Measurement Of Blind Shadow Sectors - Furuno 1751 Operator's Manual

Measurement of Blind Shadow Sectors
6. INSTALLATION
Measurement of Blind Shadow Sectors
In some shadow sectors, it should be remembered that there may not be sufficient intensity
to obtain an echo from very small targets even at close range, despite the faet that a large
vessel may be detected at a mueh greater range in non-shadowed sectors. For these reasons
the angular width and relative bearing of any shadow sectors should be determined. This
section describes how to do this. In the case of a new vessel this should be done during sea
trials. In other ships it should be done at the first opportunity after fitting a new radar set.
It should be understood that even a small shadow sector can hide another vessel if it is on a
collision course. The bearing will remain constant in the shadow area and the approach of
the other vessel may remain undetected until it is too late to avoid a dangerous situation.
There are two methods of determining the angular width of a shadow sector:
1) Turn the boat ve/y slowfy through 360 degrees while a small but clearly defined target is
observed at a distance of a mile or so. (Do not use a buoy with a reflector as this target is
too powerful to achieve the required result.)
If the echo disappears while the boat is turning, the target has entered a shadow sector and
it will again become visible when the target emerges from the shadow. Very quiet
conditions of wind and sea are essential to ensure reliable results when this operation is
carried out on a small craft since a rough sea can cause a buoy to be lost in the dutter or
to be temporarily submerged or hidden by waves. An unsteady movement may cause the
boat to swing through a shadow sector before the scanner has completed one revolution.
In any case an average of several observations of each shadow sector should be taken. It is
a waste of time to attempt the operation in anything other than very smooth water with
little wind.
2) Another method is to observe the shadow sector against a background of sea dutter. Any
shadows will show as dark sectors in the dutter. See Figure 6-24.
Note that a shadow cannot be fairly estimated in heavy dutter, as echoes from either side of
the sector may be spread into it and give an illusion that objects in the sector are being
observed. Nor can it be satisfactorily determined in confined waters, because of the prob-
ability of indirect, false or multiple echoes being produced from nearby buddings or other
vessels.
Figure 6-24 Appearance of a Shadow Sector on the Display Screen
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