|NAVAL ORDNANCE AND GUNNERY
VOLUME 2, FIRE CONTROL
Chapter 18 Spotting
A. Laws of probability in their effect on gunfire and on spotting
B. The Spotter
C. Methods of spotting
| C. Methods of Spotting
18C1. Spotting terminology
It is important that the proper terminology and order of spots be observed by the shipboard spotter.
The surface problem. For surface fire, only range and deflection are spotted. The correction necessary to bring the MPI on the target is given in the following terminology and order.
1. Deflection correction-RIGHT or LEFT, in mils.
1. Range correction-ADD or DROP, in yards.
Where no correction is necessary, the designation No CHANGE is used. Typical examples of spot transmissions by telephone are RIGHT 10, ADD 1,000, DROP 500.
The AA problem. For air targets, corrections to bring the burst on the target are needed in three dimensions. Even well-trained personnel find it almost impossible to estimate errors rapidly in three dimensions. AA spotting is therefore generally ineffective and is usually not attempted except in cases of obvious constant system errors.
The proper terminology for spotting in the AA problem is as follows:
1. Deflection correction-RIGHT or LEFT, in mils.
2. Height-of-burst correction-Up or DOWN, in mils.
Deflection and elevation spots will normally be made by the control officer. Range spots will be made by the rangefinder or radar operator, and all spots will normally be transmitted electrically.
Naval gunfire support. In shore bombardment, as in AA fire, spots in three dimensions may be necessary. The terms are the same as in the preceeding paragraph, but the units are not the same. When naval guns are used to support landing operations, joint forces are involved. The Navy, Army, and Air Force have a standardized spotting terminology for joint operations which will differ from the above, in that all corrections are spotted in yards. At the rangekeeper or computer, deflection and elevation spots must be converted to angular units before being applied to the computer.
18C2. Methods of spotting
It is evident from the preceding discussion that certain methods of spotting and certain other techniques to assist in the control of gunfire must be employed by the shipboard spotter in the difficult problem of estimating the MPI error in range.
It must be borne in mind that these methods and techniques are but general guides and are in no way intended to replace or abrogate actual procedures which are laid down in detail in Fleet and Force doctrines. Such doctrines must be perfectly known and rigidly followed by the spotter under all conditions. This demand may seem restrictive, but it is necessary that only proved and sound principles be employed. Individual untried ideas, which are likely to produce negative results, are excluded by the ruling.
Subject to the foregoing qualifications there are are two methods of spotting:
1. The direct method.
2. The bracket-and-halving method.
There are also three other techniques used in the control of gunfire which are closely related to spotting and must be discussed at this point, namely:
3. Tracer control.
The use of any of the above methods or techniques, is determined by the type of battery firing, the type of target, the range, and the visibility.
18C3. Direct methods
Spotting by the direct method is, as its name implies, the spotting of salvos (splashes) direct to the target. This is the most desirable procedure, but its use is very limited. For reasonably accurate spotting at a range of 15,000 yards, a spotting height of 120 feet is required. It is also necessary that the splash be relatively close to the target, and that the rangekeeper set-up be fairly accurate. These considerations limit this method to main-battery fire at relatively short ranges.
A thoughtful analysis of the problem with reference to the spotting diagram in figure 18B1 reveals that the greatest limitation of the direct method is the matter of range. Deflection spots can be made with equal accuracy at any visible distance. If, then, air spots are available, and the plane spots in range with the ship spotting in deflection, the direct method can be used by the main battery at any range at which a portion of the splash is visible to the shipboard spotter. Air spotters cannot spot accurately in deflection unless they have a line of sight containing the fixing ship and the target. Radar spotting is usually direct, even at considerable ranges.
18C4. Bracket-and-halving method
The bracket-and-halving method is used at long ranges by the main battery when no air or radar spot is available. At great distances it will be impossible to tell if a splash is short of or over a target, unless the two are in line. If the splash and target are not in line, or if the splash does not line up with a ship in the formation whose range is approximately that of the target, the first spot is made in deflection only. When target and splashes are in line in deflection, a range spot is made in such a direction and amount as to “cross” the target definitely. The direction of the next spot is reversed, and the size of the spot is cut in half. This “halving” is continued until a straddle is obtained, at which time it may be appropriate to shift to rapid salvo or to continuous fire. The spot should not be reduced below pattern size. Once a straddle is obtained, centering deflection spots only should be made, as long as the spotter is sure he has both short and over splashes. If, however, over splashes are obscured by the target or by shorts, every third or fourth salvo should be fired with an add spot of one pattern size, so as to lift the entire salvo over the target. Only thus can the spotter know his straddles were not all shorts in reality.
When ranging is difficult and visibility poor because of fog, smoke, or darkness, the use of ladders is of considerable assistance to a spotter. Ladders are not particularly adaptable to fast-moving targets. There are many variations of this technique. However, it is only necessary to understand its fundamental principles. Fire is deliberately opened short, and succeeding salvos are fired so as to approach the target in steps not less than pattern size. As soon as the target is crossed, the steps are reversed and halved until the target has again been crossed. After a straddle has been attained, a rocking ladder may be used in conjunction with slow timed fire or with rapid salvo or continuous fire.
This consists of moving the pattern back and forth across the target by small arbitrary successive spots such as + 100, 0, -100, . . . introduced at the computer or rangekeeper. It has the effect of increasing the pattern size, which may be valuable when firing against a target capable of rapid maneuvering. When this method is used, care must be taken to make correcting spots only when the zero salvos of the ladder series fall. The rocking ladder can be used in conjunction with air on radar spotting, so long as the spotter is kept informed that this technique is being used.
The barrage method of control consists of placing a zone of fire short or over the target, according to whether the target is closing or opening range. The gun range is kept constant until the target passes through the zone, and is then lowered or raised as necessary to force a second passage through the barrage, continuing the movement of the barrage as may be necessary. This system is used by smaller-caliber fast-firing batteries against light, fast-moving targets. It is the spotter’s function to determine when the barrage should be shifted, and in which direction and how much it should be moved.
The tracer method of control consists of spotting shots by noting the error as the tracer passes the target. It is used chiefly in spotting machine-gun fire. This method may also be used in addition to others in spotting larger-caliber projectiles (i. e., 5”/38 cal. or larger), particularly at short ranges. At longer ranges, tracers are of doubtful assistance to the spotter, because of the difficulty in keeping them in sight or in judging when they pass the target. Under certain light conditions the tracers cannot be seen clearly, and should not be relied upon except for short-range firing.
18C8. Short-range spotting
Spotting the fall of shot at very short ranges differs from other spotting problems in that range errors are not difficult to judge. However, in determining deflection errors at short ranges, consideration must be given to the travel of the target and the spotter’s position relative to the line of projectile flight. For example, with the firing ship and target on opposite courses, target to starboard, a shot fired with correct deflection but long in range will appear to the spotter to be in error to the left of the target. Special shortrange splash diagrams are constructed as an aid to the spotter in this type
of firing; instructions for their preparation and use are available in publications carried by all ships.
18C9. Radar spotting
Radar, using Type B presentation, gives accurate range errors under any visibility conditions. Therefore, it has completely replaced visual spotting as the primary means of determining range errors. Bearing discrimination by radar has become sufficiently accurate to justify its use as the primary means of deflection spotting. However, normally in a day action under favorable visibility conditions the following procedure is followed in spotting:
1. When splashes can be unmistakably identified on the radar screen, direct radar spotting in range is employed.
2. When splashes can be seen visually, the top spotter on the ship employs direct spotting in deflection.
3. Any available aircraft spots are used as a check.
In the event of a night action, or action under conditions of reduced visibility, radar will normally be employed for both range and deflection spots. The technique of radar spotting as it applies to a specific radar and method of presentation is described and illustrated in section G of chap. 20.
18C10. Pyramiding spots
Spot pyramiding is the application of a new spot before the effect of a previous spot has had time to become apparent. It can occur only at rapid fire, when the interval between shots or salvos is less than the time of flight plus the spotting interval. In that case, when a salvo lands there are one or more other salvos in the air. Suppose the spotter makes a spot on the salvo which has just landed. This spot is applied and a new salvo is fired. Then one or more of the salvos which were already in the air lands, and the spotter, forgetting that his previous spot has not had time to show its effect on the fall of shot, spots again. This spot is applied to the next salvo fired, with the result that this salvo is over-corrected and will probably miss, as will subsequent salvos until the spotter sees his mistake and spots back again to remove the pyramided spot.
In order to help the spotter avoid pyramiding, the time-of-flight signal mechanism is used. This mechanism may be incorporated in the rangekeeper or may be in the form of a separate time-of-flight clock. It is operated by the rangekeeper operator or an assistant, who presses a button when the salvo on which a spot is applied, is fired Just before the salvo lands the mechanism sounds a buzzer which informs the spotter that a spotted salvo is about to land. When this system is used, the spotter, after making a spot, waits for the buzzer before making another spot.
Spot pyramiding is a very common fault and has a disastrous effect on accurate control of fire. Every means available must be used to prevent it, and these means must be carefully and correctly operated. If they are improperly operated, they may cause worse errors than they are designed to prevent. For example, if the time-of-flight mechanism operator forgets to press the button for a spotted salvo, and the spotter waits for it, he may continue to wait after it becomes apparent that his previous spot was incorrect. In such a case the operator making the mistake should advise the spotter of the situation as soon as he himself realizes it. Some ships have a Spot Coordinator, located in the plotting room, who is responsible for the prevention of pyramiding as well as for the coordination of radar, shipboard visual, and air spots.
When modern fire control systems are being used to solve the fire control problem, the process described in subsequent chapters as rate control is used to make constant corrections to the solution. If spots are applied at the same time as rate control corrections, the effect is the same as pyramiding of spots. This provides an additional reason for not ordinarily attempting to spot bursts in antiaircraft fire.