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Basic Definitions


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Basic Definitions95k 1. BASE COURSE is the selected reference course from which the firing vessel, target (tow), and observing vessels are to man­euver.
2. GENERAL BEARING LINE (G.B.L.) is the true bearing of the enemy ship to be en­gaged by the flagship of the officer in tact­ical command, measured from that flagship.
3. The HORIZONTAL is the imaginary plane, tangent to the earth's surface, at the point instantaniously occupied by a surface ship. The ship is considered to move only in the horizontal plane. The horizontal is used as a referance plane from which to measure the relative movemnets of elevated (aerial) tar­gets.
4. The VERTICAL is a plane, perpindicular to a referance plane, passing through a desi­gnated line, point or points in the referance plane. If no referance plane is specified it is assumed to be the horizontal, and the "verti­cal" is then the true vertical plane.
5. The REFERANCE PLANE for a battery is an arbitrarily chosen, imaginary plane, from which the vertical angles of all guns and fire control instruments are measured. It is us­ually the plane whose inclination to the hor­izontal is the mean of the inclinations of the turret or mount roller path planes to the horizontal, or a selected director roller path, or the horizontal as determined while check­ing in drydock.
5a The PHANTOM PLANE is an imaginary plane resulting from combining the inclina­tion from combining the inclination of a turret's roller path to the reference plane with the angular corrections in elevation for the roller path's horizontal displacement from the reference point.
6. LEVEL ANGLE is the angle between refer­ence plane and the horizontal, measured in the plane perpendicular to the reference plane through the line of sight.
7. CROSS-LEVEL ANGLE is the angle be­tween the reference plane and the horizontal, measured in the plane perpendicular to the reference plane at 90 degrees to the plane of the line of sight. Cross-level angle is posi­tive when the right trunnions of guns or directors are depressed.
8.TRUNION TILT is the instantaneous incli­nation of the axis of the trunnions to the horizontal.
9. ROLL is the instantaneous value of the angle between the reference plane and the horizontal measured in the athwarthship plane normal to the reference plane.
10. PITCH is the instantaneous value of the angle between the reference plane and the horizontal, measured in a fore and aft plane normal to the reference plane. Note : For purpose of naval gunnery both roll and pitch are measured and recorded as rates; that is, in terms of amplitude per unit time, ordinarily as total degrees of roll (or pitch) per minute.
11. LINE OF SIGHT is the straight line join­ing the sight and the point of aim.
12. TRAGET ANGLE is the relative bearing of own ship from the target. measured in the horizontal plane from the bow of the target clockwise from 0° to 360°.
13. POSITION ANGLE is the vertical angle be­tween the line of sight to an elevated target and the horizontal.
14. RELATIVE TARGET BEARING is the bearing of the target from the firing ship measured in the horizontal from the bow of the firing ship clockwise from 0° to 360°.
15. TRUE TARGET BEARING is the true com­pass bearing of the target from the firing ship.
16. GENERATED TARGET BEARING (rela­tive or true) is the relative or true bearing of the target as determined mechanically or graphically from previous positions of own ship and target and established or estimated rates of change of bearing.
17. WIND DIRECTION is the compass direct­ion FROM which the wind is blowing.
18. WIND ANGLE is the angle between the di­rection toward which the wind is blowing and the line of sights or projection of the line of sight in the horizontal.
19. TRUE WIND is the actual wind blowing unaffected by the motion of the observing station expressed as TRUE direction and TRUE speed (velocity).
20. APPARENT WIND is the wind apparent to the observing station and is the resultant of the true wind and the motion of the observ­ing station- expressed as APPARENT di­rection and APPARENT speed (velocity). 21. BALLISTIC WIND is a weighted mean of the true winds at the various altitudes which the shell reaches in flight.
22. ALTITUDE is the height of an elevated target above the horizontal plane. It is ordinarily measured in feet.
23. DIP ANGLE is the vertical angle between the horizontal and the line of sight caused by (a) curvature of the earth, (b) differ­ence in height between eye and object sight­ed upon.
24. VERTICAL PARALLAX is the vertical angle at the target formed by a line of sight from each of two stations (such as two guns, two directors, or a gun and a director) mounted at different heights on a ship. In surface fire it is equal to the difference be­tween the two dip angles.
25. THE VERTICAL PARALLAX CORRECT­ION is the correction required to compen­sate the vertical parallax.
26. HORIZONTAL PARALLAX (CONVERG­ENCE) is the horizontal angle at the target formed by a line of two stations (such as two guns, two directors, or a gun and a di­rector on a ship.
27. The HORIZONTAL PARALLAX CORREC­TION is the correction required to compen­sate the horizontal parallax of the turret, or gun, from the gun director used as the origin in train for the system.
28. DIRECTOR CORRECTION is the vertical angle existing at the instant of firing be­tween the director line of sight and the ref­erence plane. If the director occupies a posi­tion above or below the reference plane an additional correction for vertical parallax is introduced automatically at the director.
29. BEARING RATE is the rate of change of bearing of target relative to own ship (rela­tive movement of own ship and target at right angles to line of sight). It may be expressed in "knots" (linear measure) or in angular measure per unit of time, and is measured in the horizontal plane.
30. ELEVATION RATE is the rate of change of position angle. It has significance in anti­aircraft fire only.
31. RANGE RATE is the rate of change of dis­tance to the target (relative movement of own ship and target along the line of sight). In anti-air-craft fire control, the HORIZON­TAL range rate is one component of the range rate.
32. FUZE SETTING is 'the instant measured on the fuse scale (zero of scale instant of discharge) at which the fuse is to function. In mechanical fuses this is the time of flight of the projectile. In powder fuses, the burn­ing rate of the powder in the fuze train ordinarily requires a setting differing from the time of flight. FUZE SETTING is ex­pressed in seconds, and fifth-second frac­tions, of time.
33. DEAD TIME is the total time elapsing from the instant of position (measuring range and bearing) the target until the gun is fired with data computed from that position.
34. TRANSMISSION INTERVAL is that part of dead time caused by lag in transmission of data.
35. SIGHT ANGLE is the vertical angle be­tween the line of sight of a gun and the axis of the bore o£ the gun.
36. DIRECTOR SIGHT ANGLE is the vertical angle between the line of sight of a gun di­rector (reduced to mean gun level) and the axis of the bore of the gun(s) controlled by the director.
37. GUN ANGLE is the angle between the reference plane and the axis of the bore of the gun.
38. A MIL is the unit of deflection and is the angle at the sight (gun or director) which subtends a horizontal distance at the target, at right angles to the line of sight, equal to 1/1,000 of the target distance. One mil is equivalent to 3.44' or 3'26" of arc.
39. TRUNNION T I LT CORRECTION IN TRAIN is the correction of the gun train order to compensate for the lateral errors due to the tilt of the trunnion, at the instant of firing.
40. TRUNNION TILT CORRECTION IN ELE­VATION (vertical correction for trunnion tilt) is the correction to the gun elevation to compensate for the errors in range due to the tilt of the trunnions, at the instant of firing.
41. GUN TRAIN ORDER is the signal trans­mitted by the director (or from the director to associated instruments and as corrected by them) to the guns, indicating to the guns the correct position in azimuth in reference plane.
42. GUN ELEVATION ORDER is the signal transmitted by the director (or from the director to associated instruments and as corrected by them) to the guns, indicating the correct gun angle.

RANGE

43. DISTANCE is the linear measurement be­tween two points.
44. RANGE is the same as "distance" and in ordinary use is considered to mean the dis­tance as determined by range finder, or other instrumental means, corrected for known errors. In analysis of gunnery per­formance the word "range" should not be used without a proper qualifying adjective or phrase.
45. RANGE-FINDER RANGE is the linear dis­tance to a target as read from the range­finder scale.
46. RANGE-FINDER CORRECTION is the cor­rection to be applied to the range-finder range to correct for known (or estimated) errors.
47. CORRECTED RANGE-FINDER RANGE is the algebraic sum of the range-finder range and the range-finder correction.
48. MEAN RANGE-FINDER RANGE is the mean of the corrected range-finder ranges, taken simultaneously or approximately so, from two or more range finders.
49. MEAN RANGE-FINDER CORRECTION is the correction to be applied to the mean range-finder range to correct known or esti­mated errors of the range finders in use.
50. CORRECTED MEAN RANGE - FINDER RANGE is the means of the corrected range­finder ranges of two or more range finders taken simultaneously or approximately so, or the mean range-finder range plus (or minus) the mean range-finder correction.
51. HORIZONTAL RANGE is the horizontal distance to a point directly beneath an elevated target distance multiplied by the cosine of the angle of position.
52. PRESENT RANGE is the best available estimate of the target distance. It is us­ually based on the corrected mean range­finder range at the time of opening fire, but it may be based on the spotter's estimate of the target distance. It is continuously de­termined by the rangekeeper, or by plotting.
53. ADVANCE RANGE is the value of sight bar range computed by the rangekeeper, or by plotting. It is made up on present range combined with corrections for own ship and target movements during the dead time, and time of flight, other ballistic corrections, and "spots".
54. NAVIGATIONAL RANGE is the range or distance to the target from the firing vessel. It is determined after firing, from consider­ation of all available data.
55. GUN RANGE is the range in yards (shown in the latest range table in effect) corre­sponding to the angle of elevation above the line of sight to which a gun is laid when fired. It is equal to the sight bar range when the following circumstances apply : sight scales and range converters are graduated to the proper range table, there is no con­trol ballistic and the point of aim is the desired point of impact, there is no un­corrected vertical parallax between gun and director, and erosion correctors are not used.
56. SIGHT BAR RANGE is the range in yards shown on the sight scale of the gun, the range converter or director, at the instant of firing.
57. HITTING GUN RANGE is the gun range that, if used would place the mean point of impact at the center of the target. It is equal to the gun range corrected for the error of the mean point of impact.
58. IMPACT RANGE is the distance from the firing point to the point of impact of the projectile. This can be accurately determin­ed only at calibration practices.
59. BALLISTICS are computed corrections to be applied to the measured or estimated values of range and to the midpoint of the deflection scale to compensate for known or predicted errors and for variations from selected standard conditions.
(a) GUN BALLISTIC consists of the cor­rections to compensate for the follow­ing variations from standard con­ditions:
(1) Initial velocity errors due to tem­perature of powder and to gun erosion.
(2) Errors due to atmospheric con­ditions of pressure temperature and wind.
(3) Errors produced by relative move­ments of gun and target.
(4) Errors caused by drift.
(b) ARBITRARY BALLISTIC is a cor­rection to compensate for the indeter­minate errors in total ballistic used on previous firings, obtained by analysis of those firings. When used, this is com­bined with the gun ballistic in determ­ining the gun range and deflection. A special arbitrary ballistic including correction for cold gun is often used for an opening salvo.
(c) CONTROL BALLISTIC consists of the corrections to the gun range and de­flection to obtain the sight bar range and deflection to compensate for the following:

(1) In pointer fire...

Point of aim correction.
Sight scale range table correction.
(2) In direct fire.

Point of aim correction.
Vertical parallax correction.
Range converter-range table cor­rection.
(d) TOTAL BALLISTIC is the correction determined prior to firing to be applied to the present range and to the mid­point of the deflection scale to obtain the sight bar range and deflection. It includes the gun ballistic, arbitrary bal­listic, and control ballistic.
(e) INITIAL BALLISTIC is that portion of the total ballistic which is determ­ined by computation (or graphs, etc.). It includes all elements of the total bal­listic which are not determined mechan­ically and automatically, as by the rangekeeper, is in use, the INITIAL is the correction (range and deflection) applied manually to the rangekeeper just prior to opening fire. When no portion of the total ballistic is determ­ined mechanically, the INITIAL BAL­LISTIC is equal to the TOTAL BAL­LISTIC.
(f) BALLISTIC USED is the correction in range and deflection actually applied to the present range and to the midpoint of the deflection scale; in range it is equal to the difference between present range and advance range. For the opening salvo, the ballistic in use should be the same as total ballistic, if there are no errors in application.
(g) HITTING RANGE BALLISTIC.­HITTING PRESENT RANGE BAL­LISTIC is the correction in range which, if applied to the present range, will give the sight bar range (advance range) necessary to place the mean point of impact in range at the center of the target.
HITTING NAVIGATIONAL RANGE BALLISTIC is the correction in range which, if applied to the navigational range, will give the sight bar range (advance range) necessary to place the mean point of impact in range at the center of the target. It can only be determined by past firing analysis. If this analysis shows that the present range was the same as the navigational range, then the hitting present range ballistic was the hitting navigational range ballistic.
(h) HITTING DEFLECTION BALLISTIC is the correction in deflection, which, if applied to the midpoint of the deflection scale, would place the M. P. I. (in de­flection) at the center of the target.

60. DRIFT ANGLE at any range is the angular lateral deviation due to rotation of a pro­jectile in flight.
61. SUPER ELEVATION is the angle the gun must be elevated above targets, to compen­sate for curvature of the trajectory.
62. DEFLECTION is the lateral angular correc­tion applied to the target bearing to obtain gun train order and thus offset the gun from the line of sight the proper amount to com­pensate for the lateral errors as computed or estimated. Sights and fire control instru­ments are designed for, and deflection is computed for, the application of deflection in the horizontal plane. However, unless cor­rected for trunnion tilt, the deflection is actually applied in the reference plane.
63. CORRECTED SIGHT DEFLECTIONThis term is frequently used when a separate in­strument is used, to correct for the effect of trunnion tilt, and the value so designated indicates that it is the sight deflection cor­rected for the effect of trunnion tilt.
64. SIGHT DEPRESSION is the vertical angle the gun or director sights must be set below the gun bore (or gun pilot in case of direc­tor) to secure the gun elevation correspond­ing to the computed advance range, when the gun and director are elevated to bring the line of sight on the point of aim.
65. CORRECTED SIGHT DEPRESSION This term is frequently used when a separate in­strument is used to correct for the effect of trunnion tilt and the value so designated in­dicates that it is the sight depression cor­rected for the offset of trunnion tilt in range.

ARMAMENT

66. A CAPITAL SHIP is an armored vessel of war, not an aircraft carrier, mounting a battery of a caliber greater than 8 inches.
67. ARMAMENT of a ship includes all offensive weapons.
68. TURRET GUNS are all guns of 6 inches in caliber and larger mounted in a turret, or a mount.
69. BROADSIDE GUNS are guns of less than 6 inches in caliber, except antiaircraft guns and guns for saluting purposes only.
70. ANTIAIRCRAFT GUNS are intermediate caliber guns designed for use against air­craft.
71. DOUBLE PURPOSE GUNS are guns de­signed for use against both aircraft and surface targets.
72. ANTI-AIRCRAFT MACHINE GUNS are minor caliber fully automatic guns designed for use against aircraft.
73. MAIN BATTERY The term main battery includes those guns of the largest caliber on board.
74. SECONDARY BATTERY Only turret ships are considered to have a secondary battery; the secondary battery includes all except the turret guns and those guns specifically designated for use against air­craft. In ships having no broadside guns other than double purpose guns this battery may be designated as the secondary battery.
75. ANTI-AIRCRAFT BATTERY includes all anti-aircraft guns and anti-aircraft machine guns.
76. ANTI-AIRCRAFT MACHINE GUN BAT­TERY includes all anti-aircraft machine guns.

SALVO ANALYSIS

77. A SALVO consists of two or more shots fired either simultaneously by means of a master key as in director or master-key firing, or on the same firing signal as in pointer fire. In gunnery exercises, a salvo is considered to have been fired when it is clearly the intent to fire two or more shots, but, through casualty, only one shot is fired.
78. The PATTERN of a salvo in range is the distance measured along the lone of fire between the shot of the salvo falling or bursting at the greatest distance from the firing point and the shot falling or busting at the shortest distance, excluding wild shots. In deflection it is the distance, measured at right angles to the line of fire, from the shot falling or bursting to the greatest distance to the left of the line of fire, excluding wild shots.
79. The MEAN POINT OF IMPACT is the point which is at the geometrical center of all the points of impact of the several shots of a salvo, excluding a wild shot.
80. The DISPERSION of a shot is the distance of the point of impact of that shot from the mean point of impact of the salvo. Dis­persion in range is measured along the line of fire and in deflection at right angles to the line of fire.
81. The APPARENT MEAN DISPERSION of a salvo in range (or deflection) is the aver­age of the dispersions in range (or deflec­tion) of the several shots of the salvo, excluding wild shots.
82. The TRUE MEAN DISPERSION is obtained when an infinite number of shots are fired. The mean dispersion of a small number of shots. In analysis of gun practices, to obtain the true dispersion of a salvo the apparent mean dispersion of the salvo is multiplied by a factor depending upon the number of shots in the salvo, excluding wild shots. This factor represents the probable ratio of the mean dispersion of an infinite number of shots to the mean dispersion of that number of shots.
88. ERROR OF THE MEAN POINT OF IM­PACT is the distance of the mean point of impact from the target or other reference point measured parallel to the line of fire for range and at right angles to the line of fire for deflection.
84. A STRADDLE is obtained for a salvo in range (or deflection) when, excluding wild shots and other shots of the salvo beyond the target (right and left, respectively, for deflection).
85. A WILD SHOT is a shot, the point of fall of which is abnormal as compared to the fall of the remainder of the salvo. For the pur­pose of the scoring and analysis of gunnery exercises, a wild shot is considered as a shot which fulfills one or both of the following conditions:
(a) IN RANGE the shot must fall or detonate at a distance from the mean point of impact of the remaining shots in the salvo greater than 3.5 percent of the average gun range of the salvo for full and special velocity and greater than 4 percent of the average gun range of the salvo for target velocity.
(b) IN DEFLECTION the shot must fall or detonate to an angular difference of more than 10 mils from the mean point of impact of the remaining shots in the salvo.
86. THE DANGER SPACE is the distance measured along the line of fire, in front of the target, such that, if the target were moved toward the firing point, a shot strik­ing the base of the target in its original position would strike the top of the target in its new position.
87. THE HITTING SPACE for a material target is the distance behind the target that a shot striking the top of the target will strike the horizontal plane through the base of the target. Hitting space must be computed from column 19 of the range tables or directly from the angle of fall of the shot. Hitting space is the base for computing the limits of a constructive target when such is used for scoring hits in certain forms of gunnery exercises. Allowances are made for depth of the assumed target, in range, and in some cases, for an arbitrary distance short of the point of aim target.
88. LINE OF FIRE as used in analysis of gun­fire is the true bearing of the target from the firing ship at the instant a shot (salvo) is fired.

CONTROL

89. FIRE CONTROL comprises the entire sys­tem of directing the operation of the offen­sive weapons of a vessel, including necessary material, personnel, methods, communica­tions and organization.
90. CENTRALIZED CONTROL is the control of a battery by the personnel of one control station. Sending out range, deflection, target bearing and director correction, ringing salvo signals; firing and spotting may be done at a station or stations other than the one controlling.
91. INDEPENDENT CONTROL is the control of a subdivision of a battery by the person­nel of the subdivision. It includes the duties of control and the additional duties of com­puting and transmitting firing data, target bearing and director correction, ringing salvo signals, firing and spotting.
92. LOCAL CONTROL is the control of a gun or turret by the personnel of that gun or turret. It includes the duties of control computation and spotting of firing data, fir­ing, and spotting.
93. A SPOT is the estimate made by the spotter, as a result of his observation of the fall of shot (bursts in the fire), to be applied to the gun range, deflection or fuse setting nec­essary to correct for the error of the mean point of impact (or burst) of a salvo.
94. SPOT APPLIED is the correction to the gun range, deflection or fuse setting actually ap­plied as a result of a spot received. It may be the result of the co-ordination of several spots.
95. MATCHING POINTERS is the term applied to either "following the pointer" or "match­ing zero readers", in a fire control system in which an index (pointer or zero reader) on a local station instrument is kept matched with an index (pointer or zero reader) con­trolled from a distant station.

METHODS OF FIRING

96. POINTER FIRE is the method of firing in which the gun sights are set for range and deflection and the pointing, training, and firing are performed locally at the gun.
97. MASTER KEY FIRE is a method of collec­tive firing in which guns are trained and elevated in accordance with signals trans­mitted by a director or directors to the guns (or to associated instruments and as cor­rected by them to the guns) and then are fired from the director or directors.
98. DIRECTOR CORRECTION FIRE is that method of director fire wherein the guns are laid in elevation to an angle of elevation above the reference plane corrected for a pre-selected director correction and fired when the reference plane is inclined to the line of position by the pre-selected amount.
99. SELECTED LEVEL FIRE is that method of director fire wherein the guns are laid in elevation to the correct angle correspond­ing to a pre-selected level angle, and fired when the reference plane assumes that level angle.
100. SELECTED CROSS-LEVEL FIRE is that method of director fire wherein the guns are continuously aimed in elevation (to com­pensate for changing level angle) and are trained in accordance with a gun train order corrected for a pre-selected cross-level angle. The guns are then fired when the reference plane assumes this cross-level angle.
101. DIRECTOR FIRE IN TRAIN-ELEVATE BY TELESCOPE is that method of director fire in which the director transmits the gun train angle only and the gun is elevated as in pointer fire.
102. DIRECTOR FIRE IN ELEVATION-TRAIN BY TELESCOPE is that method of .director fire in which the director transmits the gun (elevation) angle and the guns are trained as in pointer fire.
103. INTERMITTENT CRANKING is the method of operating the director in which the director pointer follows the target with his elevating gear until near the point of reversal of the ship's motion, and then short­ly after the reversal, fires when the hori­zontal cross-wire crosses the point of aim.
104. CONTINUOUS CRANKING OF CONTINU­OUS AIM is the method of operating the director in which the director pointer at­tempts to keep the horizontal cross-wire constantly on the point of aim by means of his elevating gear.
105. INDIRECT FIRE is that method of director fire employed when the target is obscured to the firing station or when for other reasons the target is not used as the point of aim. In indirect fire a vessel or object whose posi­tion with reference to the target can be determined, may be used for determining the firing point in elevation, with a bearing generated by the rangekeeper or a computed true bearing by a navigational plot of the firing ship and target.
106. BARRAGE FIRE is a rapid fire using a fixed range, or fuze setting, so that the target, if it continues its course and speed, will pass through it. The barrage must be re-laid depending upon the movements of the target. A barrage is laid in a ZONE OF FIRE ; the area covered by the zone of fire is normally the pattern size of the group or battery of guns.
107. FIXED ZONE is a method of AA barrage fire by which the approaching plane is met by a series of zone barrages, or bursts of fire, all projectiles in each zone being fired with the same fuze setting and same sight angle.
108. CREEPING ZONE is a method similar to the fixed zone, except that in each zone the projectiles are fired with successively de­creasing (or increasing) fuze setting.
109. ONE-MINUTE BARRAGE is the name given to a method of anti-aircraft fire in which a creeping zone is fired for a period of one minute prior to the time the bomber reaches the bombing point.

FIRE DISTRIBUTION

110. DIVIDED FIRE: One ship firing at two or more targets.
111. CONCENTRATION:

(a) Single fire: One ship firing alone at a single target.
(b) Double concentration: Two ships firing at the same target.
(c) Triple concentration: Three ships firing at the same target.
(d) Quadruple concentration : Four ships firing at the same target.

112. A SPOTTING STATION is a location from which fall of shot (or brust) is observed; other functions of control may be exercised from the spotting station, but only when such other functions are definitely men­tioned.
113. A PLOTTING STATION is a station for the determination of gun ranges and deflection, the reception and dissemination of fire con­trol information, and for the possible control of fire.
114. A GUN DIRECTOR is an instrument for firing a battery or group of guns from a remote position, in accordance with the prin­ciples of director firing as set forth herein.
115. THE STABLE ELEMENT is a device for indicating the true horizontal plane, and for measuring the values of roll and pitch, or level and cross-level. It may be separately mounted or incorporated in a gun director
Text submitted by Pieter Bakels.

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