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6.35 ARTILLERY CANNON GUNNERY HEADQUARTERS DEPARTMENT OF THE ARMY F M 6-40  REGRADED "APPROVED FOR PUBLIC RELEASE". SEE LAST TWO PAGES OF THIS PDF.

FM 640 1984 OBSOLETE Field Artillery Cannon Gunnery

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    6.35

    ARTILLERYCANNON

    GUNNERY

    HEADQUARTERSDEPARTMENTOF THE ARMY

    FM 6-40

    EGRADED "APPROVED FOR PUBLIC

    ELEASE". SEE LAST TWO PAGES OF

    HIS PDF.

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    Field ManualNo. 6-40

    *FM 6-40HEADQUARTERS

    DEPARTMENT OF THE ARMYWashington, DC, 7 December 1984

    FIELD ARTILLERY CANNON GUNNERY

    Table of Contents

    PREFACE

    CHAPTER 1.

    CHAPTER 2.

    CHAPTER 3.

    Section I.

    Section II.

    GUNNERY TEAM

    1-1 Observer1-2 Fire Direction Center1-3 Firing Battery1-4 Communications Links

    GUNNERY PROBLEM

    2-1 Gunnery ProblemSolution

    2-2 Requirements forAccurate Predicted Fire_

    BALLISTICS

    Interior Ballistics

    3-1 Description3-2 Factors Affecting

    Muzzle Velocity3-3 MuzZle Velocity

    Measurement

    Exterior Ballistics

    3-4 Description 3-53-5 Trajectory Elements 3-53-6 Round-to-Round

    Variations 3-7

    3-7 Dispersion Rectangles 3-73-8 Range Probable Error 3-83-9 Deflection Probable

    Error_3-93-10 Vertical Probable

    Error_3-103-11 Height-of-Burst

    Probable Error 3-10

    *This publication supersedes FM 6-40, 1 December 1978. In addition, this publication rescinds DA Form4207, October 1978.

    Page

    1-11-1

    1-21-2

    2-1

    2-1

    3-1

    3-1

    3-3

    3-4

    3-5^. r-

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    CHAPTER 4.

    CHAPTER 5.

    Section I.

    Section II.

    Section III.

    CHAPTER 6.

    Section I.

    FIRE DIRECTION CENTEROPERATIONS

    4-1 BCS/Manual FDC_4-2 FADAC/Manual FDC4-3 Manual FDC4-4 Battalion FDC4-5 3x8 Operations4-6 Firing Data Checks

    SURVEYED FIRING CHARTS

    Types of Surveyed FiringCharts

    5-1 Description5-2 Construction of Firing

    Charts

    Plotting Equipment and Preparationof Firing Charts

    5-3 Chart Operation5-4 Equipment5-5 Point Plotting5-6 Tick Marks5-7 Construction of

    Azimuth Indexes5-8 Construction of

    Deflection Indexes

    5-9 6,400-Mil Charts5-10 Methods Used in

    Plotting Targets

    Chart Data

    5-11 Chart Range andChart Deflection

    5-12 AngleT5-13 Subsequent

    Corrections5-14 Manual-to-Computer

    or Chart-to-Chart Checks_FIRE ORDER/FIRE COMMANDS/MESSAGE TO OBSERVER

    Fire order6-1 Definition__________6-2 Considerations in

    Attacking a Target6-3 Fire Order Elements_____6-4 Issuing the Fire Order_____

    FM 6-40

    Page

    4-14-34-5

    4-64-7

    4-8

    5-1

    5-1

    5-2

    5-2

    5-25-25-55-8

    5-10

    5-12

    5-15

    5-16

    5-18

    5-185-18

    5-19

    5-19

    6-1

    6-1

    6-16-26-3

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    Section II.

    Section III.

    CHAPTER 7.

    Section I.

    Section II.

    Section III.

    Section IV.

    6-5 Battalion Fire Orders

    6-6 Battery-Level FireOrders

    Fire Commands

    6-7 Definition6-8 Fire Command

    Elements6-9 Standing Operating

    Procedures

    6-10 Other Fire Commands6-11 Reports6-12 Repetition and Correction

    of Fire Commands

    Message to Observer

    6-13 Definition

    6-14 Additional Information

    FIRING TABLES

    Graphical Firing Tables

    7-1 Means of DeterminingFiring Data

    7-2 Description and Use ofthe Low-Angle GFT

    7-3 Construction of GFTSettings

    7-4 Determination of FiringData by Use of the GFTSetting

    Graphical Firing Table Fan

    7-5 Description7-6 Construction of a GFT

    Setting on a GFT Fan7-7 Determination of Firing

    Data With the GFT Fan

    Tabular Firing Tables

    7-8 Standards7-9 Elements and Purpose

    of the TFTGraphical Site Ta b l e s _ _ _ _ _ _ _ _

    7-10 Description7-11 Determination of Vertical

    Interval

    FM 6-40

    Page

    6-4

    6-4

    6-6

    6-6

    6-7

    6-86-86-9

    6-9

    6-9

    6-96-14

    7-1

    7-1

    7-1

    7-3

    7-4

    7-7

    7-7

    7-7

    7-8

    7-8

    7-8

    7-9

    7-9

    7-9

    7-11

    iii

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    FM 6-40

    Page

    7-12 Computations With theGST 7-11

    7-13 Determination of Angle of Siteand Vertical Angle Withouta GST_7-13

    7-14 Determination of VerticalInterval Without a GST _ 7-13

    7-15 Determination of ComplementaryAngle of Site Withouta GST_7-13

    7-16 Determination of SiteWithout a GST 7-14

    7-17 Average Site 7-14

    CHAPTER 8. REPLOT DATA

    Section I. Manual Procedures_8-28-1 Point-Detonating and

    Variable Time Fuze(M728) Replot 8-2

    8-2 Variable Time Fuze(M513/M514) Replot 8-5

    8-3 Mechanical Time FuzeReplot 8-6

    Section I1. Computer Procedures 8-88-4 FADAC Replot 8-8

    8-5 Battery ComputerSystem Replot 8-8

    CHAPTER 9. FIRING DATA PROCEDURESSection I. High-Explosive Munitions 9-3

    9-1 Characteristics9-39-2 Computations for High-

    Explosive Projectiles 9-39-3 Sample Mission 9-49-4 APICM Projectiles 9-6

    9-5 Smoke Projectiles 9-99-6 IlluminatingProjectiles 9-13

    9-7 Illuminating ProjectileManual Procedures ______ 9-14

    Section II. Dual-Purpose Improved ConventionalM u n i t i o n s _ _ _ _ _ _ _ _ _ _ _ _ _ 9-18

    9-8 Characteristics________ 9-189-9 DPICM Projectiles 9-18

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    FM 6-40

    Page

    9-10 DPICM Computations by Useof the GFT and TFT 9-18

    9-11 Computer Proceduresfor DPICM 9-21

    9-12 FASCAM Projectiles 9-219-13 FASCAM Employment 9-22

    9-14 FASCAM Computations 9-229-15 Aimpoint Selection

    Tables 9-24

    9-16 Location of Aimpoints 9-289-17 Number of Projectiles

    per Aimpoint 9-28

    9-18 Manual DataDetermination 9-29

    9-19 M825 Projectile 9-319-20 M825 Computation by Use of

    the GFT and TFT 9-31

    Section III. Rocket-Assisted Munitions 9-35

    9-21 Characteristics 9-35.9-22 Manual Computation 9-369-23 Registration and Determination

    of the GFT Setting 9-38

    Section IV. Nuclear Munitions 9-38

    9-24 Characteristics- 9-38

    9-25 M753 Delivery Techniques 9-399-26 Manual Data for

    K-Transfer Technique(M753) 9-39

    9-27 Met + VE Technique 9-439-28 Observer Adjustment

    Technique 9-459-29 M422A1 Delivery

    Technique 9-469-30 M422A1 Computer

    Procedures 9-489-31 155-mm Howitzer Nuclear

    Gunnery 9-509-32 155-mm Nuclear Delivery

    Techniques 9-50

    9-33 Manual Procedures for ShellNuclear by Use of the GETand TET _________ 9-50

    9-34 Nuclear Fire MissionProcessing 9-51

    V

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    FM 6-40

    PageSection V. Copperhead 9-52

    9-35 Characteristics- 9-529-36 Computations for Shell

    Copperhead 9-54

    9-37 Mission Priority 9-559-38 Copperhead SOP 9-559-39 Voice Calls for Fire 9-559-40 Message to Observer

    (Autonomous) 9-569-41 Laser Pulse Repetition

    Frequency Code 9-569-42 Fire Order 9-569-43 Computation of Firing Data 9-569-44 Angle T and Target Cloud

    Height Check 9-579-45 Determination of the Range

    Offset Correction 9-589-46 Trajectories 9-599-47 Description of M712

    Low-Angle Ballistic GFT_ 9-609-48 Low-Angle Ballistic

    GFT Setting 9-609-49 Transfer Limits 9-629-50 Application of the GFT

    Setting 9-629-51 Data Read From the

    Low-Angle Ballistic GFT_____ 9-639-52 M712 Low-Angle Glide

    Mode GFT 9-649-53 Determination of the Time

    Setting 9-649-54 Determination of Elevation,

    Time of Flight, and DesignateTime_9-65

    9-55 Glide Mode TransferLimits_9-65

    9-56 Glide Mode GFT Setting 9-659-57 Switch Setting 9-679-58 Computation of Site 9-679-59 Computation ofDeflection Correction______ 9-689-60 Computation of FADAC Data 9-689-61 Limits of the Battery

    Center Solut ion________ 9-689-62 Target Attack

    Contingencies 9-68

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    Record of FireFire CommandsEngagement CommandsHigh-Angle Fire

    High-Angle GFTHigh-Angle GFT Settings

    Data ComputationExample of a High-AngleMission

    CHAPTER 10.

    Section I.

    Section I1.

    Section III.

    Section IV.

    Section V.

    METEOROLOGICAL CORRECTIONSAND VELOCITY ERROR

    Purpose and Use of Met Messages

    10-1 Nonstandard Conditions

    10-2 Concurrent Met10-3 Subsequent Met10-4 Velocity Error

    Met Messages

    10-5 Characteristics

    10-6 Ballistic Met Message10-7 Computer Met Message10-8 Met Message Errors10-9 Met Message Space and

    and Time Validity

    Concurrent Met

    10-10 Characteristics

    10-11 Sequence for Solution of aa Concurrent Met

    10-12 Solution of a ConcurrentMet

    10-13 Position Constants

    10-110-110-210-210-3

    10-3

    10-310-310-610-8

    10-9

    10-1010-10

    10-10

    10-1110-30

    Subsequent Met 10-32

    10-14 Characteristics 10-32

    10-15 Sequence for Solution ofa Subsequent Met 10-32

    10-16 Solution of a SubsequentMet_10-33

    Subsequent Met Applications 10-35

    10-17 Eight-Direction Met 10-3510-18 Met to a Met Check

    Gage Point 10-38

    10-19 Met to a Target 10-38

    vii

    FM 6-40

    9-639-649-659-66

    9-679-689-699-70

    Page9-689-709-719-719-72

    9-739-74

    9-76

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    Section VI.

    CHAPTER 11.

    CHAPTER 12.

    Section I.

    Section II.

    Section III.

    Met Plus Velocity Error

    10-20 Application of VelocityError

    10-21 Manual Computation of aGFT Setting for an UnregisteredCharge

    MUZZLE VELOCITY MANAGEMENT

    11-1 Definitions

    11-2 Calibration11-3 Determination of Muzzle

    Velocity for Subsequent Lots-

    REGISTRATIONS ANDAPPLICATION OFREGISTRATION CORRECTIONS

    Characteristics

    12-1 Types of Registrations12-2 Assurance Tables12-3 When to Conduct

    Registrations

    Precision Registrations

    12-4 Definition and Objective

    Page

    10-42

    10-42

    10-42

    11-111-2

    11-6

    12-1

    12-112-2

    12-3

    12-4

    12-412-5 Initiation 12-412-6 Impact Portion of the

    Registration 12-412-7 Registering Piece

    Displacement 12-612-8 Time Portion of the

    Registration 12-612-9 Multilot Precision

    Registrations 12-812-10 Abbreviated Registration

    With the G/VLLD 12-812-11 Determination of a GFT

    GFT Setting From a

    Registration 12-812-12 Met + VE With a CheckRound 12-10

    HB/MPI Registrations 12-1012-13 Objective 12-1012-14 Selecting an Orienting

    Point__ 12-1112-15 Orienting the

    Observers 12-12

    FM 6-40

    viii

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    12-16 Determining FiringData

    12-17 Firing the HB/MPIRegistration

    12-18 Determining the Mean BurstLocation12-19 Determining Chart Data

    12-20 Determining Adjusted Data__

    Radar Registrations

    12-21 Employment

    12-22 Advantages12-23 Conducting a Radar

    Registration12-24 Selecting an Orienting

    Point12-25 Radar Message to

    Observer12-26 Determining Firing Data_12-27 Firing the HB/MPI

    Registration12-28 Determining the Mean

    Burst Location12-29 Determining Chart Data and

    Registration Corrections

    Registration Corrections

    12-30 Description12-31 Computation of Total Range

    Correction12-32 Computation of Total Fuze

    Correction12-33 Computation of Total

    Deflection Correction12-34 Registration Transfer

    Limits

    CHAPTER 13.

    Section I.

    SPECIAL CORRECTIONS ANDSPECIAL SITUATIONS

    Basic Correction Data 13-1

    13-113-213-313-413-5

    Piece DisplacementEstimation/PacingHasty TraverseSurveyBallistic Computers

    FM 6-40

    Page

    Section IV.

    Section V.

    12-13

    12-14

    12-1512-19.12-19

    12-35

    12-21

    12-22

    12-22

    12-22

    12-2412-24

    12-26

    12-28

    12-27

    12-27

    12-27

    12-27

    12-28

    12-28

    12-29

    13-113-213-213-213-2

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    13-6 Description of the M1O/M 17Plotting Board

    13-7 Preparing the M10/M17Plotting Board

    Position Corrections

    13-8 Types of PositionCorrections

    13-9 Registration/SpecialCorrection Work Sheet

    13-10 Registration Portion13-11 Hasty Correction Portion_ _13-12 Special Correction Portion_____13-13 Parallel Sheaf13-14 Converged Sheaf13-15 Special Sheaf13-16 FADAC13-17 Battery Computer System13-18 Hand-Held Calculator

    Section II.

    Section III.

    Section IV.

    Section V.

    Section VI.

    FM 6-40

    Attack of Large Targets13-19 Methods of Attack13-20 Target Division Method_13-21 Massed Fire Distribution

    Template Method13-22 Sweep and Zone Fires

    Final Protective Fire

    13-23 Description13-24 Manual Computation of Data

    With the GFT or TFT

    Observers

    13-25 Air Observer Considerations13-26 Observer Directions/

    Spotting Lines13-27 Ranging Rounds

    13-28 Time of Flight/Shot/Splash

    13-29 Computer Procedures13-30 Untrained Observers

    Zone-to-Zone Transformation

    13-31 Description

    13-32 Cutting and Joining Tw oGrid Sheets

    Page

    13-2

    13-3

    13-11

    13-11

    13-1213-1213-14.13-1513-16

    13-1613-2413-3213-3213-32

    13-33

    13-3313-33

    13-35

    13-3513-38

    13-38

    13-39

    13-39

    13-39

    13-4013-40

    13-4113-41

    13-41

    13-42

    13-42

    13-44

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    Section VII.

    CHAPTER 14.

    Section I.

    Section I1.

    Section I1.

    13-33 Graphic Method

    13-34 Computer Procedures

    High-Angle Fire13-35 Description13-36 High-Angle GFT

    13-37 Duties of Personnel

    13-38 HE High-Angle Missions

    13-39 DPICM High-AngleRegistration

    13-40 Computer Procedures

    EMERGENCY PROCEDURES/OBSERVED FIRING CHARTS

    Observed Firing Charts

    14-1 Emergency FiringCharts

    14-2 Construction of anEmergency ObservedFiring Chart

    14-3 Battery ObservedFiring Charts

    14-4 Determination of Direction forPolar Plotting

    14-5 Determination of Range andAltitude-ImpactRegistration

    14-6 Determination of Range andAltitude-Time Registration,Site Unknown

    14-7 Determination of Site byFiring (XO's High Burst)

    14-8 GFT Settings

    14-9 Observed Firing Chart WithIncomplete Survey

    14-10 Construction of Observed FiringChart-Position Area SurveyOnly

    Useof

    the M10/M17 PlottingBoard

    14-11 Determination of Map Data_14-12 Conversion of Map Data

    Into Fire Commands14-13 Determination of Data for

    Subsequent Rounds

    Map-Spot Surveyed FiringChart

    FM 6-40

    Page13-44

    13-47

    13-4713-47

    13-4813-4913-50

    13-5613-57

    14-114-1

    14-2

    14-4

    14-4

    14-5

    14-5

    14-614-7

    14-7

    14-7

    14-714-7

    14-8

    14-8

    14-9

    xi

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    14-14 Determination of SurveyedLocation and Azimuth of Lay.

    14-15 Map-Spot Technique14-16 Construction of a Map-Spot

    Surveyed Firing Chart(Manual)14-17 Transfer From Map-Spot to

    Surveyed Firing Chart

    APPENDIXES A. HASTY SPECIAL CORRECTIONTABLESA-1

    B. INTERCHANGEABILITY OFAMMUNITIONB-1

    C. FIRE DIRECTION CENTEREQUIPMENTC-1

    D. EMPLOYMENT OF ARMOR ININDIRECT FIRED-1

    E. INTERNATIONALSTANDARDIZATIONAGREEMENTSE-1

    F. STANDARDIZEDPROCEDURESF-1

    G. HOW TO TRAIN FIRE DIRECTIONCENTER PERSONNELG-1

    H. TARGET ANALYSISANDMUNITIONS EFFECTSH-1

    SAMPLE PROBLEM KNOWNDATA I-I

    J. NEW DEPARTMENT OF THE ARMYFORMS J-1

    K. GUNNERY TEAM PROCEDURESUSING THE GROUND/VEHICULARLASER LOCATORDESIGNATOR K-1

    GLOSSARY GLOSSARY-1REFERENCES REFERENCES-1INDEX INDEX-1

    xii

    FM 6-40

    Page

    14-914-10

    14-10

    14-10

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    FM 6-40

    Preface

    FM 6-40 provides an explanation of equipment and munitions and their

    associated fire direction procedures for the field artillery (FA) cannonsystems. This publication serves as the basis for operational procedures for

    technical fire direction and as a guide for unit training. It has beendeveloped with the five basic rules of combat applied: move, shoot,communicate, secure, and sustain.

    The target audience of this publication includes trainers and supervisors

    of cannon fire direction centers, battalion/brigade operations sections, andothers who need an understanding of technical cannon fire direction.Certain chapters are the "how to" references for 13E soldier's manual tasks.Users must be familiar with and use FM 6-20, FM 6-20-1, FM 6-30, FM 6-50,and FM 6-141-1/2.

    It is envisioned that this will be the final edition of FM 6-40. Changes tothis edition will be published as new equipment, ammunition, or procedures

    are developed. A training circular (TC 6-1-2) and a job aids have beenpublished for the battery computer system (BCS). The backup computersystem (BUCS) program will be fielded in 1985. When this program is

    completed and a decision on the hardware is made, a BUCS job aids will bepublished. A new FM containing BCS and BUCS fire direction procedureswill be published and fielded in FY 86.

    Field artillery doctrine concerning firing unit survivability requires

    technical procedures that allow the unit to occupy large areas of terrain in

    the best possible way. Occupation of a position greater than 250 meters by150 meters requires special corrections to firing data.

    a. The battery computer system automatically applies special aimingpoints and sheafs for all weapons in the unit.

    b. The field artillery digital automatic computer (FADAC) or manual

    fire direction procedures require the application of special corrections to

    firing data. Two methods may be used.

    (1) Mass fires of the platoons (plt) onto the target. This procedure

    requires plotting the platoon location on the firing charts and computing

    separate sets of firing data for each platoon (or using FADAC mass fire forthe separate platoon centers).

    (2) Use special corrections to battery (btry) center data for eachplatoon. These corrections normally are done by using hasty correctionsfrom tables discussed in chapter 13 and included in appendix A. If timepermits, individual piece corrections may also be computed manually (chap

    13).

    c. When firing data are being computed manually, the unit commandershould consider positioning one weapon at the geographical battery center.

    xiii

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    FM 6-40

    This is not a requirement; however, manually computed fires will be moreaccurate if the adjusting piece is located at the point used as the battery (orplatoon) center. In all cases, common sense and the tactical situation dictatethe emplacement of weapons and the selection of technical firing datatechniques.

    The order of technical, data computation is BCS, tactical fire directionsystem (TACFIRE) modification solution, BUCS, manual, and hand-heldcalculator (HHC). For further discussion, refer to chapter 4.

    Included in this publication are guidelines for the interchangeabilty ofammunition (app B), a listing of fire direction center equipment (app C), anda discussion on employing armor in the role of indirect fire (app D).

    Users of this publication are encouraged to submit recommended changesor comments to improve the publication. Comments should be keyed to aspecific page, paragraph, or line of text in which the change isrecommended. Reasons should be included for each comment to ensureunderstanding

    and complete evaluation. Changes and comments should beprepared on DA Form 2028 (Recommended Changes to Publications andBlank Forms) and forwarded directly to:

    CommandantUS Army Field Artillery SchoolATTN: ATSF-GAFort Sill, OK 73503

    The provisions of this publication are the subject of standardizationagreement (STANAG)/quadripartite standardization agreement (QSTAG)

    4119/220 and QSTAG 224. See appendix E.This publication contains standardized procedures relevant to field

    artillery cannon gunnery. Those procedures are denoted in the text by anasterisk (*). See appendix F.

    xiv

    When used in this publication, he, him, his, and menrepresent both the masculine and feminine genders unlessotherwise stated.

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    FM 6-40

    HOW TO USE THIS PUBLICATIONThis publication is a reference and training document for technical

    cannon fire direction. It may be used by all persons needing thisinformation. Information and tasks required by persons performing specificduties are addressed in various chapters. The entire publication defines andlists all technical fire direction tasks and provides information needed by

    the gunnery team to successfully perform technical fire direction.

    Below is a guide showing the chapters that will be of use to personnel inspecific jobs. These chapters should be used as primary references beforeother parts of this or other publications are consulted.

    ~imx.i:uu2

    UK

    - UR.OEAORULi m * ~~~jIUK

    xv

    ps 01 ia I I

    FA-UNIT':.COMM.ANID-ERSI-FIR'E DIRECTION'OFFICER.COMPUTERS SHOULD READ ALL CHAPTER

    FA BRIGADE/BATTALION"'S3sSHOULD'READ CHAPTERS 1 2 3 4"..

    13, AND 14,

    COMPUTER/BCS OPERATOR'S SHOULD READ CHAPTERS 1. 10. 11.9

    RADIOTELEPHONE OPERATORS SHOULD READ CHAPTE

    '0 .BSERVERS SHOULD READ CHAPTERSI., 2= BLJEA

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    FM 6-40

    T he field artillery gunnery team con-sists of the observer, the fire direc-

    tion center (FDC), and the firing battery,all linked by an adequate communica-tions system.

    1-1. OBSERVERa. The observer/target acquisition assets

    serve as the eyes of all indirect fire systems.An observer detects and locates suitableindirect fire targets (tgt) within his zone ofobservation. When a target is to be attacked,the observer, when necessary, transmits arequest for fire and adjusts the fires onto thetarget. An observer provides surveillancedata of his own fires and any other fires in hiszone of observation.

    b. Maneuver personnel may function asobservers. When an individual detects andlocates a suitable target, he transmits arequest for fire, adjusts the fires onto thetarget when necessary, and providessurveillance data of his fires and any otherfires in his zone of observation.

    c. Target acquisition devices alsofunction as observers and provide accurateand timely detection, identification, andlocation of ground targets to collectcombat/target information, to orient/cueintelligence sources, and to permit immediateattack on specific areas. Once combat begins,initial artillery targets will be located by theuse of the following field artillery targetacquisition assets:

    (1) Moving-target-locating radarsections.

    (2) Weapon-locating radar sections.

    (3) Sound ranging platoons.(4) Remotely piloted vehicles (RPV).

    Note. See FM 6-121 and FM 6-122 for adiscussion of these assets. I

    1-2. FIRE DIRECTIONCENTER

    The FDC is the control center for thegunnery team. The FDC personnel receivefire requests from the observer and processthat information by using tactical andtechnical fire direction procedures. They plotthe target on the firing chart or a map,determine firing data, and issue firecommands to the firing battery weaponsdesignated to fire the mission. The batteryFDC normally conducts technical firedirection by using the BCS or a backupmeans while the battalion (bn) FDC conductstactical fire direction with TACFIRE.Technical fire direction is the process ofconverting weapon and ammunitioncharacteristics (muzzle velocity, propellanttemperature, and projectile [proj] weight),weapon (wpn) and target location, andmeteorological (met) information into firingdata. The results of this process are charge,

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    FM 6-40

    fuze (fz) setting, deflection (df), and quadrantelevation (QE). Tactical fire direction is theprocess of analyzing fire requests anddetermining appropriate fire method,ammunition expenditure, unit(s) to fire, andtime of attack.

    1-3. FIRING BATTERYThe firing battery consists of the howitzer

    sections and the ammunition section.Organization and employment con-

    siderations are discussed in FM 6-50 and FM6-20-1.

    1-4. COMMUNICATIONSLINKS

    The gunnery team communications linksconsist of voice and digital communications.All members of the gunnery team mustaggressively ensure adequate com-munications are established and maintainedat all times.

    1-2

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    F ield artillery weapons normally areemplaced in defilade to conceal

    them from the enemy. Usually, thisemplacement precludes direct fire onthe target. Consequently, indirect firemust be used to attack targets. Thegunnery problem is indirect fire. Thesolu t ion to th is problem r equ i r e sw e a p o n and a m m u n i t i o n se t t ingswhich, when applied to the piece andammunition, will cause the projectile toburst on the target or at a proper heightabove the target.

    2-1. GUNNERYPROBLEM SOLUTION

    a. The steps in solving the gunneryproblem are as follows:

    (1) Know the location of the gun anddetermine the location of the target.

    (2) Determine char t (map) data(direction, range [rg], and angle T).

    (3) Compensate for nons tandard

    conditions that would have an effect on firingdata.(4) Convert chart data to firing data

    (charge [chg], fuze setting, deflection, andquadrant elevation).

    (5) Apply the firing data to the gun andammunition.

    b. The solution to the problem providesweapon and ammunition settings that causethe projectile to burst on the target or at apredetermined height above the target.

    2-2. REQUIREMENTSFOR ACCURATEPREDICTED FIRE

    In solving the gunnery problem,requirements for timely and accuratelydelivered cannon fire support must be met.When maintaining the demanding level ofcannon fires, adjustment cannot be made inevery situation. The goal, instead, is toachieve accurate predicted fires to producefirst-round fire-for-effect (FFE) missions.Such non ad ju s t ed f i r ing h a s fiverequirements that must be met to achieve thisgoal (fig 2-1). These requirements are targetlocation and size, bat tery location,

    ammunition and weapon information, metinformation, and computational procedures.If these requirements cannot be metcompletely, degraded FFE missions mayresult and may have to be replaced with time-consuming adjust fire missions that wouldreduce the effectiveness of cannon firesupport.

    a. Target Locat ion a n d Size. Toestablish the range from the gun to the target,the accurate and t imely detect ion,identification, and location of ground targets

    and their size and disposition on the groundmust be determined so that accurate firingdata can be computed. To determine theappropriate time and type of attack, thetarget size (radius or other dimensions) andthe direction and speed of movement mustalso be considered. Target location isdetermined by the use of the assets mentionedin paragraph 1-1.

    b. Battery Location. Accurate rangeand direction from the firing battery to the

    2-1

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    target also require the weapons to be locatedaccurately, and this location must be knownto the fire direction center. The position andazimuth determining system (PADS) canprovide accurate survey information on thebattery location. Survey techniques available

    to the firing battery can also help indetermining the location of each weapon. TheBCS can identify the grid location of eachpiece from the aiming circle and can computethe geographical battery center grid. TheBCS uses each weapon location individuallyto compute individual firing data. The term"battery center" indicates the geographicalcenter of the weapons and does notnecessarily correspond to any weaponlocation. No longer is there a base piece. Foran adjust fire mission, the BCS automaticallydesignates a different weapon for each

    mission based on an algorithm in themachine. The FDC, if required, may alsomanually designate an adjusting piece to thebattery computer system.

    c. Ammunition and Weapon Informa-t ion. In the battery computer system,accurate firing data are based on the actualperformance of each weapon. The actual

    performance is determined by the weapon'smuzzle velocities. The firing battery canmeasure actual muzzle velocity by using theM90 velocimeter (chap 11) and correct it tostandard for each weapon, type of propellant,and type of projectile. There is no longer arequirement to conduct a formal calibration,because calibration is conducted continu-ously by using the M90. Firing tables andtechnical gunnery procedures allow the unitto account for specific ammunit ioninformation (weight, fuze type, rocket motortemperature, or timer temperature); thus,accurate firing data are given.

    d. Meteorological Informat ion. Theeffects of weather on the projectile in flightmust be considered. Use of currentmeteorological information (chap 10) in the

    computers and in manual computationallows the firing data to compensate forcurrent weather conditions.

    e. Computational Procedures. Thecomputation of firing data must be accurate.Procedures programed in the ballisticcomputers and the manual techniquesdesigned for firing data computation yieldaccurate firing data.

    Figure 2-1 Five requirements for accurate predicted fire.

    2-2

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    B allistics deals with the motion ofprojectiles and the conditions that

    affect that motion. An understanding ofballistics is important because it is uponthe fundamentals of ballistics that firedirection procedures are built. It is alsoimportant to unders tand the factorsaffecting round-to-round varia t ions ,particularly in muzzle velocity, so thatthey can be minimized and the goal ofaccurate predicted fires can be achieved.

    Section IINTERIOR BALLISTICS

    3-1. DESCRIPTION CENTERING SLOPEInterior ballistics deals with the factors 0

    affecting the motion of the projectile within BREECH 9 MUZZLEthe tube. Interior ballistics determines thedeveloped muzzle velocity of a round when itleaves the tube. CONE

    a. Propellant . Artillery propellant is alow-order explosive that burns at a rate less

    than 1,300 feet per second.A low-order LAN

    explosive burns rather than explodes. Itburns rapidly and releases gases as opposedto exploding and causing an instantaneousrelease of energy. The melting point for alow-order explosive is lower than thetemperature required to cause it to chemicallyreact. Therefore, it requires a primer for BORE -ignition. CENTER GROOVE

    b. Cannon Tube. Several parts of thecannon tube affect interior ballistics (fig 3-1). Figure 3-1. The cannon tube.

    3-1

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    (1) The rifled portion of the tubeimparts spin to the projectile to causestabili ty in flight. The groove is thedepression in the rifling. The land is theraised portion. These parts engrave therotating band.

    (2) The caliber of a tube is the insidediameter of the tube as measured betweenopposing lands.

    (3) Caliber length is the length of thetube in terms of its caliber. Caliber lengthequals tube length divided by diameter.Weapons can be classified by caliber.Technically, mortars have 10 to 20 caliberlengths, howitzers have 20 to 30 caliberlengths, and guns have more than 30 caliberlengths. All US artillery has a uniform right-hand twist in rifling except the M102 105-mm

    howitzer, which has an increasing

    right-hand twist . This exception wasrequired by the velocity and accuracyrequirements for the obsolete 105-mm charge8 propellant.

    (4) The forcing cone is the rear portionof the main

    bore formed by the tapered lands.It allows the rotating band to be engagedgradually by the rifling.

    c. Projecti le. The projectile body hasseveral areas that affect ballistics (fig 3-2).Three of these areas affect interior ballistics.

    (1) Bourrelet. The bourrelet is thearea of maximum diameter of the projectile,slightly less than the diameter of the bore. Itallows the projectile to ride snugly along thelands of the tube.

    (2) Rotat ing band. The rotatingband is a band of soft metal (copper alloy)

    EYEBOLT LIFTING PLUG- 0

    -WEIGHT ZONE MARKING

    O G I V E . - _ 1 f

    KIND OF FILLER

    TYPE AND MODEL OF SHELL

    BOURRELET

    BO DY - "

    D54STAMPED IN THE METAL

    UNDER THE PAINT:LOT NUMBER OF EMPTY SHELLYEAR OF MANUFACTURECALIBER AND DESIGNATION OF SHELL

    GROMMET(PROTECTS ROTATINGBAND DURING SHIPMENT)

    CALIBER AND TYPE OF WEAPON

    .-. LOT NUMBER OF LOADED SHELL

    FOR DEEP CAVITY SHELL CONTAININGSUPPLEMENTARY CHARGE

    4 - DEPARTMENT OF DEFENSEIDENTIFICATION CODE

    ROTATING BAND

    OBTURATING BAND (USEDON CERTAIN PROJECTILES)

    - BASE-BASE COVER

    Figure 3-2. The projectile.

    3-2

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    d. Tube Wear. Continued firing of acannon wears away portions of the bore bythe actions of hot gases and chemicals andmovement of the projectile. These erosiveactions are more pronounced when highercharges are fired. The greater the tube wear,the more the muzzle velocity decreases.Normal wear can be minimized by carefulselection of the charge and by propercleaning of both the tube and theammunition.

    e. N o n u n i f o r m Ramming . Hard,uniform ramming is required for all rounds.Weak, nonuniform ramming causes acombined effect of escaping gases and asmaller propellant chamber. These factorsproduce an increase in the dispersion pattern.

    f. Rotating Bands. Rotating bandsallow uniform pressure buildup and provide aminimum dragging effect on the projectileonce motion has started. Dirt or burs on therotating bands cause improper seating. Thisincreases tube wear and contributes tovelocity dispersion. If excessively worn, thelands may not sufficiently engage therotating bands to impart the proper spin tothe projectile. Insufficient spin reducesprojectile stability in flight and can result indangerously erratic round performance.When erratic rounds occur or excessive tubewear is noted, ordnance teams should berequested to determine the serviceability ofeach tube.

    g. P r o p e l l a n t a n d Pro jec t i l eTemperatures. Positive action must betaken to maintain uniform projectile andpropellant temperatures. Failure to take thisaction results in erratic firing. The effect of anextreme change in projectile or propellanttemperature can invalidate even the mostrecent registration corrections.

    h. Moisture Content of Propellant.Changes in the moisture

    content ofpropellant are caused by improper protectionfrom the elements. These changes will affectmuzzle velocity.

    i. Pos i t ion of P r o p e l l a n t in th eC h a m b e r. In fixed and semif ixedammunition, the propellant has a relativelyfixed position with respect to the chamber. Inseparate-loading ammunition, however, theposition of the propellant depends on how thecannoneer inserts the charge. To ensure

    proper ignition of the propellant, he mustinsert the charge so that the base of thepropellant bag is flush against the obturatorspindle when the breech is closed.

    j. Weight of Projectile. A heavier-

    than-standard projectile causes a decrease invelocity. A lighter-than-standard projectilecauses an increase in velocity.

    k. Coppering. Coppering is the depositof a thin film of copper on the bore lands. Itoccurs in the tube when the velocity is highenough to develop sufficient friction toremove the outer surface of the rotating band.The amount of copper deposited varies withvelocity. Excessive coppering causes erraticvelocity performance by varying theresistance of the bore to projectile movement.Coppering may be reduced by firing

    lowercharges and removed by cleaning the bore.

    1. Prope l l an t Residue. Residue fromburned propellant and certain chemicalagents mixed with the expanding gases aredeposited on the bore surface. Proper care andcleaning will alleviate pitting and otherdamage to the tube.

    m. Tube Condit ioning. The tempera-ture of the tube has a direct bearing on thedeveloped muzzle velocity. A cold tube offersa different resistance to projectile movement

    and is less susceptible to coppering, even athigh velocities.

    n. Tube Memory. Tube memory is thephenomenon that occurs when the tube firesat a muzzle velocity similar to that of the lastcharge fired, even if the charge is different.For example, if tht charge fired was 5 greenbag (GB) and the next charge to be fired is 3green bag, the tube will fire at a slightlyhigher muzzle velocity than normal forcharge 3, because it "remembers" firing thecharge 5.

    3-3. MUZZLE VELOCITYMEASUREMENT

    Various methods are used to determine thedeveloped muzzle velocity of a projectile. Useof the M90 velocimeter is the preferredmethod. It gives a real-time muzzle velocitythat can be used to better realize the artillerygoal of accurate predicted fires.

    3-4

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    Section IIEXTERIOR BALLISTICS

    3-4. DESCRIPTIONExterior ballistics deals with the factors

    that affect the motion of a projectile outsidethe tube.

    3-5. TRAJECTORYELEMENTS

    The trajectory is the path traced by thecenter of gravity of the projectile from the

    origin to the level point. The elements of atrajectory are classified into three groups:intrinsic, initial, and terminal.

    a. Intrinsic Elements. Elements thatare characteristic of a trajectory, bydefinition, are intrinsic elements.

    (1) Origin-the location of the center ofgravity of the projectile when it leaves thetube. It also denotes the center of the muzzlewhen the piece has been laid.

    (2) Ascending branch-that portionof the trajectory that is traced by the center ofgravity while the projectile is rising from theorigin.

    (3) Descending branch-that portionof the trajectory that is traced by the center ofgravity while the projectile is falling.

    (4) Summit-the highest point of thetrajectory.

    (5) Maximum ordinate-the differ-ence in altitude (alt) between the origin and

    the summit.(6) Level point-the point on the

    descending branch that is the same altitudeas the origin.

    (7) Base of trajectory-the straightline from the origin to the level point.

    b. Initial Elements. Elements that arecharacteristic at the origin of the trajectoryare initial elements (fig 3-3).

    Figure 3-3. Initial elements of the trajectory.

    3-5

    COMPLEMENTARY RANGE

    LEGEND: 1. ANGLE OF SITE 4. ANGLE OF ELEVATION

    2. COMPLEMENTARY 5. QUADRANT ELEVATIONANGLE OF SITE 6. JUMP

    3. SITE 7. VERTICAL INTERVAL

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    (1) Line of elevation-the-axis of thetube extended when the piece is laid.

    (2) Line of departure-a line tangentto the trajectory at the instant the projectileleaves the tube.

    (3) Jump-the displacement of the lineof departure from the line of elevation thatexists at the instant the projectile leaves thetube.

    (4) Angle of site-the smaller angle inthe vertical plane from the base of thetrajectory to the straight line joining theorigin and the target. The angle of sitecompensates for the vertical interval.

    (5) Complementary angle of site-an angle that is algebraically added to theangle of site to

    compensate for thenonrigidity of the trajectory.(6) Site-the algebraic sum of the angle

    of site plus the complementary angle of site(comp site). Site is computed to compensate insituations where the target is not at the samealtitude as the battery.

    (7) Complementary range- thenumber of meters range correction equivalentto the number of mils complementary angleof site.

    (8) Line of site-the straight line fromthe origin to a point, usually the target.

    (9) Angle of elevation-the interiorangle at the origin in a vertical plane from theline of site to the line of elevation.

    (10) Quadran t e l e va t i on - t h einterior angle at the origin in a vertical planefrom the base of the trajectory to the line ofelevation. It is the algebraic sum of site plusthe angle of elevation.

    c. Terminal Elements. Elements thatare characteristic at the point of impact areterminal elements (fig 3-4).

    (1) Point of impact- the point atwhich the projectile strikes the target area.

    (2) Angle of fall-the vertical angle atthe level point between the line of fall and thebase of the trajectory.

    (3) Line of fall-the line tangent to thetrajectory at the level point.

    (4) Line of impact-a line tangent tothe trajectory at the point of impact.

    (5) Angle of impact-the acute angleat the point of impact between the line ofimpact and a plane tangent to the surface atthe point of impact.

    ORIGIN AdFIfoo JIIfl

    LEVEL POINT

    Figure 3-4. Terminal elements of the trajectory.

    3-6

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    3-6. ROUND-TO-ROUNDVARIATIONS

    a. When several rounds are fired from apiece under conditions as nearly identical as

    possible, the pointsof impact of the

    projectiles will be scattered both in range andin direction. This scattering is calleddispersion. The area over the points of impactis called the dispersion zone.

    b. Dispersion is the result of errors. Thereare three types of errors.

    (1) Inherent errors-built-in errorsor inaccuracies in the weapon or ammunition(for example, play in the recoil system of ahowitzer). Inherent errors cannot becorrected for and are the primary cause of

    dispersion.(2) Human errors-mistakes made by

    any member of the gunnery team (forexample, inaccurate directions sent by thefo rward observer [FO], impropercomputation of data by the FDC, or improperlaying of the howitzer by the gunners).Human errors can be corrected by training.

    (3) Constant errors-errors that areknown of and are constant throughout themission. Constant errors (for example, drift,VI, rotation of the earth) can be corrected byuse of the tabular firing table.

    c. The center of the dispersion pattern iscalled the mean point of impact (MPI). TheMPI of a small number of rounds will differslightly from that of a larger number ofrounds, since a smaller number of roundscannot be expected to give a representativesample of the dispersion.

    3-7. DISPERSIONRECTANGLES

    a. If a rectangle is constructed around thedispersion zone (excluding any erraticrounds), it is called the 100-percent rectangle(fig 3-5). If this rectangle is divided into eightequal parts by lines drawn perpendicular tothe line of fire, the percentage of points ofimpact to be expected in each part is indicatedin figure 3-6. This is called a range dispersion

    LINE OF FIRE * - . .

    0 0

    MEAN POINTOF IMPACT

    Figure 3-5. The 100-percent rectangle.

    3-7

    0 0

    0 00

    0 0 00 0

    DA

    v

    a

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    rectangle. Likewise, if this rectangle isdivided similarly by lines parallel to the lineof fire, the percentages will be as indicated infigure 3-7. This is called a deflectiondispersion rectangle. Each division of thesedispersion rectangles is called 1 probableerror. Probable error is defined as the errorthat is exceeded as many times as not whenconsidering all rounds either over or short orleft or right of the mean point of impact.

    b. If the range and deflection dispersionrectangles are superimposed, the result is theassemblage of small rectangles shown infigure 3-8. This result is called the dispersionrectangle. The percentage of points of impactin any particular small rectangle is theproduct of the percentages in two strips,range and deflection, whose intersections

    form the small rectangle.

    3-8. RANGE PROBABLEERROR

    a. In figure 3-6, the line AB through thecenter of impact is perpendicular to the line of

    fire. There are as many points of impact in thearea between AB and CD as there are beyondCD; that is, 25 percent of the points of impactare in the area AB-CD and 25 percent arebeyond CD. The depth of the area (AC) iscalled 1 range probable error (PER). Therange probable error for a given charge isdifferent at different ranges. Theapproximate value of the range probableerror is given in the firing tables. This valuecan be taken as an index of the.accuracy ofthe weapon. Firing table values for rangeprobable errors are representative ofcarefully selected ammunition. The actualprobable errors of an ammunition lot canvary from the table values. Probable error isthe value which, when added to or subtractedfrom the expected range, will produce aninterval along the line of fire that shouldcontain 50 percent of the rounds fired.Variations in muzzle velocity, angle ofdeparture, and total drag during flight affectthis value.

    b. From studying figure 3-6, you willdetermine that 50 percent of the rounds willfall within 1 range probable error,approximately 82 percent will fall within 2

    A CSHORT OVER

    2% 7% 16% 25% 25% 16% 7% 2%

    * 0

    * 0 0

    0 0 0 0*

    LINE OF FIRE* 0 0 * 00.o 0 0

    0 0 00

    1/2- - - 1/2

    B DMEAN POINT OF IMPACT

    Figure 3-6. Range dispersion rectangle.

    3-8

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    probable errors, and 96 percent will fallwithin 3 probable errors of the mean point ofimpact. For practical purposes, it is assumedthat all rounds will fall within 4 probableerrors of the mean point-of impact. Actually, asmall percentage of the rounds (about 7 in1,000) will fall farther than 4 probable errorsaway from the mean point of impact. Thisprobable distribution of rounds (includingthe fifth probable error) is given moreprecisely in the probability tables in thefiring tables.

    3-9. DEFLECTIONPROBABLE ERROR

    In the deflection dispersion rectangle (fig3-7), the points of impact to the right and leftof the long axis of the rectangle followprinciples of distribution similar to those in

    paragraph 3-7. For practical purposes, thedeflection probable error (PED) is taken as

    one eighth of the width of the dispersionrectangle. This value is listed in the tabularfiring tables (TFT).

    Figure 3-7. Deflection dispersion rectangle.

    LEFT.02 .07 . 16 .25 .25 . 16 .07 .02

    .02 .0004 .0014 .0032 .0050 .0050 .0032 .0014.0004

    1/2 .07 .0014 .0049 .0112 .0175 .0175 .0112 .0049 .0014

    .16 .0032 .0112 .0256 .0400 .0400 .0256 .0112.0032

    c .25 .0050 .0175 .0400 .0625 .0625 .0400 .0175 .0050 LINEOF FIRE D

    .25 .0050 .0175 .0400 .0625 .0625 .0400 .0175 .0050

    .16 .0032 .0112 .0256 .0400 .0400 .0256 .0112.0032

    1/2 .07 .0014 .0049 .0112 .0175 .0175 .0112 .0049 .0014

    .02 .0004 .0014 .0032 .0050 1.0050 .0032 .0014 .0004

    RIGHT BMEAN POINT OF IMPACT

    Figure 3-8. Dispersion rectangle.

    3-9

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    Different units may have different 4-1. BCS/MANUAL FDCtypes and amounts of FDC equip-ment.

    a. The battery FDC may be config-ured with one of the following threemixes of equipment:

    (1) BCS/manual.(2) FADAC/manual.(3) Manual only.

    b. The battalion will have TACFIREor will be a FADAC/manua l firedirection center.

    c. Duties and responsibili t ies ofassigned personnel will vary dependingon the type and the amount of equipmentavailable in the fire direction center.

    a. Duties of Personnel. The highestlevel of technical fire direction is found in aTACFIRE-controlled/BCS-equipped bat-tery FDC (fig 4-1). By tables of organizationand equipment (TOE), this FDC is authorizedeight people to fill four positions in two shiftsto facilitate 24-hour operations (TOE6-365J). Methods of training FDC personnelare in appendix G.

    (1) One fire direction officer (LT)-is responsible for all FDC operations. The firedirection officer (FDO) actively supervises,decides how to attack the target (app H),ensures the required level of training isattained, and ensures a check of all knownand firing data.

    Figure 4-1. BCS-equipped fire direction center.

    4-2 FOLD

    FM 6

    LEGEND: DMD DIGITAL MESSAGE DEVICEFIST FIRE SUPPORT TEAMGFT GRAPHICAL FIRING TABLEG/VLLD GROUND/VEHICULARLASER LOCATOR DESIGNATOR

    4-1

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    (2) One chief f i re direct ionspecialist (E6)-is the technical expert inthe fire direction center. He ensures smoothoperation of the FDC in 24-hour operations.He acts as a shift supervisor in the absence ofthe fire direction officer.

    (3) Two computer operators (E5)-operate the battery computer unit (BCU) andsend fire commands to the guns.

    (4) Two recorders (E4)-maintainrecords of firing data. As a minimum, therecorders should maintain the firing data of

    the piece nearest the battery center and thedata required for backup with the hand-heldcalculator.

    (5) Two chart operators (E3)-arethe horizontal control operator (HCO) andthe vertical control operator (VCO). Theymaintain a firing chart, follow each mission,and check the BCS grid coordinates withchart-and situation map coordinates. Thesedouble checks ensure that proper data arebeing used in the BCS solution. They also usethe BUCS/graphical firing table (GFT) fan to

    provide an additional check of BCS firingdata for the adjusting weapon during theadjusting and fire-for-effect portions of themission.

    b. Radio Communicat ions . Seefigure 4-2 for a diagram of the radiocommunications used in a direct support (DS)battalion equipped with TACFIRE and thebattery computer system.

    c. System Degradat ion . With theintroduction of new technical equipment,

    LEGEND: FSO FIRE SUPPORT OFFICERVFMED VARIABLE FORMAT MESSAGEENTRY DEVICEGDU GUN DISPLAY UNIT

    Figure 4-2. Direct support battalion TACFIRE/BCS communications network.

    FOLDIN 4-2

    6-40

    I4-2

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    certain situations can cause technical firedirection system degradation. Acceptablegunnery solutions are available, but theynecessitate additional actions by certainpersonnel (table 4-1) and restructuring of

    communicationsnets (fig 4-3).

    4-2. FADAC/MANUAL FDC

    a. Duties of Personnel. The FDO isresponsible for the operation of the FDC.Duties include producing accurate and timelyfiring data. The FDO analyzes the calls forfire and instructs the FDC on how to attack

    the target. He is also responsible for checkingand entering known data into FADAC/BCS/BUCS/HHC and on the graphicalequipment. He and the chief fire directionspecialist (E6) actively supervise the rest ofthe personnel as they perform their duties.The chief fire direction specialist is theprimary technical advisor to the FDO, whotrains the section. The senior fire directionspecialist (E5) operates the FADAC andissues fire commands to the guns. The otherpersonnel serve as radiotelephone operator(RATELO), recorder, and chart operator, asdirected. The FADAC/manual FDC isshown in figure 4-4.

    Figure 4-3. J-series battery fire direction nets used with the BCS.

    4-3

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    Table 4-1. Duties of personnel in the degraded FDC.

    DEGRADATION 1 TACFIRE not available.

    FDO/CHIEF Provides guidance to the computer operator for tactical fire control(BCS input). Normally, voice communications to the battalion FDCwill remain, and the battalion FDC will continue to control tacticalfire direction and issue the fire order. The battery FDO will guidethe BCU operator in filling in the FM:RFAF message format byusing the battalion fire order received by voice.

    COMPUTER OPERATOR Enters appropriate FDO instructions and required data ensuringcommunications with appropriate digital subscribers aremaintained.

    RECORDER Acts as radiotelephone operator (RATELO) if digitalcommunications are lost.

    CHART OPERATOR Duties remain the same as in the nongraded mode.

    DEGRADATION 2 BCS down/TACFIRE up.

    FDO/CHIEF Issues orders to use BCS manual backup. Ensures the battalionverbal fire orders are received and followed.

    COMPUTER OPERATOR Issues fire commands to the guns via vocal gun line. Operateshand-held calculator for a data check. Applies special correctionsto the TACFIRE solutions and records firing data or determinesplatoon's firing data based on individual platoon's chart data.

    CHART OPERATOR Determines special corrections to apply to the TACFIREcenter-of-battery to center-of-target solution if the battery is notmassing platoons. Maintains situation map and prepares firingchart for backup.

    DEGRADATION 3 BCS down/TACFIRE down.

    FDO/CHIEF Issues orders to go to BCS manual backup. Provides guidance fortactical fire direction based on the battalion FDO's fire order.

    COMPUTER OPERATOR Issues fire commands to the guns. Operates hand-held calculatorfor a data check. Applies

    siteand

    special corrections to the chartsolution and records data.

    CHART OPERATOR Determines firing data by using the GFT fan (battery or platoons).Determines hasty special corrections, if applicable, and gives allthe information to the computer.

    RECORDER Acts as RATELO. Determines site and assists computer as needed.

    4-4

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    Figure 4-4. FADAC/manual fire direction center.

    b. Mission Processing. A call for fireis received in the FDC by the'RATELO. TheFDO analyzes the target' and issuesinstructions to the FDC on how to attack the

    target.(1) The computer operator alerts the

    firing battery of a mission and operatesFADAC, which masses the individualplatoons. FADAC is the primary source fordetermining firing data.

    (2) The chart operator concurrentlydetermines firing data from the chart andfrom the GFT fan. These data provide thedata check on FADAC data. The chartoperator, maintains the situation map and,based on guidance from the FDO, announcesaverage site or determines and announces thesite for that.mission.

    (3) The recorder records the mission onthe record of fire and prepares the hand-heldcalculator to follow the mission as backup.

    (4) The chart operator double-checksthe FADAC firing data with his GFT-fan-determined firing data. If his center-of-battery to center-of-target solution checkswithin +3 mils deflection and quadrant

    elevation and 0.1 fuze setting increment, heannounces CHECK. Otherwise, heannounces HOLD. Before firing the data, theFDO ensures that data are checked. When

    fire for effect is entered, platoons are massedon the target center. If individual platoonlocations are plotted on the firing chart, a setof firing data for each platoon to the targetmay be determined instead of having hastycorrections applied to battery center data.

    c. System Degrada t ion . When theFADAC goes down, the FDC becomes amanual fire direction center. The recorderbecomes the second chart operator, and thecomputer acts as backup with the hand-heldcalculator. (The manual FDC'is Shown infigure 4-5.)

    4-3. MANUAL FDC

    a. Duties of Personnel. A manualbattery FDC is composed of the following:

    (1) Fire direct ion off icer- i sresponsible for the training of all FD Cpersonnel, supervises the operation of theFDC, establishes SOP, determines method ofattacking targets, and ensures accuracy offiring data sent to the guns.

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    Figure 4-5. Manual fire direction center.

    (2) Chief fire direction specialist-serves as the FDO's technical expert andfunctions as the FDO in the FDO's absence.He ensures that all equipment is on hand andoperational, supervises computation of alldata, ensures that all appropriate records aremaintained, and assists the FDO asnecessary.

    (3) Computer/senior fire directionspecial is t -combines data provided byother personnel. He determines andannounces firing data and fire commands.He also records mission-related data andother informat ion, as directed, anddetermines GFT settings.

    (4) Horizontal control operator-constructs the primary firing chart,determines and announces chart data, andchecks firing data with the GFT fan.

    (5) Ver t ica l cont ro l ope ra to r-constructs the secondary firing chart, checkschart data, determines and announces site,and provides corrections to firing data, byplatoon, from hasty correction tables, asrequired.

    (6) RATELO-receives/records callfor fire and maintains communications.

    b. Mission Processing.

    (1) A call for fire is received in the FDCby the RATELO. The FDO analyzes the

    situation and decides how the target will beattacked. The chart operators plot the target,announce chart data, and then determine VIand compute and announce site. Thecomputer determines and announces firingdata (the HCO checks or holds) andannounces fire commands to the guns. In firefor effect, the VCO enters hasty correction

    tables and announces corrections tocenter-to-center fire-for-effect data for eachplatoon.

    (2) The hand-held calculator (ifavailable) may be used as an alternatemethod of checking chart or firing data.

    4-4. BATTALION FDCa. Duties of Personnel. A TACFIRE-

    equipped battalion FDC is discussed in FM6-1. A battalion FADAC/manual FDC iscomposed of the following personnel:

    (1) Fire direction officer-super-vises the FDC, analyzes targets, and issuesthe fire order.

    (2) Chief computer-helps the FDOas required and trains the FDC personnel.

    (3) Assistant chief computer-is thealternate for the chief computer.

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    Figure 4-6. Employment of the fire direction nets (FADAC/manual).

    Table 4-2. Duties of the battalion (tactical)and battery (technical) firedirection centers.

    BATALION BATTERYFIRE DIRECTION FIRE DIRECTION

    CENTER CENTER

    Tactical fire direction Technical fire directionControl of mass fires Attack of other targets

    Technical backup to as assignedbattery FDCs

    Control of scheduledf res

    Data update

    (4) T h r e e c o m p u t e r s - p r o v i d eprimary communications between th ebattery and the battalion fire directioncenter. These people can provide technicalbackup to the battery if its FDC has anoverload or is neutralized.

    (5) Horizontal control operator-has the same duties as the HCO in the batteryfire direction center and he operates theFADAC.

    (6) Ver t ica l con t ro l o pe ra to r-determines and announces site and providescorrections to firing data, by platoon, from

    hasty correction tables as required.(7) RATELO-has the same duties as

    those of the RATELO in the battery firedirection center.

    b. Rad io Communica t i ons . Figure4-6 shows the radio nets normally employedin a FADAC/manual DS battalion. Table 4-2summarizes duties of the two fire directioncenters.

    4-5. 3x8 OPERATIONS

    a. FM 6-50 describes the 3x8 concept inthe direct support field artillery battalion. Ifthe battalion is equipped with TACFIRE,each four-gun platoon's FDC has a batterycomputer unit. As in 3x6 operations,TACFIRE provides tactical fire direction,while the BCS provides technical firedirection and control.

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    b. If no battalion FDC is available(autonomous operations), one of the tw oplatoon FDCs is the controlling FDC for theentire battery. All calls for fire are sent to thecontrolling fire direction center. The FDO ofthe controlling FDC reviews the message andperforms tactical fire direction. The fouroptions he may elect are discussed below.

    (1) The controlling FDO may elect notto fire the mission, and the BCU operatordeletes the fire request. If the controlling FDOelects to fire the mission, he issues a fire order.The BCU operator makes entries in theFM;RFAF message format and processes themessage in accordance with the fire order.

    (2) The target may be engaged by thecontrolling platoon only. In this case, the firerequest is processed as in 3x6 operations.

    '(3) The target may be engaged by thesubordinate platoon only, and the mission ishanded off. The controlling platoon's BC Umust have the subordinate platoon entered inits BCS;INIT HOFFADD field. The BCUoperator enters an H in the FM;RFAFhandoff field and executes (EXEC) themessage. The FM;RFAF is transmitted to thesubordinate platoon's BCU, where it isprocessed as received. The observertransmits all subsequent messages to thecontrolling platoon. When executed, themessages are transmitted to the subordinate

    platoon.(4) The target may be engaged by bothplatoons. The FM;RFAF and all subsequentmessages are transmitted to the subordinateplatoon from the controlling fire directioncenter. The messages are processed by bothplatoon fire direction centers. For adjust firemissions, one platoon must have DO NOTLOAD control until the fire-for-effect phase.

    c. If TACFIRE becomes inoperable,tactical fire control will be provided by themutual support unit (MSU) if available.Otherwise, the battalion FDC

    will performmanual tactical fire direction, or the batterywill operate autonomously.

    4-6. FIRING DATA CHECKSThere are three separate checks on firing

    data that must be done in the fire directioncenter. The FDO is responsible for seeing thatall firing data sent to the weapons aretechnically correct and do not violate any

    known restrictive measures or commander'sguidance. The three checks are as follows:

    a. K n o w n Dat a Check. In everybattery FDC, there are at least two separatefiring data sources (for example, the BCSwith firing charts or FADAC with firingcharts). The FDO directs that all known datainput 'into the computers be recalled andchecked for accuracy by someone other thanthe operator who entered the data. This checkensures that all operator-applied data(locations, met, muzzle velocities, or projectileinformation) are correct. The ballisticcomputers determine firing data based onthis information. The computationprocedures are correct, and, if known data arecorrect, the firing data determined by theballistic computers will be correct. Chartswill be checked for construction accuracy asdescribed in chapter 5.

    b. Gr id L o c a t i o n Check . Gridlocations must be checked between the BC S(or FADAC) and the firing chart. The initialgrid for the FM;FC, FM;RFAF, or FM;SUBSis plotted on the chart, and a target grid isoriented for direction over that point.Subsequent corrections for the mission areplotted on the chart, and the grid location ofthe pin is checked against the grid location inthe CORD: field of the FM;SUBS messageafter the correction has been executed.Acceptable tolerance between

    the two sourcesshould be 30 meters in range. The hand-heldcalculator may also be used to accomplishchart verification. Accuracy depends on thetolerance of the computer to the chartverification done previously. If there areunresolved discrepancies, BCS data shouldbe used.

    c. Weapon Firing Data Check. A-nother check may also be made as time andthe situation require. During the mission, theadjusting or centermost piece ballisticcomputer solution can be double-checked by

    using graphical equipment. There will bedifferences in firing data solutions betweenthe computer and the manual method. In allcases, the ballistic computers are moreaccurate than the manual solution, becausethey account for more variables. Themagnitude of the difference will depend onthe chart-check tolerance, the validity of theGFT setting being used, and whether or notthe adjusting piece location is actuallyplotted on the firing chart or is displaced fromthe location.

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    C hart data consist of range, deflec-tion from the firing unit to the tar-get, and angle T. These three elementsmay be determined by either electronic(TACFIRE, BCS, HHC, and FADAC) ormanual means. The manual methodrequires the construction and operationof a firing chart.

    a. There are two types of firingcharts that may be constructed in theFDC: the surveyed firing chart and theobserved firing chart.

    (1) The surveyed firing chart is achart on which the locations of allrequired points (battery or platoon [3x8]

    positions, registration points[reg pt],

    and observation posts [OP]) are plotted.These locations can be based on surveyor map inspection. All plotted points arein correct relation to one another andref lec t actual map coord ina tes .Addi t ional i n fo rmat ion such asloca t ions of t a rge ts , m a n e u v e r

    checkpoints, no-fire areas, and safetydiagrams (training environment) mayalso be recorded.

    (2) The observed firing chart is achart on which the relative locationsmust be established by the adjustment offire, hence, the name "observed firingchart." Details on construction and useof the observed firing chart are inchapter 14.

    b. In the BCS- or FADAC-equippedfire direction center, a firing chart ism a i n t a i n e d as a backup and issupplemented by a 1:50,000-scale mapused to determine altitude. In th emanual FDC, two firing charts aremainta ined. The vertical contro loperator maintains the backup chartand the 1:50,000-scale supplementarymap with the overlay(s), and th ehorizontal control operator maintainsthe primary firing chart.

    Section I

    TYPES OF SURVEYED FIRING CHARTS

    5-1. DESCRIPTION

    A firing chart is a graphic representation ofa portion of the earth's surface used fordetermination of distance and direction. Itmay be constructed by using a map, a

    photomap, a grid sheet, or other material onwhich the relative locations of batteries,registration points, targets, and observerscan be plotted. Additional positions, firesupport coordinating measures, and otherdata needed for the safe and accurate conductof fire may also be recorded.

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    c. Plot t ing Pins. Plotting pins areused to mark indexes and temporarypositions on the firing chart. On a1:25,000-scale chart, the thickness of theplotting pin equals 20 meters.

    d. A l u m i n u m P l o t t i n g Scale . Thealuminum plotting scale (fig 5-3) is asquare-shaped scale used to plot or determine

    grid coordinates. The scale is graduated inmeters and yards at scales of 1:25,000 and1:50,000. The meters scale is graduated every20 meters, and locations can be visuallyinterpolated to the nearest 1 meter. Care mustbe taken not to confuse the meters and yards

    scales on this instrument. Paragraph5-5

    details the procedure for plotting points withthe aluminum plotting scale.

    TOP

    0I SIDESLIDE THE PENCIL

    Figure 5-1. The 6H pencil (wedge point).

    ROTATE THE PENCIL

    Figure 5-2. The 4H pencil (conical point).

    I Z L S09 L 8 6 8S9 186000 S7 1 0001 000Z

    C SaUVA0 000 0: 1S 0 00S 0001 00S8 000Z 2000

    L 0 9

    6 00S I

    0001 2000 60 5z 0001 101E I oo 4

    V 3OHS 29 1000 1

    L 0000 1000

    6 500 90000 8

    i ,r~ .., ... 0 6

    2000 1500 1000 5001 :50 00

    METERS 32000 1000 25 000 2

    900if98765432 1 9 8 7 65 4 3 2 .Fi r.. .............. -.luminu .pl

    Figure 5-3. Aluminum plotting scale.

    5-3

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    Figure 5-4. Range-

    e. Range-Deflection Prot rac tor.The range-deflection protractor (fig 5-4) isused to measure angles in mils and distancesin meters. It is used to measure range anddeflection from a firing unit to a target anddirection and distance from an observer to atarget.

    (1) The left edge of the instrument is thearm and is used to measure range or distance.It is graduated in 50-meter increments andlabeled every 500 meters on a scale of1:25,000. Ranges and distances are visuallyinterpolated to an accuracy of 10 meters.

    (2) The 1,000-mil arc of the RDP isgraduated in 5-mil increments. The 50-milincrements are indicated by longergraduations and are permanently numbered.The arc is visually interpolated to anaccuracy of 1 mil.

    (3) There are four different RDPmodels. They differ by maximum range of thearm (12,000, 15,000, 25,000, and 30,000meters).

    -deflection protractor.

    f. Target Grid.

    (1) Description. The target grid is acircular paper or plastic device on which gridlines are printed (DA Form 4176 [TargetPlotting Grid Field Artillery Graduated inMils and Meters Scale 1:25,000]). These linesmatch the scale of the 1:25,000 firing chart,dividing a 1,000-meter grid square into100-meter squares. An azimuth scale isprinted around the outside edge of the targetgrid. It is graduated in 10-mil increments and

    is numbered every 100 mils. An arrowextends across the center of the target gridand is used to indicate the observer-target(OT) line (or other line of known direction).The target grid should be labeled as shown infigure 5-5. Tape may be applied to the reverseside of the target grid to prevent the centerhole from becoming enlarged.

    (2) Purpose and Use. The targetgrid is used for plotting the positions oftargets located by shift from a known point

    5-4

    'A,

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    (para 5-10c), plott ing subsequent 5-50 POINT PLOTTINGcorrections, and determining angle T(section To plot points that have been located by

    III). grid coordinates, proceed as follows:

    a. Grid Lines 1,000 Meters Apart.

    (1) Place a plotting pin in the upperright-hand corner of the grid square wherethe point is to be plotted (for example, grid60478 32859 [ ( , fig 5-6]). This pin willprevent the inadvertent plotting of the pointin the wrong grid square.

    (2) Place the aluminum plotting scalealong the left edge of the grid square with the0 of the scale at the lower left-hand corner ofthe grid square ( , fig 5-6).

    (3) Slide the scale to the right.Read

    easting on the scale by using the north-southgrid line as an index ( , fig 5-6).(4) With the bottom edge of the scale

    aligned precisely on the east-west grid line,plot the northing on the vertical scale with aplotting pin. This is the plotted location

    Figure 5-5. Labeling the target grid. ( ) , fig5-6).

    Figure 5-6. Plotting with the aluminum plotting scale.

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    b. Grid Lines Other Than 1,000Meters Apart.

    (1) When grid squares are smaller thannormal, plot the easting or northing byinclining the aluminum plotting scale so thatthe 0 of the scale is on one grid line and the1000 is on the other (fig 5-7). The four-pointplotting technique (para (3) below) must beused.

    (2) When grid squares are larger thannormal, incline the aluminum plotting scaleso that the 0 of the scale is on one grid line and2000 is on the other (fig 5-8). Multiply thedistance to be plotted by 2, and insert a pin atthat distance on the scale. For example, foran easting of 62681, place the pin at (62)1362on the scale.

    (3) In both cases, only easting ornorthing can be plotted at one time.Therefore, the following four-point plottingtechnique must be used to plot a critical pointwhen grid lines are less than 1,000 metersapart.

    (a) Locate the grid square in whichthe point is to be plotted.

    (b) Using plotting pins and thealuminum plotting scale, plot the easting gridin the grid squares above and below the gridsquare in which the point is to be located.Remove the pins and connect the points witha fine line drawn with a 6H pencil through thecenter of the pinholes (fig 5-9).

    Figure 5-7. Grid squares smaller than normal.

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    Figure 5-8. Grid squares larger than normal.

    Figure 5-9. Plotting the grid easting.

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    (c) Plot the grid northing in the gridsquares to either side of the grid square inwhich the point is to be located (fig 5-10).Connect the pinholes with a line drawn witha 6H pencil.

    (d) With a plotting pin, mark thepoint where the two lines meet (fig 5-11).Erase the fine lines and identify the locationwith a tick mark (para 5-6) (fig 5-12).

    (e) The four-point plotting techniquemay also be used on a standard 1:25,000-scale firing chart. Easting is plotted along theeast-west grid lines bordering the grid squarewhere the point is located. Northing is plottedon the north-south grid lines bordering thegrid square. Only when accuracy is moreimportant than speed should this techniquebe employed.

    Figure 5-10. Plotting the grid northing.

    Figure 5-11. Marking the point where thetwo lines meet.

    U I

    Figure 5-12. Identifying the locationwith a tick mark.

    5-6. TICK MARKSThe tick mark is the symbol used to mark

    and identify the essential locations on afiring chart.

    a. The tick mark is constructed in theform of a cross, with each arm beginning 40meters from the pinhole on the chart andextending 150 meters in length (1:25,000scale) (fig 5-13).

    Figure 5-13. Drawing the tick mark.

    b. The lines of the tick mark are drawnparallel to the grid lines unless the labelinginterferes with a previously labeled tick mark( , fig 5-14) or the plotted point falls on ornear (within 80 meters) a grid line ( , fig5-14). In these cases, the tick mark is plottedon an 800-mil (450) angle to the grid line. Alltick marks are drawn with a 4H pencil except

    5-8

    1______________________________________________________________

    Iare - i

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    those for targets located by firing, which aredrawn in red, and for maneuver checkpoints,which are drawn in blue.

    c. The identification of the point is placedin the upper right quadrant. The installation

    or activity is designated in the followingmanner:

    (1) Battery. The letter designation isshown in the appropriate color ( (U , fig5-14) (that is, A-red, B-black, C-blue,

    D-orange). If more than four batteries areshown, the color coding starts over againwith red. If the pieces of the battery are widelydispersed, it may be necessary to plot piece orplatoon locations (for example, platoons in3x8 operat ions) . Platoon or piecedesignations should be labeled by using thebattery's color code.

    (2) Radar. The military symbol isshown in green ( ( , fig 5-14).

    (3) Observation post. The militarysymbol for an observer and the call signnumber which will be used by the observer toidentify himself are shown in black ( SIfig 5-14). If the observation post is assigned anumber, that number is shown in black in

    01 cs.1977

    400

    place of the symbol and call sign number (forexample, 01).

    (4) Registration point. The numberassigned to the registration point is shown inblack (for example, REG PT 2 [ (" , fig5-14]).

    (5) Target. The assigned targetnumber is shown in black (for example,CB2131 [ , fig 5-14]).

    (6) Checkpoint. The checkpoint tickmark is shown in blue, while its number isshown in black and circled (for example, O[ , fig 5-14]).

    d. The altitude (alt), in meters, of theplotted point is placed in the lower leftquadrant in black ( B through , fig5-14).

    e. If the plotted point has been fired on,the fuze used in fire for effect may be placed inthe lower right quadrant ( I() , fig 5-14).

    f. If the plotted point has been fired onwith high-angle (HA) fire, the letters HA maybe placed in the upper left quadrant. Thecharge that was used in the mission can alsobe indicated (clD , fig 5-14).

    C%~ V IB424 1

    (SYMBOL INGREEN)

    5001

    Q(LABELING OF TICKMARKS SHOULDBE KEPT WITHINQUADRANTS AS ICSMUCH AS 12131

    POSSIBLE.) 4151

    0A068

    3001

    (TICK MARKIN BLUE)

    I0. 37 6

    I REGPT 24301

    0CHG 51 CS

    HA 2131

    (ALT) I VTI (FZ IN FFE)

    Note: Identification and altitude of the tick mark must always be given.

    Figure 5-14. Tick marks.

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    Figure 5-15. Numbering on the r

    5-7. CONSTRUCTION OFAZIMUTH INDEXES

    a. Azimuth indexes are constructed forpoints located on the firing chart from whichthe polar plot method (para 5-10) of targetlocation may be expected (for example,observation posts and radar sites). The RDPmust be prepared by numbering the 100-milgraduations in black as shown in figure 5-15.

    b. Azimuth (az) is always read as fourdigits. The first digit (thousands of mils) is

    read from an index that is drawn on the firingchart. The last three digits are read from thearc of the range-deflection protractor.

    c. Azimuth indexes are constructed on thefiring chart at 1,000-mil intervals throughoutthe target area except the 6000 and the 0indexes, which are 400 mils apart (fig 5-16).

    (1) Place the vertex of the RDP againstthe pin in the observation post or radarlocation, and align the arm parallel to aconvenient grid line. If you are using anorth-south grid line, plot the easting of theobserver with a plotting pin. If you are usingan east-west grid line, plot the northing of theobserver in a similar manner. When the RDPis aligned with the vertex at the observer andthe arm against the pin, a reference line isestablished at an azimuth of 0 (north), 1600(east), 3200 (south), or 4800 (west). It is notdrawn.

    (2) In (1) above, the RDP was orientedin a cardinal direction. To establish the

    range-deflection protractor.

    azimuth index, place a pin opposite thenumber on the arc corresponding to the lastthree digits of the azimuth in which the armof the RDP is oriented. The location of the pinrepresents an index. Its value is the value ofthe first digit of the azimuth in which the armof the RDP is oriented. If the arm is orientedtoward north, two azimuth indexes will beconstructed, since north corresponds to 6400and 0 mils (fig 5-16). These will be the 6000and 0 indexes. Table 5-1 summarizesindexing procedures for all orientations of therange-deflection protractor.

    Figure 5-16. Plotting the 6,400-mil and0-mil temporary azimuthindexes.

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    Table 5-1. Indexing procedures.

    If RDP is oriented NORTH (6,400 mils or 0 mils),

    (1) Place pin opposite 400 on ARC; INDEX is

    6000.

    (2) Place pin opposite 0 on ARC; INDEX is 0.

    If RDP is oriented EAST (1,600 mils),

    place pin opposite 600 on ARC; INDEX is 1000.

    If RDP is oriented SOUTH (3,200 mils),

    place pin opposite 200 on ARC; INDEX is 3000.

    If RDP is oriented WEST (4,800 mils),

    place pin opposite 800 on ARC; INDEX is 4000.

    (3) To replace the plotting pin with aconstructed index, move the RDP so the leftedge of the arm is against the pin ( , fig5-17). Remove the pin and, with a 6H pencil,draw the azimuth index through the center ofthe pinhole ( , fig 5-17). The index is afine line extending 1 inch on each side of thepinhole. For example, in @ , figure 5-17,this line can be constructed by drawing a linefrom range 8250 to range 9520 along the armof the RDP. Beginning one-eighth inchbeyond the pinhole, label the index with itsidentification and azimuth value (for

    example, OlAZO [ , fig 5-17]). Letteringis green for the radar symbol and its azimuthlabels and black for observation posts andtheir azimuth labels.

    Figure 5-17. Preparing an index.

    5-11

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    Figure 5-20. The range-deflection protractor prepared for measurement of deflection.

    Figure 5-21. Range-deflection protractor oriented to the east.

    5-13

    DEFLECTIONDEFLECTION IS LABELED

    WITHRED PENCIL

    AZIMUTH IS LABELEDWITHBLACK PENCIL

    Figure 5-22. Range-deflection protractor oriented on azimuth of lay.

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    - - -- 4P

    1 1 I l l i ll-2-70i

    Figure 5-25. Construction of supplementary deflection indexes.

    Figure 5-26. The 6,400-mil firing chart.

    5-9. 6,400-MIL CHARTS

    When firing in a 6,400-mil sector isrequired, the firing units are plotted in thecenter of the firing chart. The indexes areconstructed and numbered as shown in figure5-26. Locally fabricated larger charts may benecessary to achieve the maximum rangethroughout the 6,400 mils.

    5-15

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    Figure 5-27. Polar plot the target location.

    5-10. METHODS USED IN

    PLOTTING TARGETSa. Grid Coordinate. To plot points

    located by the grid coordinate method, followthe procedures outlined in paragraph 5-5.

    b. Polar Plot. To plot points located bythe polar method, proceed as follows:

    (1) Place the vertex of the RDP againstthe pin at the OP, with the arc opposite theproper azimuth index.

    (2) Align the RDP in the directionreported by the observer opposite the azimuthindex. In figure 5-27, the observer (01)reported a direction of 6200.

    (3) Place a pin along the left edge of thearm of the RDP at the reported distance. Thisis the location of the target. In figure 5-27, theobserver reported a distance of 8,700 meters.

    c. Shift From a Known Point. A shiftfrom a known point target is located relativeto a known point by using OT direction, alateral shift, a range shift, and a vertical

    shift. The following method is used inplotting a target location:

    (1) Place the center of the target gridover the location of the known point on thefiring chart and fix it in position with a pin.To orient the target grid, move the arrowparallel to a north-south grid line, with thearrow pointing north. Place a pin opposite 0on the scale. Then, rotate the target grid untilthe OT direction is opposite the pin. Thetarget grid is now oriented. In figure 5-28, thetarget grid is oriented in the directionreported by the observer, 1,100 mils. If it isanticipated that a location may be used as the

    known point for future shift from a knownpoint target location (for example, aregistration point or a point recorded as atarget), a north azimuth index should beconstructed by use of the range-deflectionprotractor. Position and orient the target gridtoward north as before. Instead of placing apin opposite the 0, construct an indexopposite 0. The index extends 1 inch aboveand 1 inch below the target grid (approximaterange 2050 to 3300 on the RDP). Mark theindex N as shown in figure 5-29.

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    Figure 5-28. Locating the target by shiftfrom a known point.

    (2) Plot the shift given by the observer(left or right, add or drop) on the target grid.(Be sure to note the labeling of the targetgrid.) The pin will be in the target location.

    EXAMPLE:

    Observer reported (fig 5-28):SHIFT REGISTRATION POINT 1,DIRECTION 1100, RIGHT 150,DROP 400, OVER.

    (3) If the initial shift plots the point offthe target grid, reposition the target grid sothat both points fall on the target grid. Orientthe target grid in the OT direction and plotthe shift (fig 5-30).

    (4) Recenter the target grid over thetarget location and orient it to the reportedOT direction. This will permit the plotting ofsubsequent corrections and/or refinement.Once oriented, the target grid should be

    secured at all times by at least two pins toavoid unintentional movement.

    EXAMPLE:

    Observer reports:

    SHIFT REGISTRATION POINT 1,DIRECTION 1100, LEFT 500,DROP 2500, OVER.

    Figure 5-29. Construction of a north index.

    Figure 5-30. Locating the target when the initialshift plots off the target grid.

    5-17

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    Section IIICHART DATA

    Figure 5-31. Angle T.

    5-11. CHART RANGE AND

    CHART DEFLECTIONChart range and deflection are measured

    as follows:

    a. Place the vertex of the RDP against thepin in the battery position, and place the leftedge of the arm against the pin in the targetlocation.

    b. Read the range, in meters, opposite thepin in the target location. Measure andannounce the range to the nearest 10 meters.

    c. Read the chart deflection on the arcopposite the appropriate deflection index. Toobtain the deflection, combine the 1,000-mildesignation of the index with the reading onthe arc. Announce the deflection to thenearest 1 mil.

    GUN-TARGET LINE

    V ANGLET

    0 1\

    I,

    FM 6-40 0

    5-12. ANGLE TAngle T (fig 5-31) is the smaller interior

    angle formed at the target by the intersection 'of the GT and OT lines. It will therefore neverbe larger than 3,200 mils, After the initialchart range and deflection are announced,the target grid is oriented over the targetlocation along the observer's direction. AngleT is then determined to an accuracy of 10 milsand announced by the chart operator. It ismeasured in the following manner:

    a. With the target grid centered on thetarget pin and oriented along the observerdirection, place the vertex of the RDP at thefiring unit location and place the left edge ofthe arm against the target pin. Either the 0(head of the arrow) or 3200 (tail of the arrow)will be visible on the target grid to the left ofthe arm. Measure angle T eitherhead-to-head or tail-to-tail. That is, if the 0or head of the arrow is visible, determine thenumber of mils between the head of the arrowand the intersection of the arm of the RDP,with the target grid azimuth scale fartherfrom the vertex (fig 5-32). If the 3200 or tail ofthe arrow is visible, determine the number ofigure 5-32. Determine angle T (head to head).

    5-18

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    FM 6-40

    mils between the tail of the arrow and theintersection of the arm of the RDP, with thetarget grid azimuth scale closer to the vertex(fig