they’ll show as two distinct close vessels
(resolution 49 yards).
“High definition” is a term you hear a
lot today in the world of marine radar, and
new high-def units do offer better resolution. But all of the above principles, don’t
change; the main advantage of modern
high-def systems is in the software. New
algorithms and digitizing the signals allow
for better target discrimination and detection of smaller, weaker targets. But remember, the same limiting factors still apply.
As with boats, how well a landmark
shows on radar depends on the height
of the land and the resolution of the
radar. Isolated targets such as other vessels, buoys, small islands, or drilling rigs
are easier to interpret than large, irregular
landmasses. What one must also remember is that although the size of a large target
increases on-screen as it grows closer, the
size of the echo on-screen is not always a
reflection of the actual size of the target.
Especially at longer distances, isolated targets all appear as simple dots or small line
The shape and material of the target
also influence acquisition and resolution.
Round and pointed bodies reflect only a
small part of the incoming energy back to
the scanner. The same applies to surfaces
inclined toward the horizontal, such as
the windscreens of some motoryachts.
The most common material used to build
pleasure boats today, fiberglass, provides a
far poorer target than metals.
Bear in mind that when you’re moving, the motion of any targets on the screen
is relative motion, not true motion. If
you’re moving toward a stationary buoy at
five knots, it appears on your radar screen
as if the buoy is moving toward you at a
speed of five knots. The only stationary target on a radar screen is one that is moving
in exactly the same direction and at exactly
the same speed as your boat.
Fixing The Problem
The two main uses of radar are pre-
venting collisions and getting a position
fix. When collision is a possibility, the first
thing you must decide is whether or not
a target poses a risk of collision; secondly,
you’ll have to determine what leads to this
risk. For example, it’s fairly easy to deter-
mine a target moving straight down your
ship’s heading line on a collision course
— but is this a vessel you’re going to run
into from astern, or is it a target headed
full-steam right for your bow?
This story originally appeared in Mad
Mariner’s DIY Boat Owner Magazine.
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