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CHAPTER 4

MAJOR CONCERNS OF THE FIRE DIRECTION CENTER

This chapter contains information on some of the "tools" the FDC uses to accomplish its mission. It also discusses the methods and techniques used in FDCs to convert calls for fire into proper fire commands.

4-1. TYPES OF SHEAVES

When the mortar section or platoon engages a target, different sheaves can be used, which depend on the type of target being engaged.

a. Parallel Sheaf. A parallel sheaf (Figure 4-1) is usually used on area targets. With the parallel sheaf, the distance between impacts of rounds from two or more mortars is the same as the distance between the mortars. Also, mortars all fire the same deflection, elevation, and charge.

Figure 4-1. Parallel sheaf.

b. Converged Sheaf. The converged sheaf (Figure 4-2) is normally used on a point target such as a bunker or machine gun position. It causes rounds from two or more mortars, each firing a different deflection, to impact at the same point.

Figure 4-2. Converged sheaf.

c. Open Sheaf. The open sheaf (Figure 4-3) is normally used to engage targets that are wider than a standard sheaf can cover. With the open sheaf, the distance between impacts of rounds from two or more mortars is half again the distance between the bursts of the rounds in a standard sheaf. Normally, 81-mm and 4.2-inch mortar rounds impact 40ámeters apart, and 120-mm rounds impact 75 meters apart. Thus, in an open sheaf with 60-mm mortars, which impact 30 meters apart in a standard sheaf, rounds would impact 45ámeters apart. All mortars fire different deflections for an open sheaf.

Figure 4-3. Open sheaf.

d. Special Sheaf. The special sheaf (Figure 4-4) is normally used in an attitude mission and when needed for the FPF. With the special sheaf, each mortar has a certain point to engage. The mortars may have different deflections and elevations.

Figure 4-4. Type of special sheaf.

e. Standard Sheaf. With the standard sheaf (Figure 4-5), rounds impact within the total effective width of the bursts, regardless of the mortar formation.

Figure 4-5. Standard sheaf.

4-2. COMPUTER'S RECORD

The DA Form 2399, Computer's Record (Figure 4-6), is a worksheet used to record the FO's call-for-fire and corrections, firing data, and commands to the mortars during a fire mission. The FDC uses this form for each mission received and fired by the FDC. Instructions on how to complete DA Form 2399 are discussed below.

Figure 4-6. Example of completed DA Form 2399, Computer's Record.

a. ORGANIZATION. Unit that is firing the mission.

b. DATE. Date the mission is fired.

c. TIME. Time the mission was received (the call-for-fire recorded).

d. OBSERVER ID. Forward observer's callsign.

e. NUMBER TARGET. Number assigned to the mission.

f. WARNING ORDER. Type of warning order used for the mission (adjust fire, FFE, immediate suppression).

g. TARGET LOCATION. Method used to locate target (grid, shift from, polar).

h. TARGET DESCRIPTION. Details of target (type, size, number, protection).

i. METHOD OF ENGAGEMENT. Types of adjustment and ammunition (when used). (For more information, see FM 6-30.)

j. METHOD OF CONTROL. The adjustment gun (when named by the FO) and time of delivery (when used). (For more information, see FM 6-30.)

k. MESSAGE TO OBSERVER. Space used to record any message sent to the forward observer (when used).

l. FDC ORDER. This includes the following:

(1) MORTAR TO FFE (mortar to fire for effect)--Mortar(s) that will be used during the FFE phase of the mission.

(2) MORTAR TO ADJ (mortar to adjust)--Mortar(s) that will be used during the adjustment phase of the mission. Leave blank if the mortar to adjust is the same as the mortar to fire for effect.

(3) METHODS OF ADJ (method of adjustment)--Number of rounds used by the adjusting mortar(s) for each correction during the adjustment phase of the mission.

(4) BASIS FOR CORRECTION--Point (usually the registration point) from which the correction factors to be applied are determined (surveyed chart only).

(5) SHEAF CORRECTION--Type of sheaf, other than parallel sheaf, that will be used during the FFE.

(6) SHELL AND FUZE--Shell and fuze combination that will be used for the mission. The first line is used for the ammunition that will be fired in the adjustment phase. The second line is used for the ammunition that will be fired in the FFE if it changes from the adjustment round type. If different types of ammunition will be used during the mission, the different rounds are listed--for example:

SHELL AND FUZE: HEQ in Adj, HEQ/WP in FFE

(7) METHOD OF FFE (method of fire for effect)--Number and type rounds for each mortar in the FFE phase of the mission--for example:

METHOD OF FFE: 2 Rds HEQ, 2 Rds WP.

(8) RG/LATERAL SPREAD (range/lateral spread)--Used with illumination, with one of the following:

(a) Rg Spread: 60-mm mortar, 250 meters between rounds; 81-mm mortar, 500 meters between rounds; and 4.2-inch mortar (1,000 meters between rounds) and 120-mm mortar (1,500 meters between rounds).

(b) Lateral Spread: 60-mm mortar, 250 meters between rounds; 81-mm mortar, 500 meters between rounds; and 4.2-inch (1,000 meters between rounds) and 120-mm mortar (1,500 meters between rounds).

(c) Rg/Lateral Spread: A combination of range spread and lateral spread.

(9) ZONE--Used only with the 4.2-inch mortar. The zone will be 100 or 200 meters. A platoon-size target will be a 100-meter zone, while a company-size target will be a 200-meter zone. Should the target require it, the 4.2-inch mortar can fire a larger zone.

(10) TIME OF OPENING FIRE--The fire control for the mission.

W/R = When ready
AMC = At my command (either the FO or FDC)

The chief computer/section sergeant usually completes the FDC order. This area describes how the FDC will engage the target.

m. INITIAL CHART DATA. This includes the following:

(1) DEFLECTION--Initial deflection from the mortar position to the target being engaged (plotting board only).

NOTE:When using the M16 plotting board with the drift, the drift used will be annotated in this column. "Drift" will be placed in the left column of the initial chart data (4.2 only).

(2) DEFLECTION CORRECTION--Deflection correction used for the mission (plotting board only).

(3) RANGE--Initial range from the mortar position to the target being engaged (plotting board only).

(4) VI/ALT CORRECTION--Vertical interval/altitude difference and VI correction used for the mission (plotting board only).

(5) RANGE CORRECTION--Range correction factor total range correction (TRC) used for the mission (plotting board only).

(6) CHARGE/RANGE--Charge and corrected range used for the mission.

(7) AZIMUTH--The direction from the gun position to the target.

(8) ANGLE T--Mil difference between the GT line and the OT line. (Determine to the nearest 1 mil and record to the nearest 10 mils and transmit to the nearest 100 mils.)

n. INITIAL FIRE COMMAND. This is the first fire command that is sent to the mortar section for a mission. To complete the initial fire command, the computer must use the initial chart data, plus any corrections, and the information in the FDC order.

(1) MORTAR TO FOLLOW (mortars to follow or FFE)--The mortar(s) to follow all commands or the mortar(s) that will be used in the FFE.

(2) SHELL AND FUZE--The shell and fuze combination used during the mission. If it is an adjustment mission, that is the round used during the adjustment.

(3) MORTAR TO FIRE--The mortar(s) to be used during the adjustment phase.

(4) METHOD OF FIRE--The number of rounds to be used for adjustment and in the FFE, and the type, if mixed. Any control by the FDC would be placed here--for example:

(a) One round HEQ in adjustment; two rounds HEQ/two rounds WP in FFE, AMC. Announcing the number of rounds in the FFE gives the ammunition bearer time to prepare those rounds, such as, in the event of an immediate-suppression mission.

(b) Three rounds HEQ.

(5) DEFLECTION--The command deflection to fire the first round.

(6) CHARGE--The command charge needed to fire the first round.

(7) TIME SETTING--The time setting needed on mechanical-time fuzes (normally, illumination) to obtain the desired effects over the target area.

(8) ELEVATION--The elevation used for engaging the target (800, 900, and 1065 for 4.2-inch mortar; for 60-mm, 81-mm, and 120-mm mortars, it is the elevation obtained from the FTs and M23 MBC for the range to be fired). The elevation is also the command to fire in the absence of any type of fire control.

o. ROUNDS EXPENDED. A cumulative count of the number of rounds fired for the initial fire command.

p. OBSERVER CORRECTION. This includes the following:

(1) DEV (deviation)--The LEFT/RIGHT, in meters, sent in by the observer--for example:

DEV: L 200 = The observer wants a "left 200 meters" correction.

(2) RG (range)--The ADD/DROP, in meters, sent in by the observer--for example:

RG: "Add 200" is recorded as +200, while "Drop 200" is recorded as -200.

(3) (TIME) HEIGHT--The height correction the observer wants, usually used with illumination. For corrections in height, the observer will send UP/DOWN: "UP 200" or "DOWN 200" and record the same.

q. CHART DATA. Chart data are obtained from the M16/M19 plotting boards for the observer's requested corrections. This section is used only when firing corrections are to be applied to the chart data to obtain firing data. (Disregard this portion of the computer's record when using the MBC.)

(1) DEFL (deflection)--The deflection read from the plotting equipment before any corrections are applied.

(2) CHARGE RANGE. Chart charge (or range) read from the plotting equipment before any corrections are applied. If a range is recorded, the charge corresponding to it may be written either in the lower part of the CHG box or in parentheses in the adjoining unused MORT FIRE box.

r. SUBSEQUENT COMMANDS. The command data are sent to the mortar(s) to fire the next round(s). Those commands, DEFL/CHG/ELEV, contain chart data and all firing corrections to apply. In the subsequent fire command, the only commands that are announced are any changes from the initial fire command or the previous subsequent fire command. The elevation is always given regardless of any changes.

(1) MORTAR TO FIRE--Self-explanatory.

(2) METHOD OF FIRE--The number of rounds and type of fire.

(3) DEFL (deflection)--The command deflection(s) to fire the round(s).

(4) RANGE/CHARGE--The 4.2-inch mortar: the command charge to fire the rounds; 60-mm/81-mm/120-mm mortars: the command range used for this round(s) and the charge, if different. The range is recorded and used to determine the charge that is given to the 60-mm/ 81-mm/120-mm mortars (range is not given to mortars).

(5) TIME SETTING--The time setting needed for the mechanical-time fuze.

(6) ELEV (elevation)--The elevation used for this round(s); also, the command to fire in the absence of any fire control.

4-3. DATA SHEET

DA Form 2188-R, Data Sheet (Figure 4-7), is used by the computer to record data that pertains to the mortar section or platoon and the firing data for each target engaged. (For a blank reproducible copy of DA Form 2188-R.)

Figure 4-7. Example of completed DA Form 2188-R, Data Sheet.

a. SETUP. This block is used to record the initialization data used by the firing element.

(1) TIME OUT--Amount of time selected between switch function.

(2) TGT PRFX--Target prefix used by the firing element.

(3) TGT NO.--Target numbering block.

(4) ALARM--Alarm on and off function for messages.

(5) MIN E/MIN N--Minimum easting and northing coordinates from the map sheet.

(6) GD--East or west grid declination.

(7) LAT--Latitude from the map sheet.

(8) LISTEN--Allows message transmission and reception.

(9) BIT RATE--Message transmission rates for DMD supported missions.

(10) KEY TONE--Length of time required for a communications device.

(11) BLK--Transmit block mode for DMD-supported missions.

(12) OWNER ID--Owner identification.

b. WEAPON DATA. This block is used to record the weapon initialization data used by the firing element.

(1) UNIT--Unit mortar element is assigned.

(2) __mm CAR--Weapon type and indicates either mounted or dismounted.

(3) BP--Basepiece number.

(4) E--Basepiece easting map coordinate.

(5) N--Basepiece northing map coordinate.

(6) ALT--Altitude in meters of the basepiece.

(7) AZ--Mils of the basepiece direction of fire.

(8) DEF--Referred deflection used by the firing element.

(9) ELE--107-mm requires a selected elevation.

(10) WPN/DIR/DIS--Weapon number, direction, and distance from the basepiece. Continue to fill out until all weapons have been recorded for firing section.

c. FO DATA. This block is used to record the forward observers' locations.

(1) FO--Call sign of the forward observer.

(2) ALT--Altitude at the forward observer's location.

(3) GRID--Grid coordinates of the forward observer's location.

d. AMMUNITION DATA. This section is used to monitor the rounds. This information should be updated after each mission.

(1) TEMPERATURE--Current temperature.

(2) TYPE--Check the appropriate types of ammunition issued.

(3) LOT NUMBER--List the different lot numbers of the rounds and fuzes on hand.

(4) WEIGHT--Weight difference between types of projectiles.

(5) ON HAND--The number, by lot number, the firing element has on the firing position.

(6) RECEIVED--Number and type of rounds received.

(7) TOTAL--The combination of rounds on hand and those received.

(8) ROUNDS EXPENDED--The number of rounds expended for missions.

(9) ROUNDS REMAINING--The number of rounds remaining.

NOTE:The controlling FDC will keep the data sheet.

e. TARGET DATA. This section is used to record previously fired targets.

(1) TARGET ID. This includes the following:

(a) TGT NO (target number)--Alphanumeric identifier assigned to a target.

(b) GRID--Six- or eight-digit coordinates of a target.

(c) ALT--Altitude of the target.

(2) CHART DATA. This includes the following:

(a) DEFL (deflection)--Chart (M16/M19) or initial (MBC) deflection to the target.

(b) RG/CHG (range/charge)--Chart (M16) or initial (MBC) range and charge for the mortars needed for a target.

(3) FIRING CORRECTIONS. For the 4.2-inch mortar, column (1) is used to record the total deflection correction used during the mission. Columns (3) and (4) are used on the modified and surveyed charts only. This section includes:

(a) DEFL CORR (deflection correction)--Direction (left/right) value and number of mils to apply to the chart deflection for firing data.

(b) RG CORR (range correction)--The value and amount (+/-) of meters to apply to the chart range for firing data.

(c) ALT (altitude) VI (vertical interval)--Altitude of the target and VI difference, UP (+) or DOWN (-) in meters, between the target and the mortar altitudes.

(d) ALT CORR (altitude correction)--For all mortars, this is the number and direction (UP/DOWN) of meters used for altitude corrections that are applied. For 4.2-inch mortars, charge correction is listed that is needed for the VI. For the 60-mm, 81-mm, and 120-mm only, corrections for deflection and range are used on the modified and surveyed charts.

NOTE:If the chart data and the command data are the same, do NOT repeat the data in the range/chart block.

(4) FIRING DATA. This is the base gun command data for the targets. This information contains all corrections (when used) plus chart data to get the firing data (command data) to the center mass of the target.

(a) DEFL (deflection)--Command deflection to hit the center mass of the target.

(b) RG/CHG (range/charge)--The command range and charge to hit the target.

(c) FUZE TIME SETT (fuze time setting)--Fuze/time setting on mechanical fuzes recorded to the nearest 0.1 second.

(d) ELEV (elevation)--Elevation used to fire the round: for 4.2-inch mortars, 800, 900, or 1065; for 60-mm/81-mm/120-mm mortars, the elevation from the firing tables for the command range.

(5) INTELLIGENCE. This includes the following:

(a) TIME FIRED--The time the call for fire was received.

(b) TARGET DESCR (target description)--What the target was (from the call for fire on the computer's record).

(c) METH OF ENGMT (method of engagement)--How the target was engaged (number of mortars, number and type of rounds fired in the FFE).

(d) SURVEILLANCE--What happened to the target.

(6) ROUNDS. Rounds expended for mission and amount remaining for future missions.

4-4. ANGLE T

Angle T (Figure 4-8) is the mil difference between the OT line and GT line. Angle T is not important to the FDC when computing. However, to the FO, it must be considered when making corrections to engage a target when the angle T is between 1600 to 3200 mils.

Figure 4-8. Angle T between 400 and 1600 mils.

a. To determine angle T, the computer must compare the OT azimuth and GT azimuth, subtracting the smaller from the larger. It is determined to the nearest mil, recorded to the nearest 10 mils, and announced to the observer to the nearest 100 mils when it is 500 mils or greater. GT azimuth is the azimuth that corresponds to the initial chart deflection to the target being engaged. OT azimuth is the azimuth given in the observer's call for fire or with the first correction. If a grid mission is sent, the OT azimuth may not be given in the call for fire. However, OT azimuth must be sent before or with the first subsequent adjustment.

NOTE:The FO must send the OT azimuth in the call for fire for a shift and polar.

EXAMPLE 1

Consider OT = 2950 mils and GT = 3190 mils; then, 3190 - 2950 = 240 mils (angle T).

EXAMPLE 2

Consider OT = 6210 mils and GT = 0132 mils. Because the azimuths are on either side of 6400 (0), subtracting the smaller from the larger would not yield the angle T. The computer must add 6400 to the smaller and then subtract from the larger:

0132 + 6400
6532 - 6210
= 6532
= 322, recorded as 320
NOTE:This procedure is used only when one azimuth is between 0 (6400) and 1600, and one is between 4800 and 6400.
Angle T exceeding 499 mils:
OT = 1530
GT = 810 = Angle T 720

b. Because the angle T is over 499 mils in the example above, the FDC would then send a message to the observer that the angle T exceeded 499 mils. Otherwise, there is no need to tell the FO what the angle T is unless he requests it. The observer would use this information before making any correction. When the angle T exceeds 499 mils (Figure 4-9), the FO would continue to use the OT factor to make deviation corrections. However, if it is observed that the correction is more than asked for, the deviation corrections should be reduced proportionately during the mission. Information about the angle T is automatically given to the FO only if it exceeds 499 mils. If the FO wants to know what the angle T is, then the FDC would announce it to the nearest 100 mils.

Figure 4-9. Angle T exceeding 500 mils.

4-5. FIRING TABLES

The firing tables contained in this manual include complete instructions for their use.

NOTE:Refer to appropriate firing tables for specific rounds that are not listed in this manual.

a. The 60-mm Mortar Firing Tables (Figure 4-10).

Figure 4-10. Sample pages from firing tables for 60-mm Mortar.

(1) Parts I, II, III, and IV of FT 60-P-1 contain firing data for various rounds that use propelling charges. Each part contains five tables: Table A provides the components of a 1-knot wind; Table B provides air temperature and density corrections; Table C provides variations in muzzle velocity due to propellent temperature; Table D provides basic data and nonstandard correction factors; and Table E provides supplementary data.

(2) Part I includes the M720 HE round; Part II includes the M49A4 HE round; Part III includes the M302A1 WP round; and Part IV includes the M83A3 illumination round. The appendixes contain the trajectory charts for the M720 HE round.

(3) FT-6-Q-1 contains information for M49A4 HE, M50A3 training practice, M302A1 WP, and M83A3 illumination rounds for the M31 subcaliber assembly.

b. The 81-mm Mortar Firing Tables (Figure 4-11).

Figure 4-11. Sample pages from firing tables for 81-mm mortar.

NOTE:To round off range, look for the range at the lowest charge, then round it off to the closer range.

(1) FT 81-AR-1 contains the following information:

(a) Part I contains six parts. The first of which contains data for corrections for the HE M889 cartridge. The other five parts contain firing data for a given propelling charge using the HE M821 cartridge. Tables A, B, C, D, and E are included to provide the same data for all mortar firing tables.

(b) Part II contains four parts. It provides data for the M819 cartridge, red phosphorus. All four parts contain data for given propelling charges.

(c) The appendixes contain trajectory charts. The computer uses these charts to determine the height of a round for a given charge and the nearest 100-mil elevation the round will travel to a given range. These charts assist the computer in determining what round to use in urban combat.

(2) FT 81-AI-3 contains similar data as for the FT 81-AR-1 for the M374A2 and M374 HE, and M375A2 and M375 WP, and M301A3 illumination rounds. Also included is the section containing information on range, elevation, and maximum ordinate for the M68 training round.

(3) FT 81-AQ-1 contains similar data as for the FT 81-AR-1 for the M374A3 HE rounds.

c. The 4.2-Inch Mortar Firing Tables (Figure 4-12).

Figure 4-12. Sample pages from firing table for 4.2-inch mortars.

(1) For the 4.2-inch mortar, FT 4.2-H-2 applies to the M329A1 HE, M328A1 WP, XM630 chemical, and M335A1 and M335A2 illumination rounds. FT 4.2-K-2 applies to the M329A2 HE rounds.

NOTE:The M329A1E1 has been type-classified as M329A2.

(2) Parts I, II, III, and IV of FT 4.2-H-2 give details on the different elevations that can be used with the 4.2-inch mortar, with and without extension, for various rounds and charges. These parts also provide Tables A, B, C, D, and E, which provide the same information as in all firing tables. Part I includes the M329A1 HE round and the M328A1 WP round; Part II includes the XM630 round; Part III includes the M335A1 round; and Part IV includes the M335A2 illumination round. The appendixes contain the trajectory charts.

(3) Parts 1-1, 1-2, and 1-3 of FT 4.2-K-2 provide details of the different elevations that can be used with the 4.2-inch mortar for the M329A2 round. These parts also provide Tables A, B, C, D, and E that reflect the same information as in all firing tables. The appendixes contain the trajectory charts.

d. Short-Range Training Round Firing Tables (Figure 4-13) (can be used with the 81-mm and 120-mm mortars with M303 insert). FT 81-AR-1, C7 (PROV) contains different elevations that can be used with the M880 SRTR (Figure 4-12A). These parts also provide Tables A, B, C, and E, which provide the same information as in all firing tables.

Figure 4-13. Sample pages from firing tables for the short-range training round.

e. The 120-mm Mortar Firing Tables (Figure 4-14).

Figure 4-14. Sample pages from firing tables for the 120-mm mortar.

(1) FT 120-A-O contains the following information:

(a) There are three major sections to a firing table--Parts I, II, III. These parts also provide Table A, B, C, D, and E, which provide the same information in all firing tables.

(b) Part I includes the M934 and 933 HE round; Part II includes the M929 WP round; Part III includes the M930 illumination round.

(2) FT 120-AR-1 contains the following information:

(a) Parts I and II with elevation information for use with the 120-mm NDI ammunition.

(b) Parts I and II provide general data, ground data, and correction factors for each round. Part I includes the M57 HE and M68 WP rounds. Part II includes the M91 illumination round.

4-6. BALLISTIC METEOROLOGICAL MESSAGE

The MET message (DA Form 3675) and the computer MET message (DA Form 3677) provide the means to determine the corrections needed to the firing data so that the section has better accuracy and target effect without reregistering every two to four hours. The MET message corrections are valid until a subsequent MET message is received. It provides the information to compensate for all nonstandard conditions, such as changes in powder temperatures, projectile weight, air temperature and density, and the speed and direction of the wind between the mortar platoon and the targets.

a. Use of MET Message. To be valid, the MET message must be received along with the initial registration mission. The FDC should request a MET message as soon as possible after setting up the surveyed firing chart to ensure that the first MET message will be current. This message alone is not adequate to determine firing corrections. However, it can inform the FDC of how much of the registration corrections are due to weather. After the first MET message is received and computed, a second MET message should be received within four hours. This message is computed, the two are compared, and the data are determined for updating the firing equipment.

b. Source of MET Message. The MET message is received from the corps FA target acquisition battalion and is usually transmitted by FM radio to battalion. Battalion headquarters then sends the message down to the FDC. Prior coordination with the target acquisition battalion will ensure that the FDC receives the MET in ballistic format instead of computerized format.

c. Receipt of MET Message. The MET message is broadcast in six-character groups. These groups are shown in Figure 4-15 for ease of explanation. Examples of completed DA Form 3675 and DA Form 3677 are given in Figures 4-16A and 4-16B, using the same six-character groups to show how they are entered into the form. The message has two parts: the introduction and the body.

Figure 4-15. Six-character groups.

Figure 4-16A. Example of completed DA Form 3675, Ballistic MET Message.

Figure 4-16B. Example of completed DA Form 3677, Computer MET Message.

(1) Introduction. The first four groups of six characters in the MET message are the introduction, identifying the type of message and the MET station transmitting the message. This is what the character groups mean:

(a) GROUP 1: MET B 31. (METCM) for computer MET.
MET - indicates that the transmission is a MET message.
B - type of fire; indicates that the message is a ballistic MET message.
3 - indicates that the message is for surface-to-surface fire. For use with mortars, the number 3 must appear.
1 - indicates the octant of the globe in which the MET message applies. When code 9 is sent for the octant, the area is in code and not in numbers--for example, MIF MIF.

NOTE:Octants are further defined in the firing tables.

(b) GROUP 2: 344985.
344 - indicates the latitude of the center of the area, expressed to the nearest tenth of a degree.
985 - indicates the longitude of the center of the area, expressed to the nearest tenth of a degree.

(c) GROUP 3: 071010.
07 - indicates the day of the month.
101 - indicates, to the nearest tenth of an hour, Greenwich mean time (GMT), the hour the period of validity begins.

NOTE:To convert GMT to the standard time, see FM 6-15.

0 - indicates the duration of the MET message. For US armed forces, the MET data are presumed valid until a later message is received.

(d) GROUP 4: 049982.
049 - indicates, in tens of meters, the altitude of the MET station above sea level.
982 - indicates the atmospheric pressure at the MET datum plane (MDP). This value is rounded to the nearest one-tenth of a percent of standard atmospheric pressure at sea level. When this value is 100 or greater, the initial digit 1 is omitted.

(2) Body. The next group of six-character blocks is the body, containing the MET data listed by line number. The relationship of the line numbers and zone heights to the meteorological datum plane is shown in Figure 4-17. The remaining 16 lines of the body are the same form and contain the same information. The use of all 16 lines is not applicable for mortars, because of the height that the mortars can fire. Only the first seven lines (00-006) need be recorded (Figure 4-18). The character groups that compose the body shown in Figure 4-15 are interpreted as follows:

(a) 002618.
00 - the line number indicating the standard height relative to the MDP.
26 - the direction from which the ballistic wind is blowing (measured clockwise from north). This is in hundreds of mils; that is, 2600 mils.
18 - the ballistic wind speed to the nearest knot, that is 18 knots.

(b) 009976.
009 - the ballistic air temperature to the nearest 0.1 percent of standard. The initial digit 1 is omitted when the value is 100 or greater.
976 - the ballistic air density to the nearest 0.1 percent of standard. As with temperature, the initial 1 is omitted when the value is 100 or greater.

Figure 4-17. Line number and zone height relative to meteorological data plane.

Figure 4-18. Example of completed first seven lines for DA Form 3675.

d. Recording of the MET Message. As the MET message is sent, it is recorded on DA Form 3675 (Ballistic MET Message) (Figure 4-18) and DA Form 3677 (Computer MET Message) (Figure 4-16B). If during the transmission something is missed or recorded wrong, the format of the form allows the computer to ask for that portion of the message to be repeated.

e. MET Message Computation. Using DA Form 2601-1 (Figure 4-19) after the MET message has been recorded, the FDC computes the MET and determines the corrections to apply for updating the firing equipment. Known data are recorded in the proper spaces on the form. These are data available at the mortar platoon or section (obtained from the data sheet or section sergeant) and are interpreted as follows:

Figure 4-19. Example of completed DA Form 2601-1, MET Data Correction Sheet for Mortars.

(1) CHARGE--the command charge used to hit the RP. This charge is used to determine the line number to be used for computing the message.

(2) CHART RANGE--the command range from the mortar platoon or section to the RP.

NOTE:The reason for using the command charge and range is that this puts the round at its highest ordinate for that range, which is where the round is affected most.

(3) ELEVATION--the elevation used to hit the RP.

(4) ALT OF MORTARS--the altitude of the mortar platoon or section to the nearest 10 meters.

(5) LINE NUMBER--used for the MET and can also be recorded before the MET message is received. To do so, the computer enters the firing tables as follows:

(a) For the 4.2-inch mortar, at the elevation used during the registration: go to column 2 and find the command charge, then go to column 6. The number at that charge in column 6 is the line number.

(b) For the 60-mm, 81-mm, or 120-mm mortars, at the command charge: go to column 1 (range) and find the command range, then go to column 5. The number at that range in column 5 is the line number.

(c) Once the MET message has been received and recorded, record the introduction and information from the line number being used.

(d) Since the altitude of the MDP is in tens of meters and the wind direction is in hundreds of mils, change them to read their actual values. Once this is done, determine the MET values (the corrections for this MET).

(6) DIRECTION OF FIRE--the azimuth to the RP to the nearest 100 mils.

(7) POWDER TEMP--the temperature of the propellents. If the temperature of the powder cannot be determined, air temperature at the platoon or section can be used.

(8) WT (weight) OF PROJECTILE (4.2-inch mortar)--the weight of the ammunition used during the registration mission. The weight is expressed in squares, and two squares (2[]) has been set as the standard. If the section has different types of ammunition, the same weight projectile must be used during the registration.

f. Air Temperature and Air Density Corrections. To determine the corrected values for air temperature and air density, the computer must first determine where the platoon or section is in relationship to the MDP (difference in H correction). To do so, the altitude of the section and the MDP are compared, and the smaller is subtracted from the larger. The remainder is the height of the platoon or section above or below the MDP.

NOTE:If the altitude of the section is above the MDP, the sign is plus (+); if below, the sign is minus (-).

(1) Once the distance above or below the MDP is known, the computer can enter Table B (Figure 4-20), which shows the correction that must be applied on the MET data correction sheet (Figure 4-19) to the ballistic AIR TEMP AIR DENSITY. This compensates for the difference in altitude between the platoon or section and the MDP, and determines the corrections for AIR TEMP (difference in T) and AIR DENSITY (difference in D). Those corrections modify the values of AIR TEMP and AIR DENSITY determined at the MDP to what they would be at the mortar platoon or section. Corrections for difference in T and difference in D are arranged in four double rows in the table.

Figure 4-20. Sample page from firing table for air temperature and density corrections.

(2) The numbers 0, +100-, +200-, and +300- in the left column of the table represent difference in H expressed in hundreds of meters. The numbers 0 and +10- through +90- across the top represent difference in H in tens of meters. The corrections can be found where the proper hundreds row crosses the proper tens column. The numerical sign of the corrections is opposite of the difference in H sign.

EXAMPLE

Assume that the difference in H is -30, the corrected value for the difference in H is +0.1, and the difference in D is +0.3 (enter a 0 in hundreds column, go across to +30-column). Those corrections entered on DA Form 2601-1 and the corrected values can then be determined and recorded in the proper spaces (Figure 4-19).

g. Wind Component Corrections. To determine the corrections for wind components, the computer compares the DIRECTION OF WIND (MET) and the DOF (Figure 4-19). If the direction of wind is less than the DOF, he adds 6400 mils and then subtracts the DOF.

EXAMPLE

DOF 4300, DIRECTION OF WIND (MET) 2900: 2900 + 6400 = 9300 - 4300 = 5000 mils (chart direction of wind).

The remainder (CHART DIRECTION OF WIND) is then used to enter Table A (Figure 4-21) at the CHART DIRECTION OF WIND. Table A divides a 1-knot wind into crosswind and range wind components to show the effect on a round in flight. The chart direction of wind is the angle formed by the DOF and direction of wind. The computer reads across that row to find the crosswind and range wind components. Those are recorded in the proper spaces in DA Form 2601-1. Once the wind components have been determined, the computer determines crosswind and range wind corrections.

Figure 4-21. Sample page from firing table for wind components.

(1) Crosswind (deflection correction). Multiply the component of the wind speed (Table A) by the wind velocity (MET). This yields the lateral wind. Once the lateral wind is determined, enter Table D (Figure 4-22), go to column 7 (60-mm/81-mm/120-mm mortars) or column 9 (4.2-inch mortar), and find the correction factor. Record the correction factor in the proper space. Then, multiply the correction factor by the lateral wind, carry the sign of the component (left/right), and determine the product to the nearest mil. That is the deflection correction for this MET. Record it in the proper space on DA Form 2601-1.

Figure 4-22. Sample pages from firing table for basic data and correction factors.

(2) Range wind. Multiply the component by the wind speed. Carry the sign of the component (H or T from Table D), determine to the nearest 0.1 mil, and record it in the proper space on DA Form 2601-1.

h. Range Corrections. All values should be recorded in the proper spaces except DV, which is found as follows: The computer enters Table C (Figure 4-23), which shows the corrections to muzzle velocity for various temperatures of the propellent charges. He finds the temperature closest to that recorded for the propellent; DV appears in the center column on the same line as the temperature. The computer records that value in the proper space. Then he determines the amount by which all the known values vary from the standard values upon which the firing tables are based.

Figure 4-23. Sample page from firing table for propellant temperature.

NOTE:Within the firing tables: D = decrease from standard, and I = increase from standard.

(1) Once those variations are determined, enter the firing table at Table D (Figure 4-22) (command charge and elevation, 4.2-inch mortars; command charge and range, 60-mm/81-mm/ 120-mm mortar), go to columns 8 to 15 (60-mm, 81-mm, and 120-mm) or 10 to 17 (4.2-inch mortar) and record the unit corrections for each variation.

NOTE:The sign of the unit correction must be recorded; numbers without a sign are a plus (+). If the column ends, the last listed numbers are considered to continue.

(2) Once the variations have been recorded, multiply the variations from standard by the unit corrections and place the result (rounded to the nearest whole meter) in the column with the same sign as the unit correction. Once all corrections have been multiplied, compare the minus (-) and plus (+), subtract the smaller form the larger, and use the sign of the larger. Determine the result to the nearest meter for 60-mm/ 81-mm/120-mm mortars, or to the nearest 10 meters for 4.2-inch mortars, and record in the proper space.

4-6.1 COMPUTER MET MESSAGE

The MBC, M23 does not use the same ballistic MET that FDC personnel use when plotting manually with the M16 plotting board. Instead, the computerized MET is used. Artillery also uses the computerized MET. When no MET is available the MBC uses the standard MET that is stored within the MBC, M23. The MET menu has two main options: new and current. When a new MET message is received, it is entered into the computer using the new option in the MET menu. Only after the update * option is selected does the new MET become the current MET and is applied to the firing data.

METEOROLOGICAL: NEW
QUADRANT: 0
LATITUDE: 322
LONGITUDE: 845
DATE: DAY: 02
TIME: 100
DURATION: 0
STATION HEIGHT: 014
ATMOSPHERIC PRESSURE: 003

00
01
02
03
04
05
06
07
08
231
200
230
185
000
074
057
067
070
002
008
013
009
000
013
023
027
029
2957
2937
3013
2980
2940
2935
2931
2897
2861
1003
0902
0064
0921
0868
0820
0074
0730
0688
NOTE:Refer to Figure 4-16B.

a. Press the MET switch. "MET: NEW CURRENT" is displayed. Using multiple choice entry select: NEW.

(1) Press SEQ switch. Using numeric entry, enter the quadrant. Enter 0.

(2) Press SEQ switch. Using numeric entry, enter the latitude and longitude. Enter 322 and 845.

(3) Press SEQ switch. Using numeric entry, enter the day of the month and time of the MET message (CMT). Enter 02 and 100.

(4) Press SEQ switch. Using numeric entry, enter the duration, station altitude and atmospheric pressure. Enter 0, 014, and 003.

(5) Press SEQ switch. Using numeric entry, enter for line 00 wind direction and speed. Enter 231 and 002.

(6) Press SEQ switch. Using numeric entry, enter line 00 temperature and air pressure. Enter 2957 and 1003.

b. Using the procedures above, repeat steps (5) and (6) to line 8.

c. Press SEQ switch. After line 8; UPDATE MET * is displayed. Using the multiple choice entry, select the flashing asterisk (*) to update the NEW MET stored in the MBC, placing the NEW MET in the CURRENT MET file, while retaining a copy in the NEW file. Sequence to ready.

NOTE:The MBC, M23 calculates the effect of each line of the MET on the round when determining firing data. Changes to the MET files can only be made to the new MET file, then updated to the current file.

(1) To check MET, enter MET switch and select current (CURR) and review the MET message.

(2) If a change is needed, enter NEW and make the needed corrections, then select UPDATE MET *.

4-7. THE 6400-MIL MET MESSAGE

The target area is usually larger than the transfer limits of the RP, and yet time, ammunition, and the tactical situation will permit firing only one registration.

a. By assuming negligible error in survey or maps, lay of the weapons, and preparation of the plotting boards or MBC computer, the FDC can divide the registration corrections for the RP into two parts. The first part is a correction that is only a function of the range fired, and it is constant for a given range, regardless of direction. The second part is a function of the direction fired.

b. If the amount of the concurrent MET computed for the RP is subtracted from the total registration correction, the result is an absolute registration correction that does not change with the direction fired or the weather. The FDC can then plot an imaginary RP at the same range as the original RP, but in other directions (usually 800 mils apart), compute a MET correction for each of those directions, and, by adding the different MET corrections to the absolute registration correction, determine different firing corrections for each of the imaginary RPs. The firing corrections determined for the imaginary RPs can then be applied when engaging targets within their transfer limits.

4-8. COMPUTATION OF MET CORRECTIONS FOR LARGE SECTOR CAPABILITY

A special worksheet, such as DA Form 2601-2-R, MET Data Correction Sheet 6400 Mils (Mortars) (Figures 4-24 and 4-25), is needed to compute multiple MET from single registration. The supplemental (imaginary) RPs are spaced 800 mils apart, extending to the right and left of the RP as far as needed to cover the sector of responsibility. DA Form 2601-2-R shows a full 6400-mil capacity. On the firing chart, all of the imaginary RPs are plotted at the same range from the mortar position as the real RP. Computation of the MET corrections are described herein. (For a blank reproducible copy of DA Form 2601-2-R, see the back of this publication.)

Figure 4-24. Example of completed DA Form 2601-2-R, MET Data Correction Sheet 6400 Mils (Mortars).

Figure 4-25. Example of completed DA Form 2601-2-R for a full 6400-mil capacity.

a. Complete the top section of the sheet and compute the difference in H corrections and the corrected values for AIR TEMP and AIR DENSITY in the usual way.

b. Determine the CHART DIRECTION OF WIND as on a normal MET. Copy the result into the box marked I (RP) and as many others as there are imaginary RPs (II is 800 mils clockwise from the RP, and the numbers increase in a clockwise direction to VIII, which is 800 mils counterclockwise from the RP).

c. Add the directional variations to the CHART DIRECTION OF WIND subtracting 6400 if necessary to keep the result less than 6400.

d. Copy the wind velocity into the first row of boxes under DEFLECTION CORRECTIONS and RANGE CORRECTIONS. Do not use any column that does not have the CHART DIRECTION OF WIND written on top.

e. From Table A (Figure 4-21), extract the appropriate crosswind component (record it in the DEFLECTION CORRECTIONS section) and range wind component (record it in the RANGE CORRECTIONS section) for each value of chart wind to checkpoints.

f. Multiply the velocity by the components to get values for crosswind and range wind.

g. Find the crosswind correction factor in Table D, (column 7, 60-mm/81-mm/ 120-mm mortars; column 9, 4.2-inch mortar) corresponding to the adjusted RP charge. Multiply it by the crosswind to get the MET DEFLECTION CORRECTION.

h. Find the proper range wind unit correction in Table D, (columns 10 and 11, 60-mm/81-mm mortars; columns 12 and 13, 4.2-inch mortar). Multiply it by the range wind to get the RANGE WIND CORRECTION.

i. Compute the MET RANGE CORRECTIONS for POWDER TEMP, AIR TEMP, AIR DENSITY, and PROJECTILE WT in the usual manner. The net of the four is the ballistic range correction.

j. Combine the ballistic range correction with the various range wind corrections to obtain the total range corrections.

k. Obtain the total MET corrections by bringing together the MET RANGE CORRECTION and the MET DEFLECTION CORRECTION for each of the points.

l. Determine the absolute registration correction. First, calculate the registration correction. The registration range correction is the difference between the chart range to the RP and the range corresponding to the initial range at the RP; it is plus if the chart range is smaller. The DEFLECTION CORRECTION is the LARS (left, add; right, subtract) correction, which must be applied to the initial deflection read at the RP to get the firing deflection that hit it. The RP MET correction, which has been recorded under I (RP), is then subtracted from the registration correction; the result is the absolute registration correction.

m. Add the absolute registration correction to each point MET correction to obtain the corrections to apply at the points.

4-9. METEOROLOGICAL (MET) CORRECTIONS

To place fire on a target without adjustment, the FDC must know the exact location of the target. He must be able to compensate for all nonstandard conditions. Registration and reregistration are the most accurate methods for determining and maintaining firing corrections, but reregistration is not always practical. The ballistic MET message helps to determine corrections due to changes in conditions that affect the flight of rounds during the periods between registrations. Those conditions include changes in powder temperature, air temperature, air density, and the speed and direction of the wind. The FDC assumes that all other factors remain relatively constant until the section displaces.

a. Corrections computed from the MET message are not adequate firing corrections alone. To be of value to the FDC, a valid MET message must be received along with (or within four hours) the registration. The registration corrects for all nonstandard conditions. A MET message received and computed along with the registration tells the FDC how much of the total registration correction is due to weather. By comparing the corrections from a later MET message, the FDC can modify the registration corrections to account for changes in weather. Therefore, the use of MET corrections eliminates the need for reregistration.

b. For MET corrections to be of use, the FDC must receive two MET messages. The corrections from the two are compared to determine the current corrections to update the firing corrections determined from the registration. Once the two messages are computed, the correcting areas (deflection correction and range correction) are compared, and the product is used to update the registration corrections.

EXAMPLE
(Figure 4-26)

Assume that--
MET 1:

MET 2:
Deflection correction L20
Range correction -100
Deflection correction R10
Range correction +25
Place the correction from the MET messages on a MET cross.

Figure 4-26. Updated registration corrections.

c. The MET cross helps answer three key questions:

  • Where are you? L20 - 100 (MET 1)
  • Where are you going? R10 + 25 (MET 2)
  • What is required to get there?

(1) Deflection correction. To get from L20 to an R10, first go from L20 to 0, then right to R10; in doing so, you went R20 then R10 for a total of R30.

(2) Range correction. To get from a -100 to a +25, first go from -100 to 0, then up the scale to +25; in doing so, you went +100 then +25 for a total correction of +125.

EXAMPLE
(Figure 4-27)

MET messages on the same side of the MET cross. Assume--
MET 1:

MET 2:
Deflection correction L30
Range correction +50
Deflection correction L40
Range correction +75
Deflection correction L30 + L40 = L10
Range correction +50 + +75 = +25

Figure 4-27. Deflection and range corrections.

Use the same procedure -- "Where am I?" "Where am I going?" "What is required to get there?" each time to determine the corrections. Remember, MET 1 is compared to MET 2, MET 2, to MET 3. This procedure continues as long as MET messages are received and as long as the unit remains in the same position.

d. Once the MET corrections have been determined, the FDC can then determine the corrections to use for updating. MET is based on the RP, and therefore the corrections from the MET messages are applied to corrections determined from the registration.

(1) Range correction. Compare the range correction from the RP and the MET range correction. For difference signs, subtract the smaller from the larger and use the sign of the larger for the new range correction for the RP. If signs are the same, add the values.

EXAMPLE

Range correction from the registration +150
Range correction from the MET +50
+150 + 50 = +200 range correction

(2) Range correction factor. Once the range correction has been determined, to determine the RCF, divide the initial chart range (rounded to the nearest hundred and expressed in thousandths) into the range correction.

EXAMPLE

New range correction: +200
Initial charge range: 3,050
 
(100's = 3100;
+64.5
+3.1/ +200.0
1000's = 3.1)
= +65 RCF
 
  Deflection correction from registration
Deflection correction from METs
L12
R10
L2 = DEF CORR

(a) Once the new corrections have been determined, the FDC can update the data sheet (RP and previously fired targets). Because the chart is based on the RP, the first target to update is the RP.

(b) Chart data remain the same because the known points have not moved. The MET message only told the FDC what is needed because of the weather changes. Apply the new corrections to the chart to obtain the new command data (Figure 4-28).

Figure 4-28. Example for updating target data.

(c) For previously fired targets, chart data remain the same. Apply the new corrections to obtain the new command data. To obtain the range correction, multiply the new RCF by the range (rounded to the nearest hundred and expressed in thousandths) (Figure 4-28). (For a blank reproducible copy of DA Form 2188-R.)

(d) For new targets within the transfer limits of the RP, apply the new corrections the same as the previous registration corrections.



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