Radar range as a clearing line

In low visibility time is sometimes lost by giving a wide berth to prominent headlands when rounding them or when passing along coasts with off-lying dangers. If minimum range circles are drawn on the chart from the parts of the coastline concerned and clearing lines are drawn tangentially to them, the passage may be made in safety using radar ranges to ensure that the ship does not go within the minimum distances. (Fig. 26)

It is, of course, essential if this method is to be used to make sure that the coastline will provide useful targets at the ranges needed and that contact is made with the actual coast at the targets selected and not with more prominent targets behind them. So that the near­est possible echo from the coastline will be seen, the gain should not be reduced below normal. This method of clearing dangers may be compared with the use of vertical and horizontal dangei angles in normal navigation.

In good visibility the radar clearing line may be used to clear dangers lying some distance off a coast where visual fixing facilities are limited.

"4

Fig. 26. Radar clearing line

Coasting in general

The choice of which of the above methods to use will usually be forced on the navigator by the circumstances of the case, though any or all of them may be used in conjunction. Each of them carries its load of precautions which the prudent navigator will do well to study, but which with experience will become a second nature. Fig. 27 illustrates a landfall followed by a passage along a coast, in which each of the methods described plays a part; Fig. 28 is a record of radar fixing carried out during an actual passage.

Very few situations can be imagined in which, with care and foresight, a coastal passage cannot be safely accomplished with radar assistance, whatever the visibility.

Landfall fix on three " radar ranges and ^''21 mites bearings ^ " t--.

1000

Approx. position by radar

range and radar bearing

of village with check range on pt чч⁴^

D.R. position

at 1000 Check posn by

range circles ⁰⁰ from points С and В

Ъпнлг* ¹⁷¹⁵ К* by radar ^•5 j range and radar range and radar \_ariT)Q

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Fixes by radar range MM and visual bearing

Fig. 27. Typical radar fixing during a coastal passage

ANCHORING BY RADAR

It is often necessary to anchor a ship in low visibility, and on oc­casions radar may be the only aid available. Although the position of the anchor on radar information may not be quite accurate, the assurance given by knowledge of the whereabouts of all shipping nearby will be of considerable comfort. The decision whether to seek shelter or to remain at sea depends largely on the suitability of the approach to the anchorage for using radar, no undue risk should be involved by the presence of other shipping as long as due account is taken of wind and tide and the way in which ships will be lying to their anchors. The approach course to the chosen anchorage should be made if possible with a radar conspicuous object ahead, the range

will then give a direct indication of the distance to go. Alternative­ly, the ship can be fixed by radar during the approach, using one of the methods previously des­cribed, or she can run in using radar clearing lines. In the latter case the approach courses should, where possible, be chosen so that they are parallel to the coast. An illustration of such an anchorage is given in Fig. 29. Used in this way radar can also be of the great­est assistance to anchoring in

clear weather, when visual marks are unhelpful or other ships are berthed nearby. Radar may also be used to great advantage in checking that, when anchored, the ship has not dragged. Again this method will be of greatest use in poor visibility.

RADAR FOR COLLISION WARNING

Radar has proved itself to be of very great value as an aid to avoid­ing collision in fog. The advantage which it confers, however, is not so much in assisting dramatic last-minute avoidance, as in enabling potentially dangerous circumstances to be rectified in their early stages. In studying the use of radar for this purpose it is most import­ant to appreciate that radar is not at its best in rapidly changing close-quarter situations and that much depends on the manner in which the limited contribution it can make is used. In this chapter it will be shown that sufficient intelligence can be deduced from simple radar observations to construct the situation as it would appear to the eye in clear weather but that this cannot be done without a contri­bution from the user. To emphasize this point it is necessary to com­pare the efficacy of visual observation with that of radar.