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ADXHZY (009629) FPF altitude 2990
Figure 3. Corrected Firing Data
( a) The computer decodes the grid location of the FPF, plots it on the plotting board, and indexes the FPF altitude at the index mark.
( b) The computer now plots the location of the No. 1, 3, and 4 mortars. Keeping the altitude indexed, the computer goes 40 meters above the plot and makes a small plot; he then goes 40 meters below the first plot and makes another plot, then moves 40 meters below this plot and makes another plot. The computer now has four plots in a row, which represent the impact points of the four mortars on the FPF (Figure 3).
STP 31-18B34-SM-TG
Performance Steps
( c) Upon rotating the disk and aligning the FPF with the mortar, the computer sees that there are less than 40 meters between plots on the gun-target (G-T) line, which precludes firing a parallel sheaf.
( d) To determine the deflection and range to each point, the computer now plots the location of each mortar on the plotting board.
( e) Using the altitude (direction through the long axis of the mortar position) of the mortar section, indexed at the index mark, the computer again goes 40 meters above, and 40 meters and 80 meters below the plot in the center of the hollow cross marking the mortar position. By rotating the disk back and aligning the FPF and the mortar positions, the computer can easily tell which are the No. 1, 3, and 4 mortar plots and which plot on the FPF is each mortar’s impact point (Figure 3). ( f) The computer aligns the plot for the No. 1 mortar and the No. 1 impact point, and
he reads the deflection. This procedure is then used to determine the deflection for the other three mortars.
( g) Using the edge of the computer’s record and alternate range scale, the computer determines the range for each mortar.
( h) This information is recorded, and each mortar is given its data for firing the FPF. 6. If a registration is conducted later and firing corrections are determined, the computer applies those
corrections to update the FPF data.
Evaluation Preparation: SETUP: At the test site, provide all materials, equipment, and information given
in the task condition statement. The test consists of two parts:
1. Compute data for an FPF adjustment by computing data for each mortar to place it in its position on the FPF.
2. Compute data for an FPF without adjustment using the altitude of the FPF given in the call for fire, and the altitude of the mortar section.
Brief Soldier: Tell the Soldier he will be tested on computing an FPF using two procedures:
1. Procedure 1: He will have all plotting equipment and materials, plus a computer’s record with the call for fire recorded, the FDC order completed, and all FO corrections recorded. He must compute all data required, and record the final firing data for each mortar on a sheet of paper that is to be given to each squad leader.
2. Procedure 2: He will have all plotting equipment and materials, plus a computer’s record with the call for fire recorded, and the FDC order completed. He must compute the data and record the firing data for each mortar on a sheet of paper that is to be given to each squad leader.
NOTE: For both procedures, there must be 500 to 1000 mils difference between the altitude of the FPF and the altitude of the mortars.
Performance Measures GO NO GO
NOTE: Not to be sequence scored.
1. Procedure 1: —— ——
a. Correctly decoded FPF location. b. Correctly plotted FPF location.
c. Correctly completed the heading and initial fire command. d. Correctly recorded rounds expended.
e. Correctly computed data for all FO corrections.
f. Correctly recorded each mortar’s final firing data on a sheet of paper. NOTE: All charges must be the lowest charge or elevation, deflection within 10 mils, and range within 25 meters or 1/8 charge.
2. Procedure 2: —— ——
a. Correctly decoded FPF location. b. Correctly plotted FPF location.
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Performance Measures GO NO GO
c. Correctly indexed FPF altitude and plotted impact points for No. 1, 3, and 4 mortars.
d. Correctly indexed the altitude of the mortar section and plotted the No. 1, 3, and 4 mortars.
e. Correctly computed the firing data for each mortar.
f. Correctly recorded each mortars’ final firing data on a sheet of paper. NOTE: All charges must be the lowest charge or elevation, deflection within 10 mils, and range within 25 meters or 1/8 charge.
Evaluation Guidance: Score the Soldier GO if all performance measures are passed. Score the Soldier
NO-GO if any performance measure is failed. If the Soldier scores NO-GO, show the Soldier what was done wrong and how to do it correctly.
References
Required Related
STP 31-18B34-SM-TG
Compute Data for Sheaf Adjustment Using a Plotting Board 071-078-0006
Conditions: 1. As the computer, given a plotting board (M16/M19) prepared for operation as a modified
observed chart; to include coordinate system; deflection scale; mortar and registration point (RP) plotted; firing table; DA Form 2399, Computer’s Record, completed for a registration mission; No. 2 pencil; deflection conversion table; and requirement to adjust the sheaf.
2. Given material and equipment as stated above and given a computer’s record with a call for fire requesting an open or converged sheaf, or a target description requiring a special sheaf, and final adjusted data for the mission.
Standards: 1. Determined firing data for sheaf adjustment to include—
a. Mil correction to 1 mil.
b. Deflections to the nearest 1 mil with a 10-mil tolerance.
2. Computed data for open and converged sheafs by determining deflection for each mortar to the nearest mil with a 10-mil tolerance.
3. Computed data for a special sheaf—
a. By determining deflection for each mortar to the nearest mil with a 10-mil tolerance. b. By determining range for each mortar to the nearest 25 meters with a 25-meter tolerance.
Performance Steps
NOTE: The information in this task applies to the 60-mm, 81-mm, and 120-mm mortars. For the 60-mm, follow the instructions for the No. 1 and 2 mortars only. For the 120-mm, add mortars 5 and 6 using the same procedures as used for No. 3 and 4 mortars.
1. Definition and use. Individual weapon corrections for deflections, fuse settings, charges, and elevations are sometimes computed and applied to achieve a special pattern of bursts. The term sheaf denotes the lateral distribution of the bursts of two or more weapons fired together. The width of the sheaf is the lateral distance between the centers of the flank bursts. The front covered by any sheaf is the width of the sheaf plus the effective width of one burst. A sheaf may be in any one of the following forms (Figure 1):
a. Parallel sheaf. A parallel sheaf is one in which the trajectories of all weapons are parallel. b. Converged sheaf. A converged sheaf is one in which the trajectories of all mortars intersect at
the target.
c. Open sheaf. An open sheaf is one in which the lateral distance between the centers of any two adjacent bursts is equal to the maximum effective width of 1 1/2 bursts (60 meters).
Figure 1. Types of Sheaths 2. Parallel sheaf.
a. After registration by the base mortar, the computer directs the forward observer (FO) to adjust the sheaf. Although the mortars are laid parallel with a compass or aiming circle, the
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Performance Steps
sheaf produced on the ground in the target area may not be parallel. This may be caused by the differences in the settling of the baseplates of the mortars or by improper boresighting.
(1) There are two methods to adjust the sheaf. The method used depends on the location of the FO with respect to the gun-to-target (G-T) line (whether angle T is greater or less than 500 mils). The angle T is the difference between the azimuth of the G-T line and the azimuth of the observer target (OT) line.
(2) Whenever possible, the computer selects an FO located near the G-T line so that angle T is less than 500 mils.
b. For a parallel sheaf, the computer issues a fire command to No. 1, 3, and 4 mortars to fire a section right (or left) with the same adjusted deflection and elevation obtained by the No. 2 mortar. The FO sends back individual deviation corrections in meters for any burst that needs correcting to place it in the proper position in the sheaf. Using the mil-relation formula or the deflection conversion table, the computer changes those corrections in meters to mils. Those corrections are then applied to the deflection on the mortars. The mortars are re-laid on the aiming posts with the corrected deflection. Another section right (left) may be fired to recheck the sheaf.
NOTE: Any mortar with a correction of 50 meters or more is refired. When a parallel sheaf is attained, the computer notifies the gunners to refer all mortar sights to a common deflection and to realign aiming posts. This common deflection is the deflection for the base mortar (No. 2) to hit the RP. The computer disregards range errors when adjusting the sheaf. The range determined for the base mortar (No. 2) is used by all mortars in the section.
c. For example, given a deflection of 2850 and a range of 1,200 meters, the FO’s corrections are—
NUMBER ONE, LEFT THREE ZERO END OF MISSION
SHEAF ADJUSTED
(1) Mortars No. 3 and 4. Since the FO reported no corrections for the No. 3 and 4 mortars, their position in the sheaf is correct. The computer determines the correct deflection for mortar No. 1 as below.
(2) Mortar No. 1. The FO’s correction of LEFT THREE ZERO (in meters) is equal to 25 mils at a range of 1,200 meters (using the mil-relation formula or the deflection conversion table). The left 25 mils is added (left add, right subtract [LARS] rule) to the deflection setting of 2850, because the FO’s correction was left, and becomes 2875 mils. The computer issues the command, NUMBER ONE, DO NOT FIRE, DEFLECTION TWO EIGHT SEVEN FIVE. The gunner of mortar No. 1 indexes the new deflection on the sight and traverses back onto the aiming posts. When the mortar is laid, the computer issues the following command:
SECTION REFER
DEFLECTION TWO EIGHT FIVE ZERO REALIGN AIMING POSTS
(3) The gunner of mortar No. 1 refers his sight to a deflection of 2850 mils and directs the ammunition bearer in his squad to realign the aiming posts without moving the mortar. All mortars are then laid parallel with a common deflection of 2850 mils. Therefore, to fire a parallel sheaf on any target, each mortar of the section is given the same deflection—the one determined for the base mortar.
NOTE: If any of the other mortars have a compensated sight picture at this time, they would also realign aiming posts.
d. The plotting board may be used to convert the FO’s corrections from meters to mils for mortars out of sheaf. This technique eliminates the need for the computer to convert the FO’s correction to mils using the mil-relation formula or deflection conversion table, and then apply the LARS rule to determine the deflection.
(1) Procedure. The FO’s correction in meters for the mortar out of sheaf is plotted as a shift from the RP with the disk oriented on the OT azimuth, and the deflection is read at the centerline of the mortars out of sheaf. After the deflection has been determined, the plot
STP 31-18B34-SM-TG
Performance Steps
is removed from the plotting board. The deflection determined would move the No. 2 mortar the distance specified by the FO in his correction. Since all mortars were fired with a common deflection, a deflection that would move the No. 2 mortar a specified distance would also move the No. 1, 3, and 4 mortars the same distance.
(2) Sample problem. The section, mounted on an azimuth of 2000 mils, has completed registration with a range of 2,450 meters and a deflection of 2749 mils. The section fires for sheaf adjustment and the FO reports, “Number three, right three zero.” The FO’s azimuth is 2150 mils. To plot the correction, the computer rotates the disk to the FO’s azimuth and makes a plot 30 meters to the right of the RP. He parallels the mortar position and the plot and determines the deflection (2737). This is the deflection that would move the No. 2 mortar 30 meters to the right. Since the No. 3 mortar has the same deflection on the sight as the No. 2 mortar, it also moves the No. 3 mortar 30 meters to the right.
e. Angle T over 500 mils. After the base mortar has adjusted on the RP, the computer and FO coordinate adjusting a converged sheaf. The computer determines the firing data to converge the sheaf on the RP.
(1) The FO adjusts the other mortars onto the RP, one at a time. As can be seen, this is a long, drawn-out mission. Therefore, adjusting the sheaf when the angle T is over 500 mils should only be used when no other method can be used.
(2) The computer orients the plotting board on the O-T azimuth. He plots each correction with the plotting board oriented on that azimuth. He considers the RP as the plot for the first round from each of the mortars. After each mortar completes the adjustment, he erases all plots for No. 1, 3, and 4 mortars. The computer orients the board to the G-T azimuth, then places a plot 35 to 40 meters (the distance between mortars) left and right of the final adjusted plot of the No. 2 mortar and another plot 40 meters to the left of the No. 3 plot for No. 4. The range arm of the M16 plotting board can be used to determine the deflection for each mortar to open the sheaf when plotting from the pivot point. To do this, the range arm is rotated to the desired position to the right of the registration point, and the deflection is determined for the No. 1 mortar. The process is repeated to the left for the No. 3 and 4 mortars.
(3) After each mortar is laid with the correct deflection to form a parallel sheaf, the section is referred to a common deflection and the aiming posts are realigned.
3. Converged sheaf.
a. The mortar section receives the following call for fire: F1A25
ADJUST FIRE POLAR
DIRECTION 5350, DISTANCE 600 MACHINE GUN POSITION WITH LIGHT OVERHEAD COVER; CONVERGE
The mortar section goes through a normal adjust mission until the FO’s final correction of, “Add 25, fire for effect.” At this point, the computer must determine a new deflection for the No. 1, 3, and 4 mortars to enable them to hit the same point as the No. 2 mortar.
b. To do this, the computer uses the mil-relation formula or the deflection conversion table to convert the meter distance between the mortars to mils.
NOTE: Under normal emplacement, the mortars are 40 meters apart, and the sheaf has been adjusted so the rounds impact 40 meters apart.
To use the mil-relation formula, the computer must have the range to the target and the distances between mortars to find the mils. He does this as follows:
Range to target — 1,200 meters Meters between mortars — 40 meters
(33.3 / W) / (R x mils) = 40 / 1.2 x mils = 33 mils and 400 / 12 = 33.3 40 meters = 33 mils at 1,200 meters.
c. To determine the new deflection for the No. 1, 3, and 4 mortars, the computer uses the LARS (left add, right subtract) rule.
STP 31-18B34-SM-TG
Performance Steps
No. 2 (base mortar) deflection—2735 mils
Since the No. 1 mortar must move to the left, the computer adds the 33 mils to the base mortar deflection.
(Base deflection (No. 2) — 2735 mils) + (Deflection change — 33 mils) = (New deflection for No. 1 — 2768 mils).
The No. 3 mortar barrel must move to the right, so the computer subtracts the 33 mils to the base mortar deflection.
(Base deflection (No. 2) — 2735 mils) + (Deflection change -33 mils) = (New deflection for No. 3 — 2702 mils).
NOTE: The No. 4 mortar must also move to the right, but it must be moved 80 meters to the right. Therefore, the computer must double the 33 mils to 66 mils.
(Base deflection (No. 2) — 2735 mils) + (Deflection change — -66 mils) = (New deflection for No. 4 — 2669 mils).
d. The range remains the same for each mortar, but each mortar receives and fires a different deflection to allow all rounds fired to land on the same point.
4. Open sheaf.
a. The FO has located a target that he believes is wider than a parallel sheaf will cover. He sends the following call for fire:
F1A25
ADJUST FIRE GRID 176543
PLATOON IN TREE LINE HEQ AND DELAY IN EFFECT OPEN SHEAF
DIRECTION 1450
b. The section goes through a normal adjust mission until the FO’s final correction of, “Drop 25, fire for effect.” At this point, the computer must determine a new deflection for each mortar to open the sheaf.
NOTE: An open sheaf is a sheaf opened half the distance between mortars. Normally, the mortars are 40 meters apart; thus, in an open sheaf, the rounds would land 60 meters apart.
(1) To determine the new deflection for the No. 1, 3, and 4 mortars, the procedure is as follows:
Range to target — 2,000 meters Meters between mortars — 40 meters W/R x mils = 40/2.0 x mils = 20 mils 40/20 = 2
40 Meters = 20 mils at 2,000 meters
(2) A result of 20 mils moves the strike of the round 40 meters, but the computer only wants to move the strike of the round 20 meters. To do this, he must use only half the
determined mils (10 mils) to move the strike of the round 20 meters for No. 1 and 3 mortars. However, for the No. 4, he must move the strike of the round 40 meters. He then applies the LARS rule.
No. 2 (base mortar) deflection—2815 mils.
(3) Since the No. 1 mortar must move to the right, the computer subtracts the 10 mils: (Base deflection (No. 2) — 2815 mils) + (Deflection change — -10 mils) = (New deflection for No. 1 mortar — 2805 mils)
The No. 3 mortar must move to the left, so add:
(Base deflection (No. 2) — 2815 mils) + (Deflection change — +10 mils) = (New deflection for No. 3 mortar — 2825 mils)
NOTE: The No. 4 must also move to the left, but it must move 20 mils (40 meters) to open.
(Base deflection (No. 2) — 2815 mils) + (Deflection change — +20 mils) = (New deflection for No. 4 mortar — 2835 mils)
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Performance Steps
a. A special sheaf is any sheaf other than an open, converged, or parallel sheaf. This can include a wide target that an open sheaf will not cover, or a target running parallel or diagonally to the G-T line.
b. Normally, a special sheaf requires a different deflection for each mortar, and each mortar fires a different range to engage the target. The FO may include the attitude (azimuth through long axis of target) of the target in the call for fire, or give the grid of the two ends of the target.
c. The computer receives the following call for fire: F1A25
FFE, SHIFT 251
DIRECTION 0710, R100, ADD 100 THREE TRUCKS HALTED ON ROAD 50 x 100 METERS
ATTITUDE 1100
In the call for fire, the FO locates the target by giving the location of the center of target and saying the target is 100 meters long. He also gives the attitude of the target, which allows the computer to accurately plot the target.
NOTE: The following data were used in setting up the plotting board: Grid intersection — 2551
Direction of fire — 0885 Mounting azimuth — 0900 Referred deflection — 2800 Mortar position grid — 24874976 (RP) point — 261508
Target (AA251) grid — 262513 Attitude (gun section) — 2450
d. The data for a special sheaf are determined using the plotting board. Each mortar must be plotted on the board, as well as each expected point of impact of the rounds.
e. The plotting board is set up as a modified observed chart (Figure 2).
(1) Using the grid intersection (2551), place a grid system on the plotting board.
(2) Plot the grid location of the mortar position and any other known points (RP, targets, and so forth).
(3) Rotate the disk and align the mortar plot and RP plot. (4) Determine the direction of fire.
(5) Determine the mounting azimuth.
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Performance Steps
Figure 2. M16 Plotting Board Set Up as a Modified Chart