Furuno 851 MARK-2 - Manual (Page 31)

3-1
3. INTERPRETING THE DISPLAY
As an aid to navigation, radar can be a very
valuable tool. No other electronic
navigation aid can give you the ability to
spot vessels coming at you in the fog, or
tell you the location of the inlet to the
harbor in the pitch black of night.
To help you understand what your radar
can (and cannot) do for you this chapter
covers
• the characteristics of the radar wave
• target properties and radar wave
reflection
• range and bearing resolution, and
• false echoes.
3.1 The Radar Wave and Radar
Horizon
How the radar wave travels
The radar wave tends to travel in straight
lines at the speed of light. However, it is
subject to bending or refraction in the
atmosphere, the amount depending on
region and density.
Super-refraction
Super-refraction is a condition in which
there is an upper layer of warm dry air over
a surface layer of cold, moist air. Radar
waves bend downward and thus increase
the ranges at which targets may be
detected.
Sub-refraction
Sub-refraction is the reverse condition of
super-refraction; a layer of cold air is above
a layer of warm air. Radar waves bend
upward and thus decrease the ranges at
which targets may be detected.
Radar horizon
Radar is essentially a “line-of-sight”
phenomenon. That means you have just
about the same range to horizon with a
radar as you do with your own eyes.
However under normal atmospheric
conditions, the radar horizon is 6% greater
than the optical horizon. Therefore, if the
target does not rise above the horizon the
radar beam cannot be reflected from the
target.
The distance to the horizon from the
antenna, under normal conditions, is
calculated by the following formula.
Rmax = 2.2
×
h + h
1 2
Where Rmax: Radar horizon (mile), h1:
Antenna height (meters), h2: Target height
(meters)
h1
h2
D
Horizon
Wave
path
Figure 3-1 Radar horizon