The information required

The only safe and accurate way of getting the necessary infor­mation from radar observations is to plot them. Plotting is a simple and effective process which with practice becomes quick and easy to perform. Once its elementary principles are thoroughly under­stood, a familiarity with all the cases which will arise in practice can quickly be acquired through use.

It is not possible to define dogmatically all the information'which a master will require and the sequence in which he will require it, since this will be largely a question of individual preference. How­ever, in any potential collision situation certain items of informa­tion will be regarded as essential and these can be set out typically in different stages as follows:

1. The time, bearing and range of the target on detection.

2. The time (some minutes later), bearing and range; the di­taction of the bearing's movement and the direction and rate of movement of the range.

> At this stage the master will decide whether more information is needed. If it is, he will probably wish to know what degree of risk bf collision exists. This can be ascertained from:

(3) The anticipated time, and the bearing and range of the other ship when she will be closest if both ships maintain their present courses and speeds.

If this information gives cause for anxiety, the master will prob­ably wish to visualize the exact circumstances. These can be given by:

(4) The aspect, or course and speed of other ship. Her latest range and bearing will of course also be passed. *

Here the master will decide whether any avoiding action, either of course or speed, is necessary. He may then require information such as:

5. A suitable alteration of course or speed, and the time at which to alter, assuming other ship maintains her course and speed. If an alteration is made as a result of (5) or on the master's judgement, radar observation will be required to establish that the alteration is serving its purpose and that the other ship has not also altered in such a way as to defeat it. This may be given as soon as possible after the alteration by:

6. A check on other ship's course and speed; a range and bear­ing and an indication of movement of range and bearing; a re-esti­mate on the lines of (3) and (4).

All this information can be obtained quickly and accurately enough by plotting. Experience should enable the mariner to distinguish between variations due to errors of observation and plotting and those due to alterations of the vessel's movements.

PLOTTING METHODS

There are two main methods of plotting, either of which will give all the essential information required. The plot can either be constructed as a TRUE PLOT, which is similar in conception to a geographical plot, but need only take account of the movement of both ships through the water, or it can be constructed as a RELA­TIVE PLOT on which the movement of one ship relative to another is plotted. The advantages of the relative over the true plot are that the information required can be extracted with less plotting and that it is a quicker and more accurate method of forecasting the de­velopment of a situation. Additionally, as will be shown, the best cannot be got out of the true plot without employing some of the principles of relative movement.

To give reality to the examples, the results given in this chapter were obtained by actual plotting, to a scale likely to be used at seai They differ slightly from those which would be obtained by calcui lation. i

THE TRUE PLOT

In its simplest form the true plot is made as in Fig. 31. A line is drawn to represent the course being steered by own ship along which successive positions may be plotted on a suitable scale. Suc­cessive bearings and distances of the other ship obtained from the

1006

woo

PPI are plotted to the same scale from the appropriate positions on the line. A line join­ing the other ship's positions will give her course, and her speed can be deduced from the time and distance between the positions.

In practice it is usual to mark off po­sitions on own ship's course line at regular intervals ahead; since the distance steamed in six minutes will be

one-tenth of the speed, this time interval is a convenient one. It is also simple to make the radar observations of other ship coincide in time with the intervals marked, so that they can be plotted direct­ly.

In all the examples which follow, the plot is made and deduc­tions are drawn on two observations separated by this time interval. In practice this would be unwise as the other ship might have altered course or speed between the observations. A third observation is essential to confirm the plot, and this would normally be made during the plotting interval.

It will be seen that the first information available from this plot is that required at stage (4) of the series mentioned above. A further construction is necessary to complete stage (3).

If the two course lines are extended, the anticipated positions of both ships can be marked on them, since the speeds are now both known. If there is any risk of collision the lines must intersect at a point which will be reached by the two ships at about the same time. A study of expected positions near the intersection will enable an approximation to be made of the time the ships will be closest together and of their bearing and distance apart at that time. (Fig. 32)

This rather lengthy process merely discovers the relative posi­tion which the other ship will be in when closest. It can be found more quickly and more accurately by first ascertaining the direc-

/ OtOers/iip

Estimated closest approach s($ 2 miles at W33, on bearing 327* /4? crosses ahead at 2<* mis at 1035

tion and speed of her movement relative to own ship. This movement is precisely that of her echo across the face of the PPI; if successive positions of the echo were marked on the screen, a line joining them would show the direction of the relative movement, while the dis­tance travelled along it in a given time (on the scale of the PPI) would give the relative speed. This construction can readily be done on the true plot, remembering that to reproduce the PPI picture the succes­sive bearings and distances must be laid off from a single point, which thus corresponds with the centre of the screen.

Continuing the plot from the point reached in Fig. 31, therefore, the 1000 observation would be plotted from the 1006 position (C) of own ship and marked О (Fig. 33); the 1006 observation already plotted is marked A (CO is equal and parallel to BW). The line join­ing them is then extended across own ship's course line cutting it at Z. The nearest point on this line to С is marked 7\ the angle АТС being 90°. The distance OA on the scale in use divided by the time interval between the observations (six minutes) gives the relative speed and the direction of OAT the relative course of the other ship. Her relative movement will be along this line at the relative speed; the time at which she will reach T, the point of closest approach, can be calculated and her distance and bearing at that time measured from the length and direction of CT. Thus the information required at stage (3) is obtained. That needed at stages (5) and (6) is best ob­tained by relative plotting methods. These are explained below under The Relative Plot.

_Jm ^{0mr ship}

_{/< /}

У / / from plot: ' / / Relative со. and speed 23172 15 Kn / / Ship mill cross ahead 2Л miles / / at 1035. closest approach 2 miles / / at 1033

\

\

1006 С

_//

/

WOO \/ Ovrn ship. со. 357'WAn. _m В

Fig. 33