Visual and radar observation compared

The circumstances of a daylight meeting with another ship in clear weather leap to the eye. The significant factors which are noted in the mind of the observer are the other ship's bearing and her as­pect. Aspect may be defined as the relative bearing of own ship from the other ship. It is measured from 0° to 180° and expressed as "left" (red) or "right" (green) according to whether own ship is on the other ship's port or starboard side, if other ship is steering directly towards own ship her aspect is 0°. Continuous observation of the bearing will very soon establish whether there is a risk of collision and, combined with the aspect, it will usually form the basis for a plan of action, if any is needed, within a very few minutes of the sighting. When avoiding action is taken, any untoward movement on the part of the other ship which might prejudice the manoeuvre's success will be immediately apparent if her aspect remains under close observation. The other ship's speed is not of particular interest in the general clear-weather case, since the avoiding action will presumably be taken while there is still plenty of sea room and its timing has no other significance. Nevertheless, to an experienced mariner, the re­lations between the other ship's aspect and bearing, and the move­ment of the bearing will give a good idea of her speed, this may be of value in special circumstances such as a close-range sighting or when manoeuvring is restricted.

Although radar will give bearing information of the same kind and also the range, nothing else can be obtained directly from the PPI, only in exceptional close-range circumstances will the other ship's aspect be discernible. The bearing may be seen to be steady or nearly so, indicating a risk of collision, but the degree of risk and its urgency will not be immediately evident. A slow movement of bearing and a rapid decrease of range may suggest the early develop­ment of a close-quarter situation while a similar movement of bear­ing with a gradual decrease in range may mean that the ships will pass well clear of one another. In clear weather the eye, noting the bearing and aspect, will have no difficulty in distinguishing between these extremes; radar information, however, cannot make the dis­tinction with any certainty unless it is put on paper and further de­tails are deduced from it.

Figure 30 shows how differences in the observed rate of closing imply widely differing aspects. In the example, own ship, which is steering 357° at 10 knots, detects another ship on a steady bear­ing of 040° with the range decreasing and three possible situations depending on the rate of change of range are shown. If the other ship's speed is 20 knots, her course will be about 240°; if it is 7V₂ knots, her course will be either 284° or 334°. In the first case the range will be closing at 26.7 knots and in the second case either 10.6 or 4.2 knots. Thus it will be seen that, depending on the rate of closing, the aspect

of the other ship may vary by as much as 94° in the example taken. If the detection was made, for instance, at a range of 10 miles the ships would be very close after 22 rhinutes in the first case, while in the two others it would take either 56 minutes or 2 hours 24 min­utes. One of the extremes in this example constitutes a leisurely overtaking situation, while the other is a highly dangerous crossing situation calling for lively appreciation and early action.

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The rate of closing can be worked out from successive observa­tions of range without any complicated arithmetic, but it is doubtful whether anyone but an expert in mental trigonometry would be able to deduce in his head the aspect of the other ship or construct a re­liable picture of the circumstances.