it necessary for the forward observer to be in a position other than at the mortar; however, when the forward observer is located within 100 meters of the mortar position, the deviation error that he reads in his binoculars can be applied directly to the sight without any computations. This is true because the angle that exists between the observer-burst line and observer-target line is, for all practical purposes, equal to the angle that exists between the mortar-burst and the gun-target lines. Any slight difference between these two angles is compensated for by the inherent dispersion of the weapon and the bursting area of the round. For example, when the observer, from a position within 100 meters of the mortar location, observes the burst to the left of the target and reads that it is 40 mils left on the mil scale of his binoculars, he orders a correction of RIGHT-FOUR-ZERO.

The gunner applies this connection directly to the previous deflection setting, using the LARS (left add, right subtract) rule.

Observer More than 100 Meters from the Mortar Position. It is not always possible for the observer to be located within 100 meters of the mortar position. When he cannot locate himself within 100 meters of the mortar position, he must locate himself within 100 meters of the gun-target line. It can be readily seen that this might present some difficulty in visualizing the gun-target line and getting within 100 meters of it. When the observer is attacking targets over a wide frontage, he is required to move frequently and his movement is limited. In this situation, the angle that exists between the mortar-burst and the gun-target line is not equal to the angle that exists between the observer-burst and the observer-target line, and certain computations must be made to correct the differences in these angles. For example, when the observer is halfway between the mortar and the target, the correction to be made on the sight is one half of his deviation spotting; when the mortar is halfway between the observer and the target, the correction is twice his deviation spotting. As other distances give other ratios, it is necessary to apply a correction factor to the number of mils spotted before ordering a deflection change. This factor is a fraction, the numerator of which is the observer-target distance, and the denominator of which is the gun-target distance; that is

Figure 14-24.-Plotting board and carrying case.

Correction factor is

For example, suppose the distance from the observer to the target is 1000 meters, the gun-target distance is 1200 meters, and the deviation of the burst from the target as read by the observer is 60 mils.

The correction factor is

The change in deflection equals

In this method, you should round off distances to the nearest 100 meters for simplicity and speed of computation.