When the FDC controls the fire of mortars emplaced in a section, anyone of the three forward observers can be used to adjust fire for the unit. Each forward observer is accompanied by a radiotelephone operator who carries and operates a radio. The forward observer also carries a telephone and a reel of wire. When necessary, the mortar squad leaders supplement or relieve the forward observers.

Each FO is assigned to observe and conduct fire for a particular rifle unit in the company sector or zone of operation. He is also charged with maintaining contact with the supporting unit and keeping himself and the FDC completely informed of the tactical situation. His primary activity is to watch the movement of the supporting unit and to adjust mortar fire on those targets interfering with the mission of that unit. His secondary mission is to provide military information to higher echelons through his means of communication. He does this through observing and correcting of prearranged fire and by adjusting fire on targets of opportunity that he observes or which are identified to him by others. The relative position of the OT line with respect to the mortar-target line does not affect the forward observer's procedure in adjusting observed fire. The forward observer makes his spottings and gives his connections with respect to the OT line. He determines errors and sends confections to the FDC. The FDC converts these corrections to appropriate fire commands. This is done by plotting the forward observer's corrections so the mortar section can place the next burst at the point designated by the observer. To assist in the reporting of targets, each FO may be given a freehand sketch or a suitable map showing the registration point and any other reference points whose chart locations are known at the FDC.

Terminology used by forward observers has been standardized throughout the Army, Navy, and Air Force. Therefore, the FO of a mortar section who is familiar with the basic principles of forward observation procedures is capable of adjusting the fire of any type of indirect-fire weapon. To adjust the fire of indirect weapons of units outside his own company, he establishes communication with the firing unit and maintains it throughout the adjustment.

Location of Target

The observer may report the location of a target for the FDC by one of three different methods: grid coordinates, shift from the reference point, and polar coordinates. These are listed in the order of preferred use.

GRID COORDINATES. The forward observer may send the location of a target by grid coordinates, referring to a map or photomap. In this method, the forward observer sends an eight-digit coordinate that locates the target to the nearest 10 meters.

SHIFT. The forward observer reports the location of a target through a shift or change from a reference point, which may be the registration point, a marking round (to help identify the round, the observer may request a smoke round or airburst), a numbered target, or any other point whose chart location is known at the FDC. He gives the shift as a correction in meters to the nearest 10 for deviation and 25 for range from the reference point. When either the direction or altitude of the target is the same as that of the reference point, he omits the connection. The forward observer determines the shift as follows:

1. DEVIATION. He measures the deviation in mils from the reference point to the target with binoculars and estimates the distance to the reference point. Then he determines the correction in meters from the reference point to the OT line by use of the mil-relation formula or the deflection table and the observer reference-point distance. He includes this deviation correction in his call for fire.

2. HEIGHT OF TARGET. When the difference in altitude between the reference point and the target is 50 or greater, this connection is announced in the call for fire. The height of the target may be determined as follows: Measure the angle of site to the target and to the reference point; then, by the mil relation, compute the amount so each is above or below the observation point. From these values, compute the connection for difference in altitude of reference point and target.

3. RANGE. The forward observer estimates the distance along the OT line to the target from the reference point. This distance is the range correction and is included in his call for fire.

The shift method gives accurate results for shifts of 400 mils or less and acceptable results for shifts up to 600 mils. For greater shifts in direction, the deviation error and the difficulty of estimating the distance from the reference to the target increase rapidly. For this reason, the forward observer selects and adjusts on other reference points so the large shifts to any likely targets can be kept to a minimum.

POLAR COORDINATES. When the forward observer's location is known by the FDC, report the initial location of the target by polar coordinates. The FDC plots the target along the azimuth and at the range from the observer's location as reported by the observer. This method is particularly desirable in the case of large lateral shifts and short observing (OT) distances. When the observer's location is not known at the FDC, he may send it by grid coordinates or some other means.