The Problem of Hitting a Moving Target from a Moving Ship

When an enemy ship is sighted and comes within range, it is the job of fire control to solve the fire control problem so that your ship's guns can hit the target. If the enemy were obliging enough to remain in one place and your ship were also brought to a stop, the required elevation and traverse gun settings could be determined by considering only the factors of interior and exterior ballistics for fixed gun and target discussed on the preceding sheets. No enemy ship would be foolish enough to remain still under fire, however, and your own ship doesn't want to be a "sitting duck" for the enemy's fire either. So both the target and your own ship will keep moving during battle. As a result the target range and bearing are continually changing, necessitating constant changes to gun elevation angle and sight deflection.

In order to make the proper corrections, it is necessary to know in what direction and at what speed the target ship is moving. With this information it is possible to predict the position the target will occupy after the time of flight of the projectile has elapsed. The gun is then directed so that the projectile will strike the "future position" of the target. The corrections involved are similar to those which compensate for the earth's rotation, but are much more significant and variable.

D-2

HITTING A MOVING TARGET FROM A MOVING SHIP

The Problem of Hitting a Moving Target from a Moving Ship (continued)

Another factor which must be considered in the fire control problem when both ships are moving is the fact that the velocity of the ship will be imparted to the projectile at the instant of firing and will affect its trajectory. To illustrate this effect, consider the target as standing still and the firing ship in motion. As the gun is fired the projectile has two velocities-its own I. V. and that due to the ship's motion. If the ship's velocity is not considered, the projectile will land short, beyond or to one side of the target, depending on whether the ship is leaving, approaching or moving toward one side or the other of the target. Thus the effect of ship's speed and direction on projectile trajectory must be considered to obtain accurate fire.

D-03

HITTING A MOVING TARGET FROM A MOVING SHIP

The Problem of Hitting a Moving Target from a Moving Ship (continued)

A final factor which enters into the solution of the fire control problem between moving ships is the fact that the deck which supports the gun is very seldom perfectly level. A moving ship requires that corrections be made to the gun settings to compensate for deck motion.

D-4

HITTING A MOVING TARGET FROM A MOVING SHIP

Solving the Fire Control Problem

Although the steps involved in the solution of the problem of scoring a hit on a moving target from a moving ship are many, they all can be summarized as follows:

Steps In Solution Of The Fire Control Problem

1. Determine present target position in relation to own ship
2. Predict future target position in relation to own ship
3. Stabilize the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The table below shows what must be known and what equipment is used to solve each of the above listed steps in the fire control problem.

Step in Solution

Performed By

Need to Know

1

Gun director

Range from optical rangefinder or radar Bearing from sight telescope or radar

2

Rangekeeper

Own ship course from ship's gyro Own ship speed from pitometer log Target course-estimated at director Target Speed-estimated at director

3

Stable vertical

Level angle-measured at stable vertical Crosslevel angle-measured at stable vertical

4

Rangekeeper

Future target position from step 2 Range and deflection ballistic corrections-put into rangekeeper by hand Level and crosslevel-from stable vertical

5

Synchro and servo systems, mechanical linkages, or telephoned verbal orders

Gun orders from rangekeeper

On the following sheets we will discuss in detail each of the steps in the solution of the problem. You will learn a little about the theory behind the various corrections required and how those corrections are calculated.

D-5

HITTING A MOVING TARGET FROM A MOVING SHIP

Determining Present Target Position

Present target position is the target's position relative to own ship at the instant the guns are fired. To determine the target's present position we must know its present range and bearing. In good weather, when the target is visible, the optical rangefinder in the gun director is used to determine the range to the target, and the sight telescope in the director is used to measure relative target bearing-the angle between own ship's centerline and the L. O. S. You will learn more about the rangefinder and sight telescope and how they measure range and bearing in the next section.

When the target is obscured by bad weather or in night firing, radar is used to track the target and measure its range and bearing. You will learn in a later section how radar serves in place of optical equipment under poor visibility conditions, and sometimes to the exclusion of optical equipment for certain applications.

The next few sheets discuss how the target's future position is predicted so that the initial gun settings can be corrected to fire the projectile to the position which the target will occupy at the moment of impact. In order for the target's future position to be accurately predicted, its present position must be known. Thus determining the target's present position is the first and most basic step in the solution of the fire control problem.

D-06

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position

Let's assume that the target's present position has been determined and that both ships are in motion. If we make all the corrections required to compensate for the effects of range and deflection ballistics and fire the guns based on the present position of the target, our projectiles will not hit the target. During the time it takes for the projectiles to travel from gun to point of impact, the positions of both own ship and target have changed. Thus it is necessary to predict where the target will be at the time of impact-future target position.

In order to predict future position of target we must know own ship's course and speed, as well as target course and speed. You already know that own ship's course is obtained from the ship's gyro and own ship's speed is obtained from the ship's pitometer log. Target course and speed are obtained by direct observation and estimation at the gun director. The information on own ship and target motion is transmitted to and combined in the rangekeeper which calculates future target position.

Now we've cleared the second major hurdle in the path to solution of the fire control problem.

On the following sheets we will go into details of how corrections are made for the motion of own ship and target, what "range rate" and "linear bearing rate" are and how they are used to predict range and deflection corrections.

D-7

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-True Bearing and Relative Bearing

Before we get into the analysis of how own ship's and target's course and speed affect the prediction of future target position, let's get a few definitions straightened out. We'll be talking about relative target bearing, own ship's course and speed, and target course and speed, on the next few sheets. It's important that you understand what each of these terms means and how they are represented.

First let's distinguish between relative target bearing (Br) and true target bearing (B). Br is the angle measured clockwise in a horizontal plane between the fore-and-aft axis of own ship and the line of sight. True target bearing (B) is the horizontal angle measured clockwise between the north-and-south axis and the L. O. S. Thus the direction of own ship motion affects the value of Br, but has no effect on B. Since it is more convenient to train the guns with respect to the ship's centerline, relative target bearing is used rather than true target bearing, in the solution of the fire control problem.

Own ship's course (Co) is the horizontal angle measurers clockwise between north and own ship's axis. Own ship's speed is designated So. Target angle (A) is the angle measured clockwise between the fore-and-aft axis of the target ship and the line of sight. Target speed is designated simply S. These quantities are illustrated below.

The important thing to remember is that all of these angles are measured clockwise. Depending on the positions of own ship and target, their values may vary from zero to 360 degrees. The method of measuring them, however, never varies.

D-08

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Correcting for Own Ship's Course and Speed

You know that own ship's motion affects the solution of the fire control problem and that both range and deflection are affected, depending upon own ship's course and speed.

Own ship motion can be represented graphically by a line whose magnitude is So (own ship's speed). So has two effects: that of shortening or lengthening range, and that of deflecting the projectile left or right. The deflection component Xo and the range component Yo, respectively perpendicular to and along the L. O. S., determine the effect of ship movement on the deflection and range of the projectile.

COMPONENTS OF OWN SHIP MOTION

The maximum effect of own ship motion on range occurs when the ship is either directly approaching or leaving the target; then Yo = So, Xo = 0 and Br = 0 or 180 degrees. The maximum effect of own ship motion on deflection occurs when own ship's course is perpendicular to its L. O. S. ; then Xo = So, Yo = 0 and Br = 90 or 270 degrees.

If you multiply Yo by Tf (the time of flight of the projectile) and affix the correct sign ("-" if Yo is toward target, "+" if Yo is away from target), you have the correction-to range due to own ship movement, neglecting air resistance. Similarly, the value of the linear deflection would be XoTf. Air resistance is corrected for by multiplying by a factor of less than one, so that the actual corrections for range and deflection will be less than YoTf and XoTf.

D-9

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Correcting for Own Ship's Course and Speed (continued)

When a ship moves in still air it cuts through the air and creates a head wind which is equal and opposite to own ship's speed. Thus, when a projectile is fired into still air from a moving ship, in addition to all other forces acting on it, head wind will affect its trajectory. When true atmospheric wind is present it also acts on the projectile during its flight. Head wind and true wind combine to form a resultant wind called "apparent wind."

The component of apparent wind velocity across the L. O. F. is the apparent cross wind; the component of apparent wind along the L. O. F. is the apparent range wind.

In the previous section you learned about the effects on gun elevation and train of range and cross winds respectively for fixed gun and target. The effects are the same when own ship is in motion, but apparent cross and range winds must be used in the computation of corrections to elevation angle Eg and sight deflection Ds.

D-10

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Correcting for Target Course and Speed

Target motion can be treated in exactly the same way as own ship motion. The target motion has a magnitude S (target speed) and direction A (target angle). S has the same effect on range as So, and is broken down into components along and perpendicular to the L. O. S. in the same manner. The range component is then Yt and the deflection component is Xt.

COMPONENTS OF TARGET MOTION

The maximum effect of target motion on range occurs when the target is directly approaching or leaving own ship; then S = Yt, Xt = 0 and A = 0 or 180 degrees. The maximum effect of target motion on deflection occurs when the target's course is perpendicular to the L. O. S. ; then S = Xt, Yt = 0 and A = 90 or 270 degrees.

As for own ship motion, the correction to range due to target motion, neglecting air resistance, equals YtTf. If Yt is toward own ship the correction is negative; if Yt is away from own ship the correction is positive. In the same manner the linear deflection correction is XtTf. These results must be multiplied by a factor to correct for air resistance so that the actual corrections to range and deflection due to target motion will be slightly less than YtTf and XtTf.

D-11

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Combining Own Ship and Target Course and Speed

Now that own ship and target speeds have been resolved into components along and across the L. O. S. , these components can be combined to find relative motion between ship and target in range and deflection.

The amount which range changes in a unit of time-the time rate of change of range-is called "range rate along the L. O. S. dR," and is equal to the sum of Yo and Yt (dR = Yo + Yt). Range rate dR may be positive or negative, depending on the value and sign of Yo and Yt. Thus, in the illustration below, if Yo were decreasing range at 2 yards per second, and Yt were decreasing range at 4 yards per second, dR would equal (-2) + (-4) or dR = -6 yards per second.

D-12

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Combining Own Ship and Target Course and Speed (continued)

However, if Yo were increasing range at 4 yards per second (moving away from target) and Yt were decreasing range at 2 yards per second, dR would equal (+4) + (-2) or dR = +2 yards per second. Thus dR is positive if range is increasing (when Yo + Yt is plus), and dR is negative when range is decreasing (when Yo + Yt is minus).

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Combining Own Ship and Target Course and Speed (continued)

The amount which bearing changes, measured as a change in linear distance at the target per unit time, is called "linear bearing rate RdBs," and is equal to the sum of Xo and Xt (RdBs = Xo + Xt). Linear bearing rate RdBs is considered positive if it tends to increase the relative target bearing angle Br, and negative if it tends to decrease Br.

In the illustration below both Xo and Xt tend to increase Br, so that Xo = +4 and Xt = +2. Hence, RdBs = (+4) + (+2) = +6. The linear bearing rate is converted to an angular change in bearing dBs by dividing by the range R.

D-14

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Combining Own Ship and Target Course and Speed (continued)

However, if Xo tended to decrease Br and Xt tended to increase Br, as below where Xo = -3 and Xt = +2, RdBs would equal (-3) + (+2) or RdBs = -1 yards per second, and the relative target angle Br would be decreasing. Again, angular bearing rate must be determined and depends on the range R.

Thus the linear bearing rate RdBs may be either positive or negative depending on relative motion of ship and target across the L. O. S.

Since range is changing at a rate equal to dR and the projectile remains in the air for a time of flight Tf, the total change in range due to relative target motion during projectile flight will be Rt = dR x Tf, where Rt may be positive or negative depending on whether dR is plus or minus. The value of Tf used here, however, is based on the time it would take the projectile to reach the target at present range R. Since range is changing, the future range R2 at the instant of impact will be more or less than R by the amount Rt, and so the Tf required for the projectile to travel distance R2 will be more or less than that based on present range. Thus, to find R2 we must know Rt (R2 = R + Rt); to find Rt we must know Tf; to find Tf we must know R2. This seemingly "vicious circle" problem can be solved mathematically by means of substitution and approximation, but is more conveniently solved by a mechanical computer which gives the correct value of R2 when present range R and range rate dR are known.

The target bearing angle Br changes during the projectile's flight due to relative target motion. This change must be predicted so that the guns can be trained on the position which the target will occupy after the expiration of Tf.

You know that the angle between the L. O. S. and the L. O. F. is called sight deflection Ds, and you have already learned that drift and wind contribute to Ds. Now it is necessary to introduce another correction to gun train angle. This correction is the lead angle Dt, which is the angle through which the gun must be traversed from the L. O. S. to compensate for own ship and target motion.

In the illustration above, Df and Dw represent the sight deflections necessary to correct for projectile drift (Df) and wind (Dw). The total sight deflection then is Ds = Dt + Df + Dw.

The distance which the target will travel across the line of sight during the time of projectile flight will equal linear bearing rate multiplied by time of flight (RdBs x Tf). To find the angular change in position Dt, we divide RdBs x Tf by the predicted range R2:

Dt = (RdBs x Tf) / R2

Dt is an angular correction which can be added to or subtracted from the train angle, depending on the sign of RdBs, to compensate for own ship and target motion. Thus Dt is a factor of Ds, total sight deflection.

D-18

HITTING A MOVING TARGET FROM A MOVING SHIP

Predicting Future Target Position-Range Effect of Deflection

Now you have learned how the future range and bearing of the target can be predicted. The solutions obtained were based on the fact that range is large and Dt quite small. If the distance traveled by the target during the projectile's flight (RdBs x Tf) becomes large, due to high target or own ship speeds, then Dt increases and an appreciable range error due to deflection called Rx is introduced. This error is the result of computing the range prediction R2 using the value of dR based on the present target position. Actually, however, dR changes as relative target position changes. In the illustration below dR changes from a negative value (dR1) at present target position to a positive value (dR2) at future target position. Thus, the distance traveled by the target toward own ship is less than the computed value (Rt = dR1 x Tf). As a result, the distance from own ship to target's future position is greater than R2 by an amount Rx.

The correction Rx is based on the angle Dt and predicted range R2. The deflection due to wind (Dw) is usually also considered, and Rx is found by use of the formula:

Rx = R2 x (Dt + Dw)^{2}

D-19

HITTING A MOVING TARGET FROM A MOVING SHIP

Review of Corrections Required to Predict Future Target Position

WOW!! That may very well be your reaction to the preceding sheets. "How can I ever remember all of those corrections, and how to make them?" Well, don't worry about it. You are not, as we said before, expected to become a ballistics expert and you are not expected to remember how to make all of the corrections you've just read about. It will be your job aboard ship to maintain and operate the fire control equipment which makes the corrections automatically for you. You are only expected to realize that corrections must be made for accurate gun fire and to have some idea of what these corrections are.

Let's summarize the steps required to correct for own ship and target motion, and thus predict future target position.

Now that you have some appreciation as to how future target position is predicted, let's go on and see how the third step in the solution of the fire control problem is solved-that of stabilizing the various units.

D-20

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units

Now we come to the third step in the solution of the fire control problem. We have determined present target position and predicted future target position. It would seem that all we need to do is calculate the required gun elevation and train, position the guns, and fire. This would be the case for a shore installation, or if our ship's deck were perfectly horizontal. But we are considering the problem of firing a gun from a moving ship and the deck of a moving ship is very seldom perfectly level. Roll and pitch of the ship require that the stable vertical measure "level" and "cross-level" in order to stabilize the various units and provide a reference plane from which gun elevation and train angles can be measured. What are roll and pitch? Level and crosslevel? You'll find out all about them on the following sheets.

D-21

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Roll and Pitch

Under normal conditions of operation the ship's deck is not horizontal, as it has been considered until now. Wind and ocean currents and the forward motion of the ship impart a rolling motion to the ship. This motion can be resolved into two motions: (1) "roll"-rotation of the ship from side to side in a plane perpendicular to the deck through the ship's sides, and (2) "pitch"-rotation of the ship from bow to stern in a plane perpendicular to the deck through the ship's axis. These two motions and the planes in which they are measured are shown below.

Normally the motion of a ship at sea is such that both roll and pitch are present. But regardless of how the deck moves, its motion can always be resolved into roll and pitch.

D-22

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-The Effect of Deck Motion on the Fire Control Problem

In order to illustrate the effect of roll and pitch on the solution of the fire control problem, let's start with the ship's deck horizontal. You know that to correct for wind, drift, relative target motion, etc., the gun bore axis must be elevated above the L. O. S. and traversed right or left. These angles are sight angle and sight deflection respectively.

A gun mounted on the deck must move with the deck and if roll and pitch cause the ship to move so that the deck is no longer horizontal, the L. O. F. will no longer occupy the same position with respect to the L. O. S. (which is fixed between gun and target).

If no adjustments were made to gun train and elevation, sight angle and sight deflection would change continuously as long as the ship rolled and pitched. But in order to score a hit, we know that Vs and Ds must equal the value calculated by the rangekeeper. We must continuously correct gun elevation and train to maintain the correct values of Vs and Ds, thus keeping the gun bore axis in its original position regardless of how the ship's deck may move due to roll and pitch.

D-23

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Measurement of Deck Motion

Before we can adjust gun elevation and train to correct for the effects of roll and pitch, we must be able to measure the deck's inclination. In addition, since the guns can train only in the deck plane and elevate only in a plane perpendicular to the deck, we must convert deck inclination measurement to angles in these two planes. If we were to use roll and pitch to measure deck tilt, a variation in either would affect both gun elevation and train. To simplify the computation of required corrections to gun elevation and train, the deck's inclination is measured for fire control purposes in terms of "level" and "crosslevel. " "Level" is the component of deck motion in a plane perpendicular to the deck through the L. O. S. ; "cross-level" is the component of deck motion in a vertical plane perpendicular to the L. O. S. You will learn on the following sheets how level and cross-level affect gun elevation and train.

D-24

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Level Angle

In order that you may understand more easily the measurement and effects of level and crosslevel, we will consider that the deck moves in such a way that only one is present at a time.

If roll and pitch of the ship cause the deck to move only in a plane perpendicular to the deck through the line of sight, as shown in the three illustrations on the right, only level is present. The level angle L' is measured in this plane between the deck plane and the horizontal plane (in which the L. O. S. lies for surface targets).

Although level angle is defined as the angle between the horizontal and the deck in the level plane, for fire control purposes it is measured by the stable vertical as the angle between the true vertical and a perpendicular to the deck in the level plane.

Because the guns elevate in a plane perpendicular to the ship's deck, level angle can be corrected for simply by elevating or depressing the guns, as you will see on the next sheet.

D-25

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Correcting for the Effects of Level

In order to fire a projectile the desired distance, the elevation of the gun above the horizontal, angle Eg, must remain constant regardless of the inclination of the ship's deck. The value of Eg required to attain a given range is determined by applying ballistic corrections previously discussed. The elevation of the guns, however, is more conveniently measured with reference to the deck plane. Thus as motion of the ship in level causes the deck plane to move with respect to the horizontal, the level angle L' must be continuously added to or subtracted from Eg to obtain the required "gun elevation order" E'g-elevation of the guns above the deck plane.

Motion of the ship in level, then, affects only the elevation of the guns. Corrections are made for the effects of level simply by elevating or depressing the guns through the level angle L', thus adjusting E'g to keep Eg constant.

D-26

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Crosslevel Angle

If the roll and pitch of the ship cause the deck to move only in a vertical plane perpendicular to the line of sight as shown in the three illustrations on the right, only crosslevel is present. The crosslevel angle Zh is measured in this plane between the deck and the horizontal plane.

As was true for level angle, crosslevel angle is defined as the angle between the horizontal and the deck in the crosslevel plane, but for fire control purposes crosslevel is measured between the true vertical (established by the stable vertical) and the perpendicular to the deck plane. You will learn in a later section how stable vertical measures level and cross-level.

Errors resulting from the effects of crosslevel are not as easily corrected as those produced by level, as you will see on the following sheet.

D-27

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Correcting for the Effects of Crosslevel

Just as Eg must remain constant regardless of the deck motion, sight deflection Ds must remain the same for accurate fire. Level angle does not affect Ds, but crosslevel does. When the deck rotates through the angle Zh, the position of the gun barrel is shifted as shown below, resulting in changes in gun elevation and deflection. The L. O. F. is depressed and, in the case illustrated, is deflected to the right.

Both gun elevation and train must be adjusted to correct this error. The gun must be traversed in the deck plane and then depressed in a plane perpendicular to the deck until the gun barrel is restored to its original position.

Thus crosslevel affects both gun train and elevation, while level affects only elevation.

D-28

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Combining Level and Crosslevel

Normally, of course, both level and crosslevel are introduced as the ship rolls and pitches. Thus the total corrections to gun train and elevation orders must be the sum of the adjustments required to correct for the effects of level and crosslevel individually. You can see that these corrections are important, particularly in a heavy sea. For every change in the position of the ship's deck with respect to the horizontal, there must be a corresponding change in gun orders to compensate for the change in level and crosslevel. It is the job of the stable vertical to measure level and crosslevel continuously and transmit this information to the rangekeeper which computes the required changes in gun orders. Thus the stable vertical supplies the answer to the third step in the solution of the fire control problem.

D-29

HITTING A MOVING TARGET FROM A MOVING SHIP

Stabilizing the Various Units-Your Progress in Solving the Fire Control Problem

We might pause here for a moment and take a look at our progress in reaching a solution to the fire control problem.

We have completed the first three steps in the solution. We've located the target and determined its present position (step 1); we've predicted the target's future position (step 2); and we've measured level and crosslevel at the stable vertical to stabilize the system (step 3). Our progress is indicated below, and is tied in with the equipment utilized to accomplish each step in the solution.

On completion of step 3 in the solution, all quantities necessary for the solution of the problem have been measured. The only thing remaining to be done is the calculation of corrections required, using the measured quantities, and transmission of data to the guns. Let's go on and see how this is done.

D-30

HITTING A MOVING TARGET FROM A MOVING SHIP

Calculating Corrections to Gun Train and Elevation

Now that we have found all of the quantities necessary for the solution of the fire control problem by solving the first three steps, we must convert this information into gun train and elevation orders. It is of no use to determine level angle or target speed unless the guns can be aimed in such a way as to correct for these and the other factors of the problem. So it is necessary to calculate corrections to gun train and elevation from the data obtained by completing the first three steps. These data are fed, together with information on the range and deflection ballistics discussed in the preceding section, into the rangekeeper, which utilizes various mechanisms to calculate the required corrections to gun elevation and train. You will learn more about the rangekeeper and how it works in the next section.

On completion of step 4-calculation of corrections to gun train and elevation-we have actually completed the solution of the fire control problem. The remaining step-that of transmitting the data to the guns-is not a problem in the sense of having to measure or calculate any further values. But nevertheless we must consider it as a step in the overall fire control problem, for if no means were provided for transmission of gun orders from rangekeeper to turrets, what value would there be in the first four steps? So let's see how the gun orders are transmitted to the guns.

D-31

HITTING A MOVING TARGET FROM A MOVING SHIP

Transmitting Data to Guns

Now it's time for the pay-off! The various components of the fire control system have performed their functions. All the measuring and calculating are completed and it's time to direct the guns in accordance with orders received from the rangekeeper.

Various means are available for transmitting gun orders from the range-keeper to the guns and depending on the type of ship, the battle situation and the method of fire control being employed, any one or a combination of these transmission systems may be used. The various methods include: (1) electrical synchro and servo systems, (2) mechanical linkages, and (3) telephoned verbal orders.

Synchro and servo systems may be used to actually position the guns in response to the rangekeeper outputs, or may position indicating devices which inform the gunners of the proper gun settings. The mechanical linkage and telephone systems are secondary or auxiliary systems which also inform the gunners of the required elevation and train angles.

These systems for transmission of information are also used to link the other units of the fire control system-the director, stable vertical and rangekeeper. Their function here is just as vital as the transmission of the final gun orders, for the gun orders could not be determined if any one of the fire control system components were cut off from the others.

D-32

HITTING A MOVING TARGET FROM A MOVING SHIP

The Completed Solution to the Fire Control Problem

It may have seemed like a long and complicated procedure-and we must admit that it isn't a simple job-but at last we have reached the end. We've located the target and determined its present position, we've predicted the target's future position, stabilized the various units, calculated the corrections required to gun train and elevation, and finally, we've transmitted the gun orders to the guns. We have completed the five steps in the solution of the fire control problem, and all that remains to be done is to fire the guns.

Shown below is the tie-in between the five steps in the solution and the equipment used to complete each step.

We have considered the fire control problem, for our purposes of explaining its solution, as one isolated problem. It must be remembered, however, that conditions are constantly changing during combat. Own ship's and target's course and speed, wind, level and crosslevel, as well as other factors, are constantly changing value. So the fire control system must continuously solve the problem, taking account of the varying conditions. This it does, amazingly, and provides continuously changing gun orders which keep the guns properly aimed regardless of any change in one or all of the factors affecting the problem's solution.

D-33

HITTING A MOVING TARGET FROM A MOVING SHIP

Other Factors Affecting Accuracy

Now you have found out about almost all of the factors which enter into the solution of the surface fire control problem. In earlier sections you studied interior and exterior ballistics, and the problem of hitting a fixed target from a fixed position. In this section you have learned about the additional factors which must be considered when own ship and target are moving.

There are certain factors which affect accuracy of fire but do not properly fall into the classification of interior or exterior ballistics. They are functions of the particular ship in question, combat conditions, and/or the skill of operating personnel.

The fire control and gunnery systems on a large ship are very complex and contain many mechanical and electrical devices used to solve the fire control problem, aim, and fire the guns. This equipment must be kept in good working order, properly aligned and accurately calibrated. Malfunction of or lost motion in this equipment can cause serious errors in setting the guns in elevation and train so as to score a hit. Your responsibility in keeping the fire control equipment functioning properly is very important in order to keep errors due to malfunction of equipment to the absolute minimum.

Another factor which affects accuracy of fire is the skill of operating personnel. The gunnery and fire control equipment is only as useful as the operators are skillful in using it. Here, too, it will be your job to operate the fire control equipment effectively to keep human errors to the absolute minimum.

Combat conditions also affect accuracy of fire. All the most elaborate equipment in the Fleet will be useless unless the target can be located and its course and future position accurately determined. Evasive tactics by the enemy, therefore, often reduce the accuracy of fire.

D-34

HITTING A MOVING TARGET FROM A MOVING SHIP

The Importance of Spotting

From your study of interior and exterior ballistics, you will recall that not all of the factors which affect the flight of a projectile can be precisely evaluated in advance of firing. Some deviation from the predicted fall of the projectile is to be expected, even with the best fire control equipment available, experienced gun crews and efficient fire control personnel. Thus, it is entirely possible that the first shots fired may not hit the target. So it is not enough to track the target, operate the fire control equipment and direct the guns according to the computed gun settings required. The most carefully calculated settings may result in the projectile missing the target. If you were to sit back and assume that you were hitting the target because you had made all adjustments and calculations very carefully, you would probably score many more misses than hits.

It is essential that the fall of each round be observed so that it can be determined whether the projectiles are falling beyond, short of or to one side of the target. Corrections can then be applied to gun elevation and deflection to cause the projectiles to land on the target.

This procedure of observing where projectiles land and making corrections to the gun settings for subsequent rounds in order to score hits is called "spotting."

If it is noted that the first round fired falls short and to the right of the target, despite all corrections to compensate for interior and exterior ballistics and relative motion of ship and target, the guns must be elevated and traversed to the left. If this adjustment results in a hit, fine! But the second round may fall beyond the target, in which case the elevation angle must be reduced. Thus spotting is a continuous process during combat and is the final correction to the gun settings to compensate for errors which cannot be evaluated prior to firing.

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HITTING A MOVING TARGET FROM A MOVING SHIP

Why Fire Control Instruments are Necessary

Picture yourself on the deck of a modern warship, charged with the responsibility of directing the ship's gunfire. The target is visible on the horizon and it is up to you to tell the gunner how he must elevate and train the guns in order to score a hit on the enemy. You are familiar with the corrections which must be applied and you know how they are calculated. So first you determine range and bearing (by eye); then you must correct for drift, wind, air resistance, earth's curvature and rotation of the earth. You must determine the target's course, predict its future position, allow for your own ship's motion, predict future range and deflection, and correct for level and crosslevel. You work rapidly, and after a half-hour or so you come up with the answer!! The only difficulty is that by that time the target has either disappeared or blown you out of the water!

No, the solution of the fire control problem cannot be reached rapidly nor accurately enough to obtain effective gunfire without the help of various fire control instruments and related equipment. If it were not for the gun director, rangekeeper, stable vertical, other auxiliary devices and their cross connections, the many computations and corrections involved in setting the guns for accurate fire would have to be abandoned and a "hit or miss" method of gunfire employed. This would hardly be suitable in modern warfare where often the target can't be seen and when the first round is the important one. So the fire control equipment you will learn about on subsequent sheets is all-important to the effective operation of your ship in combat.

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HITTING A MOVING TARGET FROM A MOVING SHIP

Why Basic Mechanisms and Electrical Circuits are Necessary

You have seen that it would be next to impossible to solve the fire control problem effectively without the help of fire control instruments and equipment. The measurements and computations involved are too many and too complex. You might ask, "How does the fire control equipment solve the problem? What's inside, a mathematical genius?"

No, even a mathematical wizard couldn't calculate and measure fast enough to be of use in directing your ship's gunfire. Fire control equipment, like most complex machinery, is composed of simple component mechanisms and electrical circuits. Various component mechanisms can add, subtract, multiply or divide numbers; other devices solve vector and component problems. Still others record, transmit, interpret, convert or retain information. There are other devices which measure required quantities with a high degree of precision. And all of these functions are performed continuously within the fire control equipment. All of these basic mechanisms and circuits are tied together, and when operating as an integrated unit, give the answers to the fire control problem almost instantaneously. Thus the component devices together make up the piece of equipment, and the various pieces of equipment operating together form the fire control system which acts as the "brain" for your ship's guns.

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HITTING A MOVING TARGET FROM A MOVING SHIP

Review of Fire Control Problem Solution

Now you know what factors are involved in solving the problem of hitting a moving target from a moving ship and why fire control equipment and devices are required to solve the problem. The function of each of the major components of a typical fire control system is probably more clear to you now than it was when it was introduced earlier in these sheets. Let's take another look at a simplified diagram of a typical system and review how each component fits into the picture.

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HITTING A MOVING TARGET FROM A MOVING SHIP

Review of Fire Control Problem Solution (continued)

As shown in the diagram on the preceding sheet, the gun director measures present target range, bearing, course and speed. This information is continuously transmitted to the rangekeeper. By combining this information with own ship's course (from ship's gyro) and speed (from pitometer log), the rangekeeper is able to correct for the motion of own ship and target, and thus predict future range and deflection. True wind speed and direction are also supplied to the rangekeeper, which combines this with information on wind caused by own ship's motion and computes the corrections required for the effects of apparent wind.

The stable vertical measures level and crosslevel and transmits this information to the rangekeeper so that the necessary corrections to gun train and elevation can be computed.

On the preceding sheets you have seen what corrections to gun elevation and train orders are required to achieve accurate fire, and here you have reviewed the function of each component of a typical fire control system. Now let's go on to learn more about how each of the major components of a fire control system works.