RELATIVE MOTION-COMBAT INFORMATION CENTER

RELATIVE MOTION

Note: Plotting symbols and standards may be found in RADFIVE, The Surface Plotting Manual.

RELATIVE MOTION

All of us at one time or another have solved problems in relative motion, probably without recognizing them as such. Crossing a busy street safely, intercepting the player carrying the ball in a football game, running to catch a ball, all of these activities involve a problem in relative motion, even though we do not realize it. In a more complex form, all combat tactics, their determination and execution, deal with relative motion; that which is seen on the radar's PPI scope is relative motion. Hence, it is necessary that all personnel attached to the Combat Information Center, from the operator to the Captain, have a thorough understanding of the varied applications of this important principle.

The basic method of solution of relative motion problems utilizes the maneuvering board, and it is therefore required that all personnel should be capable of using it and applying its solutions.

What is meant by relative motion? It is the resultant of two actual motions, but let us break this down further. The word motion implies direction, speed, and distance; relative motion, then, would

These three definitions then, collectively make up relative motion.

Realizing that relative motion is caused by two actual movements, let us analyze the movement of two objects within the same vicinity. Since we are concerned principally with relative motion as applied to moving ships, imagine two ships in the positions shown in figure 5-2, moving in a general easterly course, during a period of 20 minutes.

The initial position of Ship 1 is at the point A. Moving on a due easterly course it arrives at point B 20 minutes later. Picture Ship 2, starting from initial position a, which is 2,000 yards astern of Ship 1, and arriving 20 minutes later at position b, 2,000 yards broad on the starboard beam of Ship 1.

Figure 5-3.

Point O, figure 5-3, represents your position at all times, point M₁, the relative position of Ship 2 at time 00, and point M₂, is the relative position of Ship 2 after 20 minutes. By drawing the line from M₁ to M₂ you establish an imaginary line that indicates the apparent movement of Ship 2 with reference to yourself; therefore, the direction of the line from M₁ to M₂ would indicate the direction of relative movement. The length, or distance, of the line would indicate, according to definition, the distance of relative motion during a period of 20 minutes.

If the distance from M₁ to O or M₂ to O is to represent 2,000 yards, you can measure M₁M₂ using the same scale, and find it to be about 2,800 yards.

Using the formula: distance divided by time equals speed, you have:

speed = 2800/200

speed = 140 yards per minute

Multiply yards per minute by 0.03 and you have miles per hour, or knots. speed = 140 x 0.03
= 4.2 miles per hour.

Therefore, the speed of relative motion would be 4.2 knots.

Examining the direction of the line M₁M₂ you find it to be 135 degrees T. Listing of the information found from the relative plot you have the following:

Direction of relative motion, 135 degrees T
Distance of relative motion, 2,800 yards
Speed of relative motion, 4.2 knots.

You have, therefore, found the three parts of relative motion from the relative plot.

Leaving the relative plot, let us analyze a second method of determining relative motion, which is called the vector diagram. The vector diagram is a picture in which we combine the movements of two objects, with respect to a third object, to find their movement with respect to each other. The third object is usually the earth, so that we are really combining two geographical movements to find relative motion. To show these movements, geographical or relative, as pictures, we use single lines, called vectors. The slope or direction of the vector indicates the direction of the movement it portrays; the length of the vector indicates the speed of the movement. By properly combining two vectors, showing how two objects are moving with respect to the earth, and then drawing the resulting vector necessary to complete a triangle, the resultant side will be the relative movement of the two objects involved. Let us see why that resultant does actually establish the relative motion. Let us suppose you have two objects, the first, moving on a course of 090 degrees at a speed of 12 knots, and the second, in the same vicinity, moving on a course of 150 degrees at a speed of 15 knots, as shown in figure 5-4.

If you should form a relative plot of these two moving objects for a full hour, as shown in figure 5-5, you would get the line XY.

Figure 5-5

From what you know of the relative plot, you realize that the direction of XY, is the direction of the relative motion; the length of XY, is the distance of relative motion; and the time interval in this case is one hour.

But what is the distance per hour? It is speed. Therefore, the length XY, is the speed of relative motion. Suppose you move the position of the second ship so that it moves from some other position with the reference to Ship 1, but remains on the same course and at the same speed, as shown in figure 5-6.

Figure 5-6.

Figure 5-7 is made by forming the relative plot of figure 5-6 and we get the line X₁Y₁.

Figure 5-7.

same speed as XY. So, let us start Ship 1 and Ship 2 on the same courses and speeds as before, but from a common point, as shown in figure 5-8.

Figure 5-8.

If you establish the relative plot, you find that X₂Y₂ is in the same direction as XY and X₁Y₁ and the same length as XY and X₁Y₁. Likewise, if you connect the terminal positions of Ship 1 and Ship 2 (points g and m) you find that the line is also in the same direction and the same length as XY and X₁Y₁. Therefore, it appears unnecessary to form the relative plot, when course and the speeds of two moving objects are known. Instead, you can draw two lines which indicate direction and speed, that is, draw two vectors from a common point; then by connecting the ends of those vectors you will find the resulting vector, which represents the direction and speed of relative motion. This triangle, which is called the vector diagram, and labeled egm, as shown in figure 5-8, is the solution of two actual motions, namely eg and em, which produces the resultant gm. This resultant is the direction and

eg represents the course and speed of the unit to which the relative motion is referred, or the guides course and speed. The course being indicated by the direction of eg, and the speed being indicated by the length of eg.

em represents the course and speed of the second moving object, or the maneuvering unit's course and speed, the course being indicated

gm represents the relative motion with reference to the guide ship, that is, the direction from g to m represents the direction of relative motion, and the length of gm indicates the speed of relative motion.

USING THE MANEUVERING BOARD

Figure 5-9 is a standard Navy maneuvering board. Notice that it is a polar coordinate system,

The maneuvering board is made up of ten concentric circles, that is, ten circles having the same center. You can therefore indicate any speed or distance desired, by choosing the proper scale. If a 1/1 scale is chosen, you can indicate any speed from 1 to 10 knots. If you choose a 2/1 scale, you can indicate speeds up to 20 knots. etc.

Figure 5-10.

In representing distances, use scales such as 1 unit is equal to 2,000 yards, or 1 unit is equal to 3,000 yards, etc.

The scales on the left and right of the board, that is 2:1, 3:1, 4:1, and 5:1, are simply aids to be used in computation in accordance with whatever scale is chosen.

The logarithmic scales, at the bottom of the chart, that indicate time, distance, and speed, are so placed as to enable you to solve problems of time, distance, and speed, merely by drawing a straight line. For example, if you know that an object has traveled 3 miles in 10 minutes, you can place a point on the distance scale at the position that indicates 3 miles, and on the time scale at the position that indicates 10 minutes. By drawing a line through those points and extending it until it intersects the speed scale, you will find that it crosses the speed scale at 20 knots. That is, the object is traveling at a speed of 20 knots. Obviously, the care with which you establish your points and draw your line has a direct bearing on its accuracy.

If you are requested to draw the vector diagram egm on a maneuvering board, obviously, the simplest procedure is to place one of the angles of the triangle at the center of the chart. For the sake of consistency, let us say that we will always place point e at the center of the chart. This point e has no meaning by itself; it is merely the point at which two vectors are joined. It is absolutely incorrect to think of the guide as moving from e to g during the maneuver; he does not. The line eg shows the direction and speed of the guides movement over the earth; it in no way indicates his starting point or final position. Naturally, whatever scale is used to represent the speed of the guide, must be consistently used in the representation and interpretation of eg, em, and gm. Going back to the relative plot, you realize that you have established the position of M₁ and M₂ with

reference to the guide's position, ignoring the movement of the guide, it would be possible to draw this relative plot on the same maneuvering board, even though the relative plot and the vector diagram are two different problems. Since they are different problems, it is possible to use a scale in the relative plot different from that which was used in the vector diagram. In order to keep in mind which scale you are using for egm (the vector diagram) and which for the relative plot, it will be helpful to label the scales. In the illustrations, notice that the distance scale is marked D and the speed scale is marked S.

Examining figure 5-10, which is a relative plot and a vector diagram, notice that the line gm is parallel gm represented the direction of relative motion. It to the line M₁M₂. It has been shown that the line

In geometry if you have two sides and the included angle of a triangle, you are able to establish the third side. That is, if you know the direction of two sides of a triangle and the length of those sides, it is an easy matter to find the third side. Therefore, knowing eg and em, gm can be found by connecting the points g and m. Likewise, if you know eg and

In cruising at sea, you should always know or be able to find one line of the vector diagram, since you always know your own course and speed. You could establish or start a vector diagram by drawing the line eg to represent your course and speed, choosing an appropriate scale in accordance with the speed. If you pick up a contact bearing 280 degrees at 24,000 yards, you could establish point M₁, and report a contact at that position with reference to your ship.

How can that be determined? If the relative motion between your own ship and the contact is known, you can draw the line gm and therefore establish the line em. Then you can report what the contact is doing. How can you find the relative motion? Having established a relative plot, you can find M₁M₂, which tells all you need to know about relative motion. You recall, that to form the relative plot, you need to know a series of successive positions of the contact with respect to your ship. Where can you get this information? The answer is the radar. The radar tells where the unit is with reference to your ship, so you can establish in a short period of time a relative movement line M₁M₂ (M₂ being a later relative position than M₁). Thus, M₁M₂ forms the relative plot. Let us say that M₁M₂ is the direction of relative motion. Therefore, you are able to draw the line from g parallel and in the same direction as M₁M₂. Since the length of gm is determined by the speed of relative motion, you must next find the speed of relative motion. By measuring M₁M₂. which gives the distance of

ILLUSTRATED EXAMPLES

Finding course and speed of the maneuvering ship.

Suppose you are cruising along on a course of 290 degrees, at a speed of 18 knots. At 1506 you pick up a contact at a bearing of 213 degrees and at a range of 16,000 yards At 1515² the contact has moved to a position of 200 degrees, 12,400 yards away from you. Your problem is to find the contact's course and speed.

Let us consider some of the disadvantages of figuring course and speed of a radar contact by this method. For one thing, the solution is correct only when our ship remains on a steady course and speed throughout the period of time used to form the relative plot. Furthermore, if the relative speed is slow, it requires a greater period of time to establish a relative plot that is large enough to measure with any degree of accuracy. In forming a relative plot, a change of speed is not easily detected. This increases the possibility of error in the solution of the contact's speed.

Maneuvering to attack position.

Suppose you have picked up a contact and have determined that it is traveling on a course of 060 degrees at a speed of 12 knots. The contact bears 200 degrees, at a range of 18,000 yards from your ship. Your task is to figure out a course to maneuver your ship to an attacking position, 70 degrees off the port bow, at a range of 4,000 yards, using a speed of 24 knots, and the time that will be required to accomplish this maneuver.

Assume you are on the necessary course and traveling at the proper speed to arrive at a position 5,000 yards and 60 degrees off the starboard bow of an enemy vessel, which has been determined to be on a course of 163 degrees at a speed of 15 knots.

Upon arriving at that position, your ship is to fire a 27-knot torpedo at the enemy ship. What course should the torpedo be set on?

A convoy is on a base course of 030 degrees at 10 knots. Your position is 40 degrees relative, at a range of 2,000 yards from the guide. At 1400, guide makes an

The following problems of average difficulty are for practice in use of the maneuvering board.

1. You are traveling on course 120 degrees at 16 knots. At 0405 you pick up a contact at 236 degrees, range 16,000 yards. At 0411 the contact bears 239 degrees at 19,000 yards. What is the course and speed of the contact?

2. Your ship is on course 070 degrees at a speed of 18 knots. At 0316 there is a contact at 050 degrees, range 18,000 yards. After tracking the contact for 9 minutes, he then bears 058 degrees at 13,000 yards.
REQUIRED:

3. A freighter is traveling on a course of 025 degrees at a speed of 10 knots, An enemy destroyer is on a course 115 degrees at a speed of 20 knots. Visibility is 6,000 yards. The destroyer bears 300 degrees at a range of 15,000 yards at 0115.
REQUIRED:

4. Your ship is traveling at 16 knots. On a course 325 degrees with an escort 2,000 yards dead ahead. At midnight the escort picks up a contact at 291 degrees, 15,200 yards from him and reports it. Five minutes later you find the contact bears 283 degrees, 14,800 yards.
REQUIRED:

5. You have determined that a contact is on course 138 degrees at a speed of 17 knots. You are now approaching a point 60 degrees off the port how at a range of 5,000 yards in order to fire a 36-knot torpedo at the contact.
REQUIRED:

6. You have found an enemy transport which is traveling on a course of 235 degrees at a speed of 14 knots. You wish to use a speed of 24 knots to maneuver into a position 70 degrees off the nearest bow at a range of 6,000 yards. At the time of the start of the maneuver the enemy bears 116 degrees, range 16,200 yards.
REQUIRED:

7. A convoy is traveling on a course of 255 degrees at a speed of 12 knots. You wish to take it under fire from a position broad on the beam, from a range of 10,000 yards. At present it bears 150 degrees, 18,000 yards.
REQUIRED:

8. You are stationed 060 degrees relative to the guide, at a range of 6,000 yards. Base course is 300 degrees, speed 11

9. You have a contact south of you on a course of 080 degrees at a speed of 14 knots. You wish to come to a point 5,000 yards from the enemy to fire a 27-knot torpedo so that it will have a 90 degree track angle when

10. You are stationed on a true bearing of 101 degrees, 3,000 yards from a guide on a course of 340 degrees, speed 10 knots. You are asked to patrol out on your bearing for one hour at 20 knots and then return to station.
REQUIRED:

COMBAT INFORMATION CENTER

Since you will he a member of the Combat Information Center team (abbreviated CIC), it is essential that you understand the objective and functions of this organization. Entire books have been written on CIC, but in this manual only a brief description of the objectives and functions of this "nerve center" will he given in order to help you to adjust yourself to the part you will play as a member of the CIC organization. This description is followed by typical layouts of different type ships such as, BB's, CV's, CL's, and DD's.

OBJECT OF COMBAT INFORMATION CENTER

The object of the Combat Information Center is to assist the command in planning a correct course of action, and to assist the command and armament control in the execution of that plan.

FUNCTIONS OF COMBAT INFORMATION CENTER

The Combat Information Center, is briefly. an agency for the collection, evaluation, and distribution of combat information, and for facilitating the use of that information. It is not something strange and complex, nor is it merely a radar plot or an antisubmarine plot under a new name. It provides, however, a marked clarification and simplification of work for the command.

The following paragraphs in this section show how complex a Captain's life formerly was, and how

For purposes of functional analysis, the Combat Information Center may be considered as divided into two sections, evaluation and control.

To aid in the realization of the surprising mass of information available, the following lists are included. Obviously, much of the data will still go directly to the Captain, but details of sifting and correlating all of it need no longer distract him.

(a) Position information:

To make full use of this information in the CIC, it is displayed graphically on the DRT, summary, and air plots.

At first thought, this identification may seem rather out of place in CIC. But the plot can show more readily than any other way, for instance, whether a plane sighted and reported by a lookout has previously been detected and identified by radar. It can show me easily if the maneuvers of an otherwise unidentified ship or plane are hostile.

(c) Reports from outside the ship:

These reports are filtered and sifted in CIC and the Captain receives, not a series of disconnected facts, but an evaluated report fitted into the entire tactical or strategical picture in its proper perspective.

To indicate the increased effectiveness of a ship, where all this information is available in one place and is sifted with greater thoroughness than the Captain has normally been able to do it, a hypothetical example is given: A destroyer is escorting a seaplane tender, and aircraft patrols have reported an enemy destroyer of Togo class on a course such that she will intercept during the night. The CIC can determine that the enemy carries a few small guns, but has many torpedoes. Intelligence reports indicate effective enemy gun and torpedo ranges. Knowing this, the Captain can plan how best to keep the enemy

CIC is the source to which the Captain turns for a specific tactical or operational fact, for a general summary, or for an opinion or suggestion. The CIC through the evaluator, should provide the Captain with the following data:

(a) Filtered contact, position and identify information presented, according to the Captain's requirements at the time.

(b) Tactical and strategical summaries, which include facts necessary for the Captain's information and decisions, but which omit all irrelevant material.

(c) Pertinent reports and background data which contribute to his understanding of the problem at hand, and to the resultant decisions.

(d) Evaluated comment, suggestions, and opinions, based upon the greater availability of information in the CIC.

The control function of the CIC, with its detecting, tracking, and communication equipment, can be of inestimable assistance in matters concerning both own armament and coordination with other units.

(a) With respect to coordination with, or control of own armament, the CIC can:

(b) With respect to the coordination with, or control of other units, the CIC can:

TYPICAL COMBAT INFORMATION CENTER

The Combat Information Center will he somewhat alike on all combatant ships, but will differ as to size, number of pieces of radar equipment, and personnel allowance, depending on the ship type. Some typical CIC layouts for BB's, CV's, CL's and DD's are illustrated on the following pages.