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Advanced Navigator Functions
5-9
Interpreting and Adjusting the Radar Picture
Navigational echoes may be large, small, bright or faint, depending
not only on the size of the object but also on its orientation and
surface. Different objects reflect the radar signals in different ways.
For example, vertical objects such as cliffs reflect signals better than
sloping ones such as sandbanks.
High coastlines and mountainous coastal regions can be observed at
the longest radar range. However, the first sight of land may be a
mountain several miles inland from the coastline. The actual coastline
may not appear on the radar until the vessel is close to the line of sight
distance.
The radar indication may not be similar to your visual observation; a
nearby small object may appear to be the same size as a distant large
object on the radar. However, with experience the approximate size of
different objects can be determined by the relative size and brightness
of the echoes.
Some targets, such as buoys and small boats, can be difficult to
differentiate, since they bob and toss about in the waves and do not
present a consistent reflecting surface. Consequently, these echoes
have a tendency to fade and brighten and, at times, to disappear
momentarily. Buoys and small boats often resemble each other, but
boats can usually be distinguished by their motion.
Not all radar echoes are produced by hard navigational items. Some
echoes may be received from irregularities on the surface of the water,
particularly at close range (sea clutter), or from rain or snow either
around the vessel or in the distance. In addition, some echoes may be
indirect returns to the radar scanner, providing false echoes or multiple
echoes.
The effects of many false echoes can be minimized, and the target
presentation clarified using the controls summarized below. This
section explains how to interpret the radar picture and how the
controls affect the display. It provides instructions for adjusting the
settings, describes the circumstances in which they should be adjusted,
and how they interact with each other.