TenMile
Well-Known Member
...everything you'd ever want to know...
http://theradarreflectorsite.org/
...from the site in the articles and blogs section good overview on "Can you be detected"
Summary
Pleasure vessels are not strong radar targets, multipath pulse cancellation limits detection
range in calm sea conditions, shadowing limits detection when the waves are larger than the
freeboard, and the radar horizon is limited because the target is on the surface. Fog and rain
attenuate the signal and reduce the probability that a pleasure vessel is detected. Clutter from
waves and rain may make detection impossible regardless of the scanner power.
As skipper of a pleasure vessel in good weather and calm seas, you may think in terms of
being detected by low-power recreational radar at half a mile or so and by professional radar on a
large ship at four miles or so. Detection range improves about three to one as the seas build to
seven feet or if a good radar reflector is mounted on the vessel. Other than that, darkness, light
drizzle, moderate fog, and waves to three feet have little effect on detection range.
Detection in bad weather is problematic. Thick fog and drizzle extending over large
areas may attenuate the radar signal greatly and reduce detection range severely. If the rain
surrounds your vessel, clutter restricts detection to ranges shorter than the rain clutter limit range,
which is about one mile in light rain, half a mile in moderate rain, and even shorter in heavier
rain. This is independent of the radar. Detection range may be better if your vessel is large or if
you have a good radar reflector, but the skipper of a recreational vessel should not count on
being detected at more than half a mile in moderate rain.
Waves generate a clutter limit. Outside the clutter limit, wave clutter has little effect on
detection. Inside, detection is possible only if the target is larger than the wave clutter. Three-
foot waves are not much of a problem. Five-foot waves require a moderately large radar
reflector; eight-foot waves require about 10 m2; ten-foot waves probably preclude detection
entirely. The sea clutter limit is proportional to antenna height and sea state; the larger ships, i.e.
higher antennas, experience the larger clutter rings, up to several miles. Detection range may be
better if your vessel is large or if you have a good radar reflector, but the skipper of a recreational
vessel should not count on being detected at less than a mile in eight-foot waves.
It does not take much in the way of rain or waves to limit detection of a pleasure boat to a
mile or less. A vessel approaching at 20 knots would have no more than three minutes to detect
you, identify you, track you and determine risk of collision, and decide on a course of action. A
large vessel would not be able to maneuver to avoid you under these time constraints. Your best
option is to proceed with extreme caution in bad weather.
Radar Reflectors Improve Detection
A radar reflector is a small device that is a very strong reflector of radar waves. (A one-
foot diameter radar reflector is often as strong a radar target as an entire sailboat.) One benefit
from a radar reflector is that it overcomes the pulse cancellation effect of smooth seas, as long as
you mount it high enough, because the radar target is no longer close to the surface. As a rule of
© Philip G. Gallman, 2009 7
thumb, mounting a radar reflector of the same RCS as the vessel hull 16 feet (5 meters) above
the water overcomes the flat-sea pulse cancellation effect. With a good radar reflector, correctly
mounted, the detection range table is applicable even in calm seas. A second benefit is that radar
reflectors mitigate wave shadowing and improve detection reliability when the waves are about
the same height as the target vessel’s freeboard. A radar reflector mounted 16 feet above the
waterline would be above the waves and would not be shadowed except in extreme conditions.
A third benefit is extension of the radar horizon because the reflector is some distance above the
waterline.
Unfortunately, nothing is free, and one must understand the second effect of pulse
cancellation by smooth water. While the radar reflector generally increases the maximum range
at which radar first detects you in smooth water, it also makes you invisible to radar in rather
broad rings, or blind zones, around the radar scanner. However, since the blind zones are at
ranges greater than the maximum detection range without a radar reflector, there is still a net
improvement from mounting a radar reflector.
On balance, a radar reflector is a relatively inexpensive device that improves detection in
many situations. In my opinion, every pleasure vessel should have one.
Radar Reflector
• Improves detection in all conditions if it has larger RCS than
the vessel it is mounted on
• Increases maximum detection range in calm seas, even if it’s
RCS is no larger than the vessel’s
• Overcomes intermittent detection caused by waves shadowing
the hull
• Extends the radar horizon
• Is invisible to radar in broad rings around the scanner in calm
seas, depending on the heights of the scanner and radar
reflector
TenMile
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