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Chapter 12

TERRAIN GUN POSITION CORRECTIONS AND SPECIAL CORRECTIONS


To enhance survivability on the battlefield a unit must take maximum advantage of the natural cover and concealment offered by the terrain and vegetation (Figure 12-1). When pieces are so positioned corrections may be required to obtain an acceptable burst pattern (sheaf) in the target area. These corrections compensate for the differences in muzzle velocity between pieces and terrain positioning of the weapons. When FFE rounds impact in the target area, the results, to a large extent, depend on how well the sheaf fits the size and shape of the target.


Section I

Types of Corrections


Artillery fires can be computed to fit the size and shape of the target by computing corrections for the following:

  • Individual piece displacements (position corrections).

  • Shooting strength of each piece (muzzle velocity corrections).

  • Target size, shape, and attitude (irregularly shaped targets).

12-1. Overview

a. Terrain gun position corrections (TGPCs) are individual piece corrections applied to the gunner's aid on the panoramic telescope (pantel), the correction counter on the range quadrant, and the fuze setting of each piece.

b. Special corrections are individual piece corrections applied to fuze settings, deflection, and quadrant elevation to place the FFE bursts in a precise pattern on the target.

c. Special corrections and TGPCs include corrections for the location of each weapon in the firing unit area (position corrections) and for the shooting strength of each weapon (muzzle velocity corrections).

(1) The goal of TGPCs is to compute corrections to obtain an acceptable sheaf in the target area.

(2) The goal of special corrections is to compute aimpoints tailored to fit the target size, shape, and attitude.

12-2. Piece Displacement

a. To determine position corrections, the relative position of pieces in the firing unit area must be known (piece displacement). Piece displacement is the number of meters the piece is forward or behind and right or left of the base piece. It is measured on lines parallel (forward or behind) and perpendicular to (right or left) the azimuth of fire. (See Figure 12-2.)

b. Piece displacement (Figure 12-2) can be determined by estimation and/or pacing, by hasty traverse, or by survey. Usually, estimation and pacing are not accurate enough for the large displacement distances involved in firing unit positions. The hasty traverse technique is a quick, accurate means of determining piece displacement by using the M10 or M17 plotting board. The survey technique provides grid coordinates for each weapon location.

(1) Estimation is the least desirable method to determine piece displacement. Using this technique, the XO or platoon leader estimates the displacement of the pieces from the base piece both parallel and perpendicular to the azimuth of lay. The pacing technique is fairly accurate in small open areas, but it is time consuming. The XO or platoon leader uses this technique to determine displacement by pacing from the base piece both parallel and perpendicular to the azimuth of lay.

(2) The hasty traverse technique is a graphic solution of piece displacement that uses the M10 or M17 plotting board. The advance party provides the FDC with the initial lay deflection, distance and vertical angle to each howitzer position from the aiming circle (AC).

(3) The survey method is the most accurate method. Field artillery survey crews provide a surveyed grid and altitude to each weapon position. Piece displacement is computed by determining the difference between the grid coordinates from the base piece to each weapon position.

12-3. Sheafs

a. A target is covered by fire through controlling the pattern of bursts (sheaf) on the target.

b. When firing a parallel sheaf, the rounds impact at the target in generally the same pattern formed by the howitzers in the firing unit area. The width and depth of the unit's sheaf are always measured on a line perpendicular to the line of fire. As the line of fire changes, so does the width and depth of the unit's sheaf. (See Figure 12-3.)

c. A parallel sheaf does not require TGPCs or special corrections. All weapons fire the same deflection and quadrant.

d. There are three basic types of sheafs that may be obtained with TGPCs and special corrections.

(1) Converged sheaf. All weapons have the same aimpoint.

(2) Open sheaf. Aimpoints are separated by one effective burst width. Figure 12-4 shows sheaf widths for an open sheaf. The open sheaf width equals the number of howitzers multiplied by the projectile effective burst width. See Figure 12-5 for burst widths.

NOTE: For manual computations of TGPCs and special corrections with an M17 plotting board, a width of 30 meters is used for the 105-mm howitzer for ease of computation.

(3) Special sheafs. Special sheafs are sheafs other than parallel, converged, or open.

(a) Linear. The sheaf is described by a length, and attitude or by two grids. Aimpoints are evenly distributed along the length of the sheaf along the attitude specified.

(b) Rectangular. The sheaf is described by a length, width, and attitude. Aimpoints are evenly distributed along two lines equal to the length and parallel to the attitude specified.

(c) Circular. The sheaf is described by a grid and a radius. Aimpoints are evenly distributed on a concentric circle half the radius specified.

(d) Irregular. The sheaf is described by a series of grids. Aimpoints are evenly distributed along the length of the sheaf.


Section II

The M17/M10 Plotting Board


The M17 and M10 plotting boards are versatile pieces of the fire direction set. The M17 and M10 plotting boards are similar and used to determine TGPCs and special corrections. The differences are as follows:

  • The M17 has a snap-type pivot in the center; the M10 uses a small screw.

  • The M17 has a map scale graduated in metric measure; the M10 scale is graduated in both meters and yards.

NOTE: In further discussions, the term "M17" will refer to the M17 and M10 plotting boards.

12-4. Description

The M17 plotting board consists of two parts--the gridded base and the clear disk.

a. The gridded base is a white plastic board. The center area of the board is a circular gridded area called the target area. The grid pattern divides the target area into squares. The scale assigned to the grid pattern is at the discretion of the user, but most common scales for various operations are as follows:

OPERATION

SCALE

Terrain Gun Position Correction

1 Square = 10 meters

Special Corrections

1 Square = 10 meters

Laser Adjustment of Fire

1 Square = 100 meters

Target Location

1 Square = 100 meters

b. A red arrow is printed from the bottom to the top of the target area. This arrow represents the direction of fire. The arrow points to a vernier scale. The center graduation forms the vernier index, which is used to determine direction from the scales on the clear disk. The vernier scale may be used to determine directions to an accuracy of 1 mil. Along the top edge of the plotting board is a measuring scale graduated in millimeters. On the bottom is a scale in meters. On the right side there is a scale in inches.

c. The clear disk is a transparent circular plastic board that snaps or screws into a center pivot on the gridded base. A single black line is engraved in the clear disk. This line represents the 0-3200 line when weapons are plotted.

d. The edge of the disk is engraved with scales that are graduated every 10 mils. The outermost scale is a black numbered scale (the outer black scale). The scale is numbered every 100 mils starting at 0 and ending at 63. The outer black scale is used to represent azimuth and lay deflection. Immediately inside the outer black scale is a red numbered scale (the red scale). This scale is numbered every 100 mils from 0 to 32. Past the 32 graduation the numbering continues from 1 to 5. This scale is used to orient the disk on chart deflection. The innermost scale is a black numbered scale (the inner black scale). This scale is numbered every 100 mils from 0 to 3200 and is used to orient the disk for lay deflection for the M12-series pantel when lay deflection is measured from the line of fire.

e. The screw or rivet secures the disk to the base and maybe used to represent one of the following:

  • Base piece.

  • Target.

  • Observer location.

  • Location of the last burst.

NOTE: In the following sample problems, the M17 plotting board is viewed with the curved edge to the operator's left and the description "top of the plotting board" refers to the side of the plotting board with the vernier scale.

12-5. Plotting Piece Locations for Weapons Equipped With the M100-Series Sight

a. The following is an example of the platoon leader's report for the M100-series sight:

b. Table 12-1 shows the steps required to plot piece locations for weapons equipped with the M100-series sight.

12-6. Plotting Piece Locations for Weapons Equipped With the M12-Series Sight

a. The M12-series sight is capable of determining deflection to a maximum value of 3,200 mils. The sight is graduated from 1 to 3200 twice to form a full circle. Consequently, every deflection has a "back deflection" of equal value in the opposite direction. This arrangement is indicated on the plotting board by the inner black scale. This scale is a lay deflection scale graduated from 0 to 3200. Care must be taken when setting up the plotting board to prevent the use of the wrong scale and thereby creating a "mirror image" of the battery. The use of the scales is dictated by the location of the howitzers in relationship to the position of the aiming circle. In the XO's report, the XO must indicate whether each piece is to the left or right of the aiming circle in respect to the azimuth of lay. If the lay deflection for a howitzer is exactly 3200, the XO's report must indicate whether that piece is forward (down range) or behind (the aiming circle is down range) in comparison to the aiming circle. The rules for the use of the scales areas follows:

(1) If the piece is left of the 0-3200 line as viewed from the aiming circle, use the inner black scale.

(2) If the piece is right of the 0-3200 line as viewed from the aiming circle, use the outer black scale.

(3) If the piece is on the 0-3200 line (lay deflection 3200) and behind as viewed from the aiming circle, use the inner black scale.

(4) If the piece is on the 0-3200 line (lay deflection 3200) and forward as viewed from the aiming circle, use the outer black scale.

b. For ease in determining whether a piece is left or right and forward or back, the XO need only realize that if the piece is laid using the red graduations on the aiming circle it is to the left or forward of the aiming circle as you face the azimuth of fire.

c. A simple alternative to the use of these rules is to have the XO report all lay deflections as they would be determined by using values read from the black numbered graduations of the aiming circle. It is recommended that this method be unit SOP in an attempt to avoid confusion with the plotting board.

d. The plotting of the pieces and establishment of an azimuth index are done by using the same procedures as described for the M100-series sight.

e. Piece displacement is determined by using the same procedures as described for the M100-series sight.

f. The following is an example of the platoon leader's report for the M12-series sight:

NOTE: Howitzers 3 and 4 are left of the aiming circle. Azimuth of lay (AOL) equals 4800.

g. Use the steps in Table 12-2 to plot piece locations for weapons equipped with the M12-series sight.

12-7. Determination of Base Piece Grid Coordinates

After the pieces have been plotted on the M17, the base piece grid can be determined by using the steps in Table 12-3.

NOTE: The grid determined can be used to plot the base piece on the firing chart.


Section III

Terrain Gun Position Corrections


Terrain gun position corrections are the precomputed corrections carried on the howitzers to compensate for terrain positioning and muzzle velocity differences to achieve acceptable results in the target area. TGPCs should be computed each time the firing unit occupies a position. The use of TGPCs will allow the unit to effectively engage targets whose size and orientation requires a sheaf other than a parallel sheaf.

12-8. Transfer Limits and Sectors of Fire

a. Terrain gun position corrections are most accurate at the range and direction for which they are computed. They are considered valid 2,000 meters over and short of the center range and 400 mils right and left of the center azimuth of the sector. (See Figure 12-9.)

b. Terrain gun position corrections will provide an acceptable effect on the target provided the firing unit's position is within a box 400 meters wide and 200 meters deep. This box is centered over the firing unit center and oriented perpendicular to the azimuth of lay. If the firing unit is spread out more than 400 meters by 200 meters, a degradation in effectiveness of sheafs can be expected as fires are shifted throughout the sector for which they were computed.

c. If a firing unit's area of responsibility covers an area larger than the TGPC transfer limits, the unit should compute TGPCs for other sectors. Ranges to the centers of the other sectors may be different. Overlapping sectors for different charges may be necessary. (See Figure 12-10.)

12-9. Fire Order and Fire Commands

a. The FDO must establish a fire order SOP that indicates the corrections for the primary sector are standard. This is done in the special instructions section of the fire order SOP.

b. Fire command standards should direct that the primary TGPCs sector is used unless otherwise specified. The special instructions block of the fire commands will indicate which TGPC sector will be used for a mission if other than the primary sector. The absence of any instructions in the initial fire commands indicates that corrections for the primary sector will be fired. The command LEFT (RIGHT) SECTOR in the special instructions block of the initial fire commands indicates that the corrections for the left (right) sector are to be set on the howitzers. The command CANCEL TERRAIN GUN POSITION CORRECTIONS indicates that all TGPCs are to be zeroed for the mission. After end of mission is announced, the primary sector TGPCs are reapplied to the howitzers.

12-10. Determination of Terrain Gun Position Corrections

a. It is recommended that TGPCs be computed for a converged sheaf. This will generally provide an acceptable sheaf within the transfer limits. TGPCs can be computed by using other sheafs, but the dispersion of bursts can be expected to increase as the target range varies from the range to the center of the sector. It is preferred that base piece carry no corrections. To do otherwise causes the base piece to "zero" the corrections on adjustment and reapply them during fire for effect, which may lead to error. Therefore, the aimpoint of the base piece should be the center pivot when computing TGPCs.

NOTE: DA Form 4757 (Registration/Special Corrections Work Sheet) is completed regardless of the sheaf used. Before computing any TGPCs, plot the howitzers on the M17.

b. Table 12-4 lists the steps for determining TGPCs and completing DA Form 4757.

c. The note on the bottom of DA Form 4757 lets you know to use the chart range to target wherever there is an * displayed.

d. The steps in Table 12-5 are used to determine TGPCs for all sheafs.

NOTE: Step 1 will be different for each type of sheaf. Steps 2 through 17 are common to all sheafs.

12-11. Hasty Terrain Gun Position Corrections

a. Even with well-trained FDC personnel, computing TGPCs is time-consuming. Corrections are required for firing shortly after occupation of a position. If the advance party has determined displacement and computed TGPCs, corrections will be available immediately. If the advance party has not been able to do this, hasty TGPCs must be determined and used until accurate TGPCs are computed. For the hasty solution, piece displacement is estimated and fuze corrections are ignored.

b. Hasty TGPCs computations are designed to provide a converged sheaf at the center range of the TGPCs sector.

c. Tables 12-8 through 12-13 show data for hasty TGPCs and special corrections. The data presented in these tables are:

  • Range in 1,000-meter increments.

  • Charge most likely to be fired at the listed range.

  • Deflection correction, in mils, to compensate for lateral piece displacement. These values have been determined to the nearest mil with the GST for each 20 meters of lateral displacement (20 to 200 meters).

LATERAL DISPLACEMENT (D SCALE) = HASTY TGPC DF CORR RANGE IN 1,000s (C SCALE)

  • Position range correction, in mils, to compensate for piece displacement in range. QE corrections to the nearest mil for each 20 meters of front or rear displacement (20 to 100 meters) were computed by using the range change per mil for the listed ranges.

DISPLACEMENT                   = HASTY TGPC POSITION RG CORR
RANGE CHANGE PER MIL

  • MV correction in mils to compensate for the difference in shooting strengths (battery comparative VEs). MV corrections were determined by using the following formula.

MVUCF X BTRY COM VE    = HASTY TGPC MV CORR
RANGE CHANGE PER MIL

NOTE: MVUCF is the muzzle velocity unit correction factor from the TFT, Table F, Columns 10 and 11.

NOTE: The following tables are to be used for hasty TGPCs and special corrections. They are separated by weapons system.

12-12. Determination of Hasty TGPCs

Table 12-6 gives the procedures for determining hasty TGPCs, and Table 12-7 gives the procedures for completing DA Form 4757. Figure 12-11 shows recorded hasty TGPCs.


Section IV

Special Corrections


The corrections determined by using TGPCs are valid only within the specified transfer limits and produce the sheaf for which they were computed. If a target falls outside the transfer limits or is irregularly shaped, it is necessary to compute special corrections.

12-13. Definitions and Use

Special corrections are individual piece corrections applied to time, deflection, and quadrant elevation to place FFE bursts in precise location on a target. Special corrections are used for:

  • Individual piece locations (position correction).

  • Shooting strength of each piece (calculated correction).

  • Target shape and size.

a. Knowing when to compute special corrections is as important as knowing how to compute them. Some factors that influence the use of special corrections are:

  • Time available for computation.
  • Target size, shape, and proximity to friendly troops.
  • Accuracy of target location.

b. Special corrections should be applied when and where they will increase the effectiveness of fires on the target. Because of the time required for computation, they are used only for FFE missions.

c. The special corrections are computed in a similar manner to TGPCs, the major difference being the plotting of the target. The following types of sheafs may be computed:

  • Converged sheaf.

  • A target described by grid, length, and attitude.

  • A target described by two grids.

  • A target described by three or more grids.

  • A circular target.

12-14. Computation of Special Corrections

Table 12-13 provides the steps and procedures for the computation of special corrections.


Section IV

Use of Plotting Board for Fire Mission Processing


When the use of a firing chart is not possible, the M10 or M17 plotting board and GFT or TFT may be used to compute firing data. The observer transmits the call for fire to the firing unit and describes the target location by using any of the methods of target location.

12-15. M17 Plotting Board

The steps in Table 12-14 are used to process fire missions with the Ml7 plotting board. See Figure 12-12 for the Ml7 format for processing fire missions.

12-16. Determination of Subsequent Corrections for a Laser Adjust-Fire Mission

Table 12-15 shows the steps and procedures to determine subsequent corrections for a laser adjust-fire mission.

12-17. Examples of TGPCs

a. The following is an example of the platoon leader's report for the Ml00-series sight:

NOTE: Howitzer Number 3 is the base piece.

b. The following is an example of piece displacement.

c. A completed DA Form 4757 for each sheaf (converged, open, and circular) containing TGPCs using the data listed above are shown in Figures 12-13 through 12-15.

12-18. Examples of Special Corrections

a. Using the data listed below, determine special corrections for a linear target described by a grid, length, and attitude.

GIVEN:

(1) Example of the platoon leader's report for the M100-series sight:

NOTE: Howitzer Number 3 is the base piece.

(2) Example of piece displacement.

(3) Target Grid: 432275

(4) Length: 300 M

(5) Attitude: 1,300

(8) Chart data to the center grid: Chart range 4260 Chart deflection 3452

b. A completed DA Form 4757 for the special corrections and the Ml7 plotting board are shown in Figures 12-16 through 12-18.

c. Using the data listed below, determine special corrections for a linear target described by two grids.

GIVEN:

(1) Example of the platoon leader's report for the M100-series sight:

NOTE: Howitzer Number 3 is the base piece.

(2) Example of piece displacement.

(3) Target Grids: 424275 and 427273

   (4) (Easting)
 42400
-42700
    -300
(Northing)
    27500
   -27300
      +200

(6) Center Grid:

     (Easting)
  42400
-(-150)
  42550
(Northing)
 27500
-(+100
 27400

(7) Target Length: 360

(9) Chart data to the center grid: Chart range 4920 Chart deflection 3438

d. A completed DA Form 4757 for the special corrections and the M17 plotting board are shown in Figures 12-19 through 12-22.




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