CHAPTER 14
SPECIAL CONSIDERATIONS
This chapter discusses the special procedures applied to some missions to effectively engage targets.
141. REGISTRATION AND SHEAF ADJUSTMENT
Firing the registration is the first mission that will be completed if time and the tactical situation permit.
a. Firing Coordinated and Noncoordinated Missions. Two types of registration missions are fired on the surveyed chart: coordinated and noncoordinated.
(1) Firing coordinated missions. The FDC and FO coordinate the location of the RP before the FO joins the unit to support it. Once the FO is in position, the FDC sends a message telling the FO to prepare to register RP 1. The FO sends the OT direction to the RP.
(2) Firing noncoordinated missions. The FO, upon joining the unit to support it, checks the area of responsibility and selects a point to be used as the RP. This point must be identifiable both on the ground and on the map to allow a valid eightdigit grid to be determined. The FO then sends the call for fire to register the RP.
b. Constructing Surveyed Firing Chart. The surveyed firing chart is the most accurate chart that can be constructed. It can be used to determine all the correcting factors that are needed to fire more firstround FFE missions than the other firing charts. Three items must be known to construct the surveyed chart: a grid intersection to represent the pivot point, a surveyed mortar position, and a surveyed registration point. (The construction of the surveyed chart is similar to the modifiedobserved chart.)
(1) To obtain the DOF after constructing the chart, align the mortar position with the RP. Determine the DOF to the nearest mil.
(2) To determine the mounting azimuth, round off the DOF to the nearest 50 mils.
(3) To superimpose the deflection scale, the referred deflection is received from the section sergeant. Then, construct the deflection scale in the same manner as for the modifiedobserved chart.
NOTE:  The procedure to obtain the firing data is the same as with all firing charts. 
(4) Determine correction factors after the registration has been completed. Apply these factors to all other targets within the transfer limits of the RP.
c. Obtaining Firing Data. Obtaining the firing data is the same as with any mission, except that the FO continues to adjust until a 50meter bracket is split and the last fired round is within 25 meters of the target (Figure 141). Refinement corrections are sent to the FDC and the mission is ended. Table 141 provides information to be used in setting up the plotting boards to fire a surveyed registration. (See FM 630.)
Figure 141. Splitting of a 50meter bracket.
MORTAR GRID:  0086 6158  ALT: 0520 
RP 1 GRID:  9953 5884  ALT: 0470 
GRID INTERSECTION:  01/59  
DOF:  3660 MILS  
MAZ:  3650 MILS  
REF DEF:  2800 MILS  
INIT DEF (1ST RD):  2790 MILS 
Table 141. Plotting of a surveyed registration.
d. Adjusting the Sheaf. The purpose of adjusting the sheaf is to get all mortars firing parallel. Mortars are positioned with gun No. 1 through 4 for the 81mm mortars, No. 1 through 6 for the 4.2inch and 120mm mortar (when employed as a platoon), or No. 1 through 3 for the 4.2inch and 120mm mortar (when employed as a section) from right to left as seen from behind the guns. There is normally a 10second interval between rounds. The FO needs that time to observe the impact of the rounds and to determine corrections. If the corrections are 50 meters or more (deviation left/right only), the mortar must be refired. The corrections can be plotted on the board, or the DCT (Figure 142) can be used to determine the number of mils to add or subtract from the base mortar deflection.
Figure 142. Deflection conversion table.
NOTE:  If the target has been mechanically surveyed, enter the DCT at the initial range plot. If the target is a nonsurveyed target (even if it is an eightdigit grid), enter the DCT at the final range plot. 
EXAMPLE
The sheaf of a 81mm platoon is being adjusted. No. 2 mortar conducted the registration. The FDC has requested to prepare to adjust the sheaf. The FO requests section right. The entire platoon then fires, in order, starting at the right (No. 1, 3, 4) with 10second intervals between rounds. The mortar that was used to register (No. 2) will not fire. The sheaf is adjusted perpendicular to the guntarget line. The observer notes where each round lands and sends back deviation corrections in meters; range corrections are ignored if less than 50 meters. If a deviation correction is 50 meters or more, it must be refired. Corrections to be refired should always be transmitted first by the FO.
If angle T is greater than 499 mils, each piece is adjusted onto the registration point, and the FDC computes the data for the sheaf. In adjusting the sheaf, all rounds must be adjusted on line at about the same range (within 50 meters) and with the lateral spread between rounds equal to the bursting diameter of the ammunition used. The spottings from the FO are No. 4, right 20, No. 3, left 60, and No. 1, left 30 (Figure 143). The FO then sends these corrections to the FDC; No. 3, right 60 (because it needs to be refired), No. 4, left 20 No. 4 is adjusted, and finally No. 1, right 30 No. 1 is adjusted. The No. 3 mortar is now fired, and the round impacts 10 meters right of the desired burst point. The FO would then send: No. 3, left 10, No. 3 is adjusted, sheaf is adjusted, end of mission. 
Figure 143. No. 1, No. 3, and No. 4 mortars out of sheaf.
e. Obtaining Corrections Using the Deflection Conversion Table. The computer enters the DCT at the initial chart range rounded to the nearest 100 meters: 3050 = 3100. (Remember, the RP is at a surveyed grid, and it has not moved.) Using the deflection in meters line at the top of the table, the computer finds the meters needed to correct the sheaf. Where the range line and the correction line meet is the number of mils needed to apply. He applies the mils to the base deflection. (When working with deflections, use the LARS rule.) Once the FO has given EOM, "Sheaf adjusted," the section is given, "Section, refer deflection two eight zero one (2801), realign aiming posts," (2801 was the base mortar's hit deflection). This procedure allows all mortars to be fired with the same data, and the resulting sheaf will be parallel.
f. Determining Firing Corrections. Once registration is completed, the firing corrections (range correction factor and deflection correction) are determined for use within the transfer limits of the RP (Figure 144). The computer applies correction factors to correct for nonstandard conditions (weather and equipment wear) affecting the round.
Figure 144. Transfer limits for one registration point.
Range correction is the difference in meters between the initial chart range and the final chart range for the RP. As the registration mission is fired and completed, the rounds on the plotting board may not be plotted at the point where the RP was plotted. Because of wind and weather, the rounds may have to be fired at a greater or lesser range and to the right or left of the target to hit it. As shown in Figure 145, the initial chart range to the RP was 3,050 meters; the final adjusted chart range (range used to hit the RP) was 3,200.
Figure 145. Plotting of rounds.
g. Determining Range Difference. The computer compares the initial chart range and the final adjusted chart range and subtracts the smaller from the larger. This gives the range difference. If the initial chart range is larger than the final adjusted chart range, then the range correction is a minus (). If it is smaller, then the range correction is a plus (+).
EXAMPLE
Initial chart range smaller: 3,050; final adjusted chart range: 3,200 Then, 3,200  3,050 = +150 meters. Initial chart range larger: 3,200; final adjusted chart range: 3,050 
(1) Range correction factor. The RCF is the number of meters per thousand to be applied to the initial chart range of a target within the transfer limits resulting in a range correction for that mission. Continuing the preceding example, since the ranges to other targets will not be 3,050 (range to RP), the RCF (+150) will not be correct. Therefore, other range corrections must be determined and used for other targets. Once the range difference has been determined, round the initial chart range to the nearest 100 and then express that in thousandths.
EXAMPLE
Initial chart range: 3,050. Round to nearest 100 = 3,100 expressed in thousandths = 3.1. Divide the range in thousandths into the range difference: +150. 
48.3 = +48 RCF 3.1/+150.00 124 260 248 120 93 27 
Determine to the nearest whole meter and use the sign of the range correction.
(2) Deflection correction. The deflection correction is the number of mils needed to correct the deflection to hit the target since nonstandard conditions again caused the plots on the board to be either left or right of the initial chart deflection (Figure 146). Compare the initial chart deflection and the final chart deflection and subtract the smaller from the larger.
Figure 146. Example of completed DA Form 2399 for firing a total range correction mission on the surveyed chart.
RULE: Final chart deflection (hit) larger = LEFT deflection correction; final chart deflection (hit) smaller = RIGHT deflection correction.
EXAMPLE
Hit Larger Hit deflection: 2801 Initial chart deflection: 2790 (2801  2790 = L11) EXAMPLE Hit Smaller 
Range and deflection corrections are applied to all other targets within the transfer limits of the RP.
h. Firing of a Total Range Correction Mission. The procedure for a mission on the surveyed chart is the same as with the modifiedobserved chart. However, now the firing corrections are applied to chart data to obtain command data (firing data sent to the mortars). For example, the computer assumes that the board is still set up on the information for the registration mission, and the mission in Figure 146 has been received. It is within the transfer limits.
i. Applying Firing Corrections. Once the chart data have been determined, the computer applies the deflection correction by either adding or subtracting the deflection correction to the chart data determined. When working with deflection corrections, the computer uses the LARS rule. The deflection correction must be applied to each chart deflection throughout the mission.
EXAMPLE
2715 + L11 = 2726
(1) Range correction. Determine the initial chart range, then round to the nearest hundred and express it in thousandths; for example, 2975 = 3000 = 3.0. Multiply the range in thousandths times the RCF and use the sign of the RCF: 3.0 x +48 = +144. This gives the total range correction for this target.
(2) Total range correction. The total range correction (TRC) is the total correction that must be applied to get the command range to fire the target. TRC is the range correction (RCF x range in thousandths) plus or minus the altitude correction.
EXAMPLE
Range correction = +144  25 (altitude correction) = +119 TRC
The two factors (RCF and altitude correction) are compared. If one of these factors is a negative, subtract the smaller from the larger. The sign of the larger is used for the TRC. If both factors are negative or positive, then add the two factors to get the TRC. This must be applied to every chart range to obtain command range. To enter the firing tables, the command range is rounded to the nearest 25 meters.
j. Firing Reregistration. The FDC must consider weather changes to ensure the accuracy of the firing data (firing corrections) from the surveyed chart. Two methods can be used to do this: reregistration on the RP or MET message. Of those two methods, reregistration is the better because all the unknown (nonstandard) factors are fired out. However, due to countermortarradar, determining and applying the MET messages may be safer. The choice is dictated by the tactical situation and the availability of MET messages.
(1) Fire the reregistration at the established RP using only the mortar that originally fired the registration (Figure 147). (The FDC assumes that the sheaf is still parallel; therefore, the sheaf should not need adjusting again.) The chart data are the same as with the initial registration. Apply the firing corrections to obtain the command data (Figure 148). (A blank reproducible copy of DA Form 2188R, Data Sheet, is located at the back of this manual.)
Figure 147. Example of completed DA Form 2399 for a reregistration.
Figure 148. Example of completed DA Form 2188R.
(2) The chart deflection plus or minus deflection correction equals command deflection. The chart range plus or minus the range correction plus or minus the altitude correction equals the command range.
(3) Carry out the mission the same as with the initial registration. Once the EOM, "Registration complete," has been given, determine firing corrections again.
(4) In the initial registration, the FDC compared the initial chart range and the final chart range difference. Determining the range difference after the reregistration is the same; however, now determine the final adjusted range. During the reregistration, firing corrections were applied for each round. Now apply those same corrections.
(5) Adjusted range is the final range with the correction for altitude correction deleted.
EXAMPLE
Final command range: 3,100 meters; altitude correction: 25. Final adjusted range: 3,100 + 25 = 3,125. 
The altitude correction is added since it was initially subtracted. If the altitude correction had been a plus (+), then it would have been subtracted to obtain the final adjusted range.
(6) Once the final adjusted range has been determined, compare the initial chart and the final adjusted range. Subtract the smaller from the larger to determine the RCF. The sign (+/) would be determined as with the initial registration. Again, divide the range (initial chart range rounded to the nearest 100 expressed in thousandths) into the new range correction to determine the new RCF.
(7) To determine the deflection correction, compare the initial chart deflection and the final command deflection. Subtract the smaller from the larger and determine the sign (L or R) to apply.
(8) Apply the new firing corrections to all targets that have been and are fired within the transfer limits. For those targets that are already plotted on the board, apply the new firing corrections and update the target data. (The chart data do not change. The target has not moved; only the weather has changed.)
142. MEAN POINT OF IMPACT REGISTRATION
The FDC uses MPI registration during darkness and on featureless terrain to determine firing corrections. Two M2 aiming circles or radar must be used to conduct an MPI registration. (MPI registration can also be used for reregistration.)
a. Conduct of an MPI Registration. To fire the MPI registration, the FDC must proceed as follows:
(1) Set up the M16/M19 plotting board as a surveyed firing chart (eightdigit grids to the mortar position and RP).
(2) Plot the location and altitudes of the two FO points to be used.
Because the FOs will be sending azimuth readings for the impact points of the rounds, they must see the area of the RP using the M2 aiming circle.
(3) Record all data on DA Form 2188R. To determine each FO's direction to the RP, rotate the azimuth disk until the FO's position is aligned with the RP. Read the azimuth scale to the nearest mil. To determine each FO's vertical angle, compare the altitudes of each FO's location and the RP, and subtract the smaller from the larger. This remainder is the VI, which is used to determine the vertical angle and carries the sign of the larger. Determine the range from each FO's location. Round the range to the nearest 100 and express it in thousandths. Divide the range expressed in thousandths into the VI and determine the product to the nearest whole mil. The sign (+/) of the vertical angle (VA) is the same as the VI sign (+/).
EXAMPLE
Assume that the VI is 80 for FO 1 and +50 for FO 2. The range for FO 1 is 2,525 meters; for FO 2 is 3,000 meters.  
 32 FO 1: 2525 = 2500 = 25/800 
VA: 32 mils  
+ 16.6 FO 2: 3000 = 3000 = 30/+500.0 
VA: +17 mils 
Send the direction and vertical angle to the FOs so they can set up their M2 aiming circles.
(4) To determine the firing data, align the mortar position with the RP. Determine the chart data and apply the range correction for altitude between the mortar and target. During the registration, only the range correction for altitude is used. Give the firing command to the base mortar. Three to six rounds will be fired at 10second to 20second intervals. The FO uses this interval to give himself time to determine the azimuth readings to each round. If the azimuth for one or more rounds is determined to be 50 or more mils different, then another round may be fired for each erratic round. Six rounds are needed for the most accurate MPI registration, but as few as three rounds give correction data.
(5) As the rounds are fired, the FO reads the azimuth to each round and records it. When the last round has been fired, he sends the data recorded to the FDC. Once the rounds have been fired and the readings recorded in the FDC, plot the MPI as follows:
(a) Determine the total by adding all the readings from each FO.
(b) Divide the total by the number of readings to determine the average of the readings to the nearest mil.
EXAMPLE
1 2 3 4 5 6 TOTAL 
FO 1 6104 6110 6105 6106 6107 6109 36,641 mils 
FO 2 0400 0402 0404 0405 0401 0400 2412 mils 
6106.8 = 6107 mils (average azimuth) 6/36641 
402 = 402 mils (average azimuth) 6/2412 
NOTE:  FO may send the average azimuth. 
(c) Once the average azimuth for each FO has been determined, index the average azimuth and draw a line from each FO position toward the top of the board; where the lines intersect is the MPI. Determine and record the eightdigit grid coordinates and altitude of the MPI.
b. Determination of Range Correction Factors. With the MPI and RP on the board and the altitude determined, correction factors to be applied to other targets within the transfer limits of the RP must be determined. Again, because of the effects of interior and exterior ballistics on the round, the MPI may not be plotted in the same location on the plotting board as the surveyed point. Therefore, the corrections to hit that surveyed point must be determined. These corrections are noted on DA Form 5472R, Computer's Record (MPI) (Figure 149). (A blank reproducible copy of DA Form 5472R is located at the back of this manual.)
Figure 149. Example of completed DA Form 5472R, Computer's Record (MPI).
(1) Range difference. Compare the command range to the MPI point (minus the altitude correction) and the initial chart range to the RP.
EXAMPLE
Command range MPI = M Alt 500 mils, MPI Alt 450 mils, VI = 50, Alt Corr 25. Adjusted chart range to the MPI = command range 2,650 M + 25 (to delete altitude correction, reverse the sign) = 2,675 adjusted chart range to the MPI. The sign of the range difference is determined by how the move from the MPI to the RP must be made. If the RP range is larger, the difference is a plus (+); if smaller, it is a minus (). 
EXAMPLE
Initial chart range to the RP is 2,600 meters; adjusted chart range to the MPI is 2,675 meters. 2,675  2,600 = 75 range difference 
(2) Range correction factor. Once the range difference has been determined, divide it by the chart range to the MPI rounded to the nearest 100 expressed in thousandths and round it to the nearest whole meter. The sign is the same as the range difference.
EXAMPLE
Range difference  75; chart range to MPI 2,675 meters. 
27.8 = 28 meters = RCF 2.7/75.00 54 210 189 210 
(3) Deflection correction. Compare the chart deflection of the MPI and the chart deflection of the RP (Figure 149) to determine the deflection correction. The sign of the deflection correction will be determined by how the move from the MPI to the RP must be made.
RULE:  RP deflection is greater than the MPI deflection = LEFT deflection correction. RP deflection is less than MPI deflection = RIGHT deflection correction. 
EXAMPLE
MPI chart deflection = 2810; RP chart deflection = 2790. 2810  2790 = L20 (correction to apply R20) 
The application of the correction factors to other targets, within the transfer limit of the RP, is the same as with the other registration corrections except that the sign of the corrections must be reversed.
NOTE:  The only time the corrections will be applied with the signs as determined is when the corrections are being applied to move the strike of the round from the MPI to the RP. 
143. VERTICAL INTERVAL CORRECTION FACTORS
When the mortar position is known to surveyed accuracy and a map is being used, the computer can work with altitude differences and the correction factor for those altitude differences. As noted earlier, the term used for altitude difference is vertical interval (VI).
a. Determination of Vertical Interval. The computer compares the altitude of the mortar position and the altitude of the target being engaged. If the altitude of the target is higher than that of the mortar position, then the VI will be a plus (+); if lower, it will be a minus () (Figure 1410).
Figure 1410. Altitude correction.
b. Correction for Vertical Interval. Because of the VI, a range correction must be applied to the chart range to obtain the range to be fired (command). The range correction to apply is half of the VI; it is determined to the nearest whole meter.
EXAMPLE
VI = 75 meters 1/2 = 38 meters (altitude [range] correction) 
The altitude (range) correction must be 25 meters or more to be applied. The range correction is then added to or subtracted from the chart range. If the target is higher than the mortar, the computer adds the range correction; if lower, the computer subtracts to get the altitude to be fired (command). The altitude correction is applied to every chart range throughout the mission.
NOTE:  A VI of less than 50 meters is not used when working with the modifiedobserved chart. 
c. Determination of Vertical Interval for Different Missions. When there is a difference in altitude between the mortar position and the target, a range correction is made. Since the mortar round has a steep angle of fall, corrections are made only when differences of 50 meters or more in altitude exist. The chart range is corrected by onehalf the difference in altitude expressed in meters. The correction is added when the target is above the mortar, and subtracted when the target is below the mortar. Difference in altitude can be determined from contour maps, by estimating, or by measuring the angle of sight, and by using the milrelation formula.
(1) Grid missions. The target is plotted on the map and the altitude determined. If the altitude of the target cannot be determined, then the computer assumes that it is the same as that of the mortars.
(2) Shift missions. The target is assumed to be the same altitude as the point being shifted from unless, in the call for fire, the FO sends a vertical shift (up or down). Therefore, that shift is applied to the point being shifted from, and that is the altitude of the new target.
(3) Polar missions. The altitude of the target is assumed to be the same as that of the FO's position if no vertical shift is given. If one is given, then the computer applies the shift to the FO's altitude, and that is the altitude of the new target. Once the computer has determined the altitude of the target, then it is possible to determine the VI for the mission and, finally, the altitude correction to apply. Remember, VI is the difference in altitude between the mortars and the target.
144. RADAR REGISTRATION
Radar registration is another method used by the FDC to obtain firing corrections to apply to the firing data to obtain better accuracy.
a. Two types of radar units can be used: AN/PPS5, which gives direction and distance to impact; and AN/PPS4, which gives grid of impact. The one used will determine which method the FDC will use during the registration. At the unit level, the AN/PPS5 will probably be used for the 60mm and 81mm mortars; the AN/PPS4 for the 4.2inch and 120mm mortars.
NOTE:  Registration of the AN/PPS5 is explained here for the 60mm and 81mm mortars. 
b. The M16/M19 plotting board must be set up as a surveyed firing chart. That is, the mortar position, RP, and radar site must be plotted to surveyed accuracy. The procedure for obtaining firing data is the same as with a regular registration mission. The altitude correction is the only firing correction used. Because this is a polartype mission, the VI is now obtained as with a polar mission. The firing corrections are obtained in the same manner as with the regular registration mission.
c. After the board is set up and the direction and distance from the radar to the target have been determined, the FDC informs the radar operator of this information. The radar operator then orients the radar set using the information and calls the FDC when the set is ready. Once the radar is ready, the FDC then gives the initial data to the mortar section. The base mortar will adjust and then the sheaf will be established.
(1) When the first round impacts, the radar operator sends the FDC the direction and distance to that round.
(2) The FDC then indexes that direction and plots the round at the distance sent (the plot is made from the radar position plot, using the distance sent).
(3) The FDC indexes the mortar RP azimuth and determines the spotting by comparing the round's impact plot with the RP plot. The FDC, acting as the FO, determines all spottings (Figure 1411).
Figure 1411. Determination of a spotting.
(4) Once the spotting has been determined, the FDC converts the spotting into a correction to fire the second round. He does this by reversing the signs of the spotting. He then applies that to the registration point on the azimuth of the radar position (Figure 1412).
Figure 1412. Application of correction to fire the second round.
(5) The firing data are then obtained by aligning the new plot with the mortar position.
(6) The spottings for additional rounds are spotted from the initial RP, but the corrections (spotting reversed) are applied to the last fired plot. This procedure is repeated for all adjustment rounds until a range correction of 50 meters is split.
145. FINAL PROTECTIVE FIRES
The highest priority mission for the mortar section is FPF. The FPF is a barrier of steel designed to stop the enemy. It is integrated with the other weapons of the unit being supported to cover dead space or likely avenues of approach. The FPF is a lastditch effort to stop the enemy force from overrunning the unit. Normally, it is placed not more than 200 meters in front of friendly forces; however, the exact position of the FPF is based on the tactical situation.
a. The M16/M19 plotting board can be set up as any one of the three firing charts for FPF. With regard to the area of an FPF, the 60mm and 81mm mortar platoons can fire up to three FPF (one for each mortar).
b. The target location given in the call for fire is not the location of the FPF. A 200to400meter safety factor is added to the location of the FPF by the FO. This is the location given in the call for fire.
NOTE:  The computer never adds a safety factor. 
c. An FPF adjustment can be fired in three ways:
(1) Adjust each mortar onto the FPF (most desirable method).
(2) Adjust only the dangerclose mortar, using the attitude of the target and mortar position to compute data for the other mortars.
(3) Using the attitude of both the mortar section and the FPF, compute only the data for the FPF, with no rounds being fired (least desirable method).
d. Obtaining the firing data is still performed by aligning the mortar location with the plot being engaged and using the azimuth disk and vernier scale.
NOTE:  If the FPF is within 200 meters of friendly troops, the FO should call for HE delay in adjustment (preferred method) and use the creeping method of adjustment. 
e. When adjusting each mortar, the FO may (in the call for fire) give a section left (SL) or section right (SR) to determine the dangerclose mortar. The dangerclose mortar is the one impacting closest to friendly troops.
(1) Once the danger mortar is known (Figure 1413), it is adjusted onto the FPF line.
Figure 1413. Determination of danger mortar.
(2) Once the danger mortar has been adjusted, the next mortar (No. 2) is given the danger mortar data and fired. The firing of the same data should put the impact of the next mortar 40 meters left or right of the adjusted mortar.
(3) This procedure is used for the remaining mortars until each is on the FPF line. As each mortar is adjusted to the FPF line, the data are then given to each mortar and placed on the mortar after each mission. Also, the predetermined number (unit SOP) of rounds are set aside ready to fire (Figure 1414).
Figure 1414. Example of completed DA Form 2399 for computing FPF missions.
f. When adjusting only the dangerclose mortar, the computer is given the attitude of the target in the call for fire.
(1) The FDC can determine the dangerclose mortar by indexing the target attitude and drawing a line from the initial FPF plot (given in the call for fire) 50 meters above and below (Figure 1415).
Figure 1415. Drawing FPF symbol with attitude indexed.
(2) After drawing the FPF line, the computer rotates the azimuth disk and aligns the mortar plot with the FPF plot to see which side of the line is closest to the friendly troops (Figure 1416).
Figure 1416. Determination of danger mortar.
(a) To use this method, the frontline trace of the supported unit must be plotted on the board.
(b) Once the danger mortar has been determined, that danger mortar is fired and adjusted to the FPF line.
(3) After the danger mortar is adjusted to the FPF line, the computer then indexes the FPF attitude and erases all but the last plot.
(4) Using the last plot, the computer draws the FPF symbol by extending a line 90 meters long toward the top of the board and 10 meters long from the plot towards the bottom of the board. This shows the full 100meter width of the FPF.
(5) The remaining plots for the No. 1, No. 2, and No. 3 mortars are then plotted 40 meters apart (Figure 1417).
Figure 1417. Plotting of No. 1, No. 2, and No. 3 mortars.
(6) Once the plots are on the plotting board, the computer determines the firing data for each mortar by aligning each mortar plot with its intended impact plot (Figure 1418).
Figure 1418. Alignment of each mortar with its impact point.
(7) Again, these data are placed on the mortar after each mission, and the rounds are readied to fire.
g. To compute data for FPF without adjustment, the computer indexes the attitude of the FPF line and makes a plot 40 meters above and below the FPF starting plot.
(1) The computer then indexes the attitude of the mortar section and plots the No. 1, No. 3, and No. 4 mortars 40 meters above and below the No. 2 mortar plot.
(2) Once the FPF and mortars have been plotted, each mortar is aligned with its impact plot, and the data determined.
(3) These data are given to the mortars and, again, are set on the mortars between missions.
(4) This method is used when ammunition is low and time or the tactical situation does not permit the adjustment of the FPF.
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