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Radar ranges plotted as position circles

When it is possible to select three prominent isolated objects, separated in bearing by, say, 45°, an extremely accurate fix can be obtained. Usually when coasting, however, the navigator's prob­

lem is to select suitable and identifiable portions of the paint of a coastline and plot their ranges as position circles. Since all parts of an indented coastline, except those lying at right angles to the ob­server's line of sight, will be to some extent distorted because of the beam-width of the aerial, care is necessary in selecting suitable points and in using the resulting observations. With 3 cm. radar the effect is not great but it should nevertheless be borne in mind when the choice of objects on which to range is made. The most suitable points on a coast-line which lie at right angles to the line of sight will often be those nearest the ship.

Lost in paint

Not painted

The distorting tendency of beam-width is illustrated in Fig. 25. The general effect is to broad­en headlands, to fill in narrow indentations along the line of sight and to merge islands into the adjacent coast. The existence of shadow areas thrown over portions of bays hidden from the radar beam by high land adds to this effect. The general radar appearance of a strongly marked coastline will still resemble its actual form, but the detail will largely disappear and what re­mains will change considerably as the angle of view alters. As already remarked, portions of the coastline which are roughly at right angles to the line of sight will not be distorted and in the example illustrated, points Ay В and С should be suitable from this point of view for fix­ing by range circles. The ranges observed are laid off on the chart with compasses from the three points, using the appropriate part of the latitude scale, and the

circles will intersect or form a cocked hat at the ship's position. Naturally points should be chosen which are identifiable as accu­rately as possible and which are steep-to, so as to reduce to a mini­mum the errors due to the location of the source of the echo, and on this depends the advantage in accuracy of this method over the one which follows.

Radar range and radar bearing

The use of a radar range and radar bearing is listed as the method with the lowest accuracy of the three given because of the difficulties which have been referred to.

When the target is isolated and of small extent it is possible to make a fair estimate of the bearing of its centre, and if it is steep-to, an accurate range can be obtained. The same applies to the bear­ing of a narrow promontory jutting out in the direction of the ship.

When taking a radar bearing of the side of a prominent land fea­ture, a fair degree of accuracy can be obtained by applying & correc­tion of half the beam-width to the observed bearing in the direction of the land. This is necessary because the edges of a headland cause an echo to paint while they are illuminated by any portion of the beam. When the edge of the beam is touching the headland, the edge of the echo on the PPI will appear on the bearing of the centre of. the beam at that instant. The error will, therefore, be half the beam-width. As the effective beam-width is not a quantity which can be calculated with any accuracy it is not possible to give an exact cor­rection, but the gain should be reduced when taking the bearing so as to reduce the error as much as possible. It should be noted that the direction of the extension of the land has nothing to do with the di­rection of rotation of the scanner.

It is necessary, therefore, to exercise considerable discretion when fixing by this method.

This will only make for safety if there are no dangers close to the track on other side. Whenever possible, confirmation of the po­sition should be obtained by additional information such as a radar range of another point.