Bill Mullarkey & Rob Warne
A VESSEL that has poor radar visibility is a danger to all
onboard and to other vessels in her vicinity. Radar reflectors
make a considerable improvement to the strength of a vessel’s
echo but there are large differences in performance between
models and they work badly if not carefully installed.
IMO regulations REQUIRE you to fit best radar reflector you can.
MCA guidance is that almost every vessel can and must carry a
reflector.
The recent tragic loss of the yacht Ouzo has re-energised the
discussion regarding seeing and being seen at sea. Whilst the
reason for the loss is still under investigation, it seems that
detecting yachts on radar in difficult conditions is once again
under the microscope, and this article discusses some of the
fundamentals of the radar visibility of leisure vessels, the
improvements offered by radar reflectors and the importance of
having good radar visibility.
What is radar visibility?
Conventional marine radars work by firing a short pulse of
energy from a directional antenna, listening for the echoes from
everything that reflects it and then slightly rotating the
antenna before repeating the transmission. Knowing where the
antenna is pointed and the speed that the transmitted pulse
travels is all that is needed for the supporting electronics to
produce a chart-like display of every RADAR REFLECTING OBJECT
within radar range. Note, however, that it is chart of
reflectors that is produced, not a chart of vessels, buoys, et
cetera.
GRP, plastic, wood and dry sails are such poor reflectors that
even military-grade radars find them difficult to detect.
Perhaps worse, whilst most metals are better reflectors, they
tend to concentrate the returned energy in a direction away from
the radar that illuminated them. Consider shining a torch on a
flat mirror: the reflection will be bright but will only return
to the torch if the mirror is pointed exactly at it. Wet sails
may act as a curved mirror but will scatter the radar energy
over a wide range of vertical and horizontal angles. Very
little, if any, will find its way back to the radar. Vertical
metal masts are inherently good reflectors because their shape
but, unless the mast is EXACTLY vertical, the energy will be
reflected into the sky or the sea. The idea may seem bizarre but
there is evidence that for many vessels their best reflector is
the hole they make in the water.
Unless the vessel carries a radar reflector, radar visibility is
a matter of chance. The only onboard echoes, apart from the
hole, will be from cookers, engine blocks, tools inside tool
boxes, port-lights, windows, et cetera. If she does carry a
reflector, then her visibility will depend upon how good a
reflector is fitted and how it is installed.
The term “radar visibility” has two metrics: the first is Radar
Cross Sectional Area (RCS) and the other Detection Probability.
For the purpose of this article it is enough to know that, for
both, the bigger the better.
It is estimated that the RCS of a small yacht of 30ft or less,
will be between 1m2 and 10m2 depending upon her heading relative
to the radar. In many cases, her best reflector will the hole
she makes in the water. Figure 1 illustrates the RCS pattern and
also shows the average RCS of the market-leading passive radar
reflector and the RCS value which is due to be required by the
next edition of the IMO requirements for radar reflectors. If
correctly mounted and at least 4m above the water line, the
reflector helps to fill in the low RCS areas of a yacht’s own
cross section.
Be aware that there are large differences in performance between
different equipment from different manufacturers. Figure 2 shows
the polar diagrams of two typical reflectors that have been
tested at a radar ranging facility. They are about the same size
and cost about the same amount of money but have significantly
different performance. Many others have not been rigorously
tested, and some that the authors have seen defy any scientific
explanation of why they should work!
It would be inappropriate for RSYC or the authors to recommend
one commercial product against another. The advice is to compare
the published polar diagrams from different manufacturers AND to
ensure that they have been validated by third parties: the
results of some tests are available on the Internet..
There is a current international standard (ISO 8729) which
describes required performance and how it is to be measured. Be
cautious of any manufacturer’s claims which do not refer to it.
A further complication
It would have been reassuring to suggest that leisure mariners
should determine their radar visibility by looking for each
other on their own radars. Sadly, the most important visibility
requirement is that to large commercial vessels, and there are
three fundamental reasons why a yacht may be visible to another
yacht but not to, for example, a ferry:
• the antennas are at different heights so “see” another vessel
differently (which may mean that one sees it clearly and the
other not at all);
• the commercial vessel will have a wider sea-clutter zone and
within that zone will be able to detect only strong and
repeatable targets; and
• leisure-marine radars generally have a much wider beamwidth
than commercial ones (perhaps 3 degrees versus 0.6º).
As a consequence of these differences, there is little that one
yacht can do to determine the radar visibility of another.
Why does radar visibility matter?
The leisure mariner has a potential problem if those navigating
in his or her vicinity have good radar visibility whilst his
vessel does not. The authors have heard people say that it does
not matter if they cannot be seen by radar if the natural
visibility is good because ColRegs will defend them. Nothing
could be further from the truth. The MCA states:
“ Many large vessels rely on radar for navigation and for
spotting other vessels in their vicinity. So, whatever size your
boat is, it’s important that it can be seen by radar.”
Put yourself in the position of the Officer of the Watch (OOW)
of a commercial vessel in heavy traffic. He is bound, as a
matter of professional conduct, to use every means at his
disposal to develop an awareness of the situation around him.
Even in good visibility, the OOWs of commercial ships will use
their radars to help them assess collision avoidance manoeuvres
several minutes in advance. Their understanding of what
manoeuvres will be necessary and possible is, in large part,
determined by what is visible on their radar.
Many leisure-marine radars have a MARPA function (Mini Automated
Radar Plotting Aid) and most mariners have decided that, whilst
they are useful, they are not to be relied upon. High-seas
radars have an ARPA, which is far from “Mini” and is excellent
at tracking all visible targets but ONLY if they are visible on
about 75% of antenna rotations. It is no use suddenly producing
a strong echo from a conveniently aligned wet sail only to be
missing the next time the antenna goes around because the sail
has moved by a degree or so: if not carrying a good radar
reflector most yachts cannot be tracked on ARPA. The same
caution applies to the polar diagrams of reflectors such as
those illustrated in Figure 2. What matters is not just their
maximum reflectivity but how well it is maintained at different
bearings.
Consider Figure 3, which shows the Detection Probability of a
yacht, with and without a radar reflector, at different
distances. Remembering that an ARPA requires repeated
detections, how easy should you feel crossing the Channel or
manoeuvring in the vicinity of a ferry without one?
If you are invisible, you actively prevent professional mariners
from having a complete picture of what is around them, and
increase the odds of their misunderstanding or misinterpreting
the situation.
As leisure mariners we have an obligation to make ourselves
visible for at least three reasons:
• it is sensible (and polite) to make things easier for other
mariners;
• our safety is reduced if we cannot be seen by the radar on a
commercial vessel; and
• we are obliged to carry the best radar reflector we can.
Even if the first two do not cause concern then the third
should. IMO SOLAS Chapter V now applies to pleasure vessels.
Regulation 19.2.1.7 requires that a vessel over 15m is fitted
with an IMO-compliant reflector: smaller vessels are not excused
but are required to fit the best reflector they can.
There is nothing voluntary in those requirements.
• We are not required to “Carry a radar reflector if we are in
such and such waters or engaged in such and such an operation”.
• We are not required to “Carry a radar reflector if it is easy
to do so”.
• Perhaps more worryingly, we are not required to “Carry a radar
reflector that someone claims works”.
The requirement is absolute and unbending. Regardless of your
vessel’s size, if by any practical means you can carry a radar
reflector then you must carry one: if over 15m you must carry an
IMO-compliant one; if below 15m, you must carry the best you
can. In either case, it is your responsibility to have reason to
believe that it works. Make no mistake: be in a collision with a
larger vessel whilst not carrying a correctly installed
reflector and, whether or not you survive the experience, the
subsequent Enquiry will place much of the blame on you.
So what should the leisure mariner do?
Perhaps most importantly, decide what would be the best radar
reflector you could fit and then fit it in the best way that you
can. Having done so, be aware of where any blind spots may be
and then do not “show” them to vessels that you would like to
avoid you.
In other word, be as visible as possible, for as many degrees as
possible and engage with other vessels knowing where your
visibility is reduced.
Active Devices
It is probably also worth mentioning active devices, although it
is recognized that yachts tend to be short of constant electric
power when cruising. A number of transponders are on the market,
which when pulsed by a radar, effectively send back an amplified
radar signal to the other vessel that will produce a “large”
echo on the display. They have their place but like everything
else, need to be used with caution. Southampton Water, for
example, is under constant radar surveillance by Southampton
Port Control and most leisure mariners keep outside of the main
channel. Imagine the consternation that would be if several
large-ship sized echoes were in grounding positions.
Conclusion
Regardless of the time of day, nature of passage, or natural
visibility, radar visibility is a safety issue and a legal
obligation. The authors respectfully recommend that all leisure
mariners carry the best radar reflector they can, understand its
imperfections and conduct themselves in the vicinity of other
vessels in the full knowledge of those imperfections.
DANGERS AT SEA
