Targets to be plotted

If time is to be employed to the best advantage, the selection of targets for plotting is an important matter.

When in relatively narrow waters or following traffic routes, ships will be mainly on parallel or opposite courses, and it will usual­ly be obvious which of them is worthy of close attention. In open waters and particularly at route junctions or crossings, it may not be so. If sea room is unrestricted and ships are converging from a variety of directions, a commendably bold alteration of course to avoid one ship may put a new complexion on the relative movements of others. A ship which has been harmlessly keeping station well out on the quarter may become a considerable danger if a turn of, say, 70° has to be made towards her.

It is a useful rule never to permit a ship echo to remain for long on the screen without making at least two observations of its range and bearing. Even if nothing further is done, the information thus made available may be of great value should later movements of own ship make it desirable to plot the other. Conversely, if there is an opportunity to plot the observations when they are made, and to establish the course the other ship is steering and her speed, time may be saved later when it is short. Naturally, attention will be con­centrated on echoes which are closing on steady or nearly steady bearings, and the higher the rate of approach, the more urgently will they be considered. Thus, echoes of targets on ahead and bow bearings will be thought of more immediate importance but, as already re­marked it will be unwise to neglect all others.

The selection of targets for plotting will also depend on factors such as the visibility and the speed of own ship in relation to the speed of vessels likely to be met with. The slower the speed of own ship, the greater will be the interest taken in echoes on after bearings. A reduction of visibility may render the use of radar desirable and yet be unlikely to cause ships to reduce speed much below their normal.

In this case rates of approach may be high and the importance of early plotting will be increased.

A useful guide on the advisability of plotting radar targets may

be:

Be aware of the bearings of all echoes on the screen and of their trends.

If the time permits, plot all of them, even if only to the extent of obtaining the relative track between two observations.

Always plot those whose bearings are nearly steady and whose ranges are closing, and obtain their courses, aspects and the distanc­es and times of closest approach.

Range scale to use

No seaman will regard the onset of a close-quarter situation in fog with equanimity. It has been shown that plotting takes time and that radar is not a rapid detector of change of movement. These factors point towards the desirability of ascertaining the facts as early as possible and acting upon them in good time, and suggest the use of a long range scale.

If ship targets can be detected at 15 miles it would be wise to take advantage of the facility. At the same time it should be remem­bered that, although the same echo may appear on the PPI on more than one range scale, it may be more conspicuous on the shorter scales, while the better discrimination of the latter may disclose more than one target in a given area. These and other factors make it desirable to change the scale in use at regular intervals to ensure that all re­quirements are satisfied.