Raymarine ST50 plus Specifications download pdf (Page 27)

ST50 PLUS RADAR Operation and Installation Handbook

Buoys and small boats are an example of targets that are sometimes

difficult to differentiate between. Since they bob and toss about in the

waves, they do not present a consistent reflecting surface.

Consequently, their echoes have a tendency to fade and brighten and

at times to disappear momentarily. Very often buoys and small boats

resemble each other, but usually the motion of one target to the other

identifies the boat from the buoy.

High coastlines and mountainous coastal regions can be observed at

the longest range of the radar. However, the first sight of landfall on

the radar display may be a mountain several miles inland from the

coastline. The actual coastline may not appear on the radar until the

vessel has closed the range to the land near the line of sight distance.

3.2.4 Sea Return

Not all radar echoes are produced by hard navigation items such as

boats, buoys and land. Some Radar echoes may be received from

irregularities on the surface of the water, particularly

atclose

range by

breaking wavecrests in heavy seas. These echoes appear on the PPI

screen usually on the very short range scales as multiple small echoes

not in a repetative or consistant position. Under high winds and

extreme conditions the echoes from sea clutter may appear as dense

background of clutter forming the shape of an almost solid disc, as far

as one to three miles in all directions from the display center.

3.2.5 Storm and Rain Squal Returns

The Radar can also see echoes from rain or snow. Returns from storm

areas and rain squalls consist of countless small echoes, continuously

changing in size, intensity, and position. These returns sometimes

appear as large hazy areas on the display depending on the intensity

of the rainfall or snow in the storm cell. The cells usually are visible for

long distances due to their extreme altitude and are very helpful for

observing bad weather warnings. If the returns from storm areas and

rain squalls are not desired, the RAIN control can be adjusted to

minimize them.

Section 3. Operation

3.2.6 Blind Sectors or Shadow Effect

Not all echoes on the radar are direct returns to the radar antenna.

There are many types of echoes that can appear on the display if

certain conditions occur. The sections that follow briefly describe the

echo patterns that may be produced by these false echoes and their

likely cause. It should be noted that the Radar operator, through

observation, practice, and experience can detect these conditions

generally very quickly.

Funnels and masts, (when located near the antenna array) may cause

shadows. In the shadow area beyond the obstruction there will be a

reduction of the beam intensity, although not necessarily a complete

cutoff. However, if the subtended angle is more than a few degrees

there will

a blind sector.

In some shadow sectors the beam intensity may not be sufficient to

obtain an echo from a very small

object even

at close range, despite

the fact that a large vessel can be detected at a much greater range.

For this reason the angular width and relative bearing of any shadow

sector must be determined at installation. Sometimes shadowing can

be seen by increasing the Radar Gain until noise is present. Dark

sectors indicate possible shadowed areas. This information should be

posted near the Display Unit, and operators must be alert for objects

in these blind sectors.

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Raymarine ST50 plus Specifications Page 27

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