Radar Reflectors

[s/v Jedi]

Originally Posted by Paul Elliott

If I'm not mistaken, the Tri-Lens should have good performance on a monohull, too.

I'm afraid not. I remember reading a test where it wasn't good when heeling. The three lenses are to cover the 360 degrees around you, so each does 120 degrees. All passive reflectors work by bundling the reflected energy into a beam which is affected by heel.

cheers,
Nick.

[chala]

Test hum!
I recently read a test on active reflector, the Sea-Me did not fare that well.
Start reading tests on anchors and you will knows less at the end that when you started unles you pick the one you like.

[Paul Elliott]

Originally Posted by s/v Jedi

I'm afraid not. I remember reading a test where it wasn't good when heeling. The three lenses are to cover the 360 degrees around you, so each does 120 degrees. All passive reflectors work by bundling the reflected energy into a beam which is affected by heel.

cheers,
Nick.

Here's one report that compares several different reflectors, including the Tri-Lens: http://www.westmarine.com/images/wa/...ors_report.pdf (the download is about 2 MByte)They tested up to 20 degrees of heel (which is a fair amount), and the Tri-Lens performance was virtually unaffected over this range.

Passive radar reflectors work by reflecting the radar energy directly back to the source. A standard corner reflector such as the octahedral reflector works well over a fairly wide range of heel angle. There are peaks and nulls due to changes in the effective surface reflective area, but the signal is always reflected back to the source.

I've got no vested interest here -- my reflector is a Davis octahedral -- but the Tri-Lens is an ingenious design and based on the tests I've seen and looking at the fundamental design, I am impressed. As I said before, it's heavy and expensive. An active "reflector" will generally give a even better results (as long as it responds to the radar frequency in use). The Tri-Lens is *not* the same design as the Lens-Ref, which needs to be kept virtually horixontal.

[Paul Elliott]

And here's the article I was looking for: http://www.theradarreflectorsite.org...PassiveRTE.pdf (from this very interesting website: The Radar Reflector Site). In tha article, he does a mathematical simulation of several different reflectors based on their geometries. The Tri-Lens performs spectacularly well at large angles of heel (he tests the large Tri-Lens, but the issue of angle is the same for the smaller units).

This is a simulation, not a live test, but I think I trust the simulations more than most on-the-water tests (which are usually poorly controlled).

I apologize for being obsessive -- I can't help it, that's just the way I am!

[bewitched]

A few thoughts:

A passive reflector, regardless of brand or design, is not a guarantee that you will be seen.

A passive reflector may be suitable for those who only sail locally in areas where poor visability is not a common occurance (you can always choose not to go out). I'd guess the majority of readers of this forum fall into this category.

Active radar reflectors and AIS transponders considerably increase the chances of being seen, but they still do not guarantee it.

Active radar reflectors and AIS transponders still are complementary systems - It's not a case that one is better than the other.

Also worthy of note is that the carrying of an AIS Transponder is now an ISAF requirement for CAT 1. The carrying of an active radar reflector never has been a requirement, neither has the carring of a radar.

So is it a case that:
1. AIS & Active combined give best protection
2. Either AIS or Active give medium protection (with AIS being favoured)
3. Passive systems offer least protection, but could be sufficient for a large percentage of sailors

[Paul Elliott]

Bewitched, good summary -- but --

As good as AIS is (and I'm usually the guy promoting it), it is useless unless the other vessel is also AIS-equipped in some manner. Most smaller vessels still don't have even an AIS receiver. This is where a reflector (passive or active) has value. Of course many smaller boats don't have (or run) radar, either, so in that case even our reflectors are useless.

The problem simplifies nicely if we are mainly concerned with not being hit by big ships.

(And let's pretend that we've already had the obligatory "MkII Eyeball, False Sense of Security" lecture, OK everyone? We're talking "in addition to", not "instead of".)

[s/v Jedi]

Originally Posted by chala

Test hum!
I recently read a test on active reflector, the Sea-Me did not fare that well.
Start reading tests on anchors and you will knows less at the end that when you started unles you pick the one you like.

Pls. post the link as all the tests that I am aware of praise the Sea-Me.

An active reflector works under any degree of heel because it transmits an electronically amplified signal. The only way for them to fail a test would be a malfunction or bad installation.

cheers,
Nick.

[Paul Elliott]

Nick, I recall seeing a test of an active reflector in which it performed poorly in short-range conditions. I believe it had to do with overload and/or timing of the regenerated signal. It might have been this article: http://www.theradarreflectorsite.org...gActiveRTE.pdf, from the website I previously mentioned. It does seem plausible and is another way the active reflector might test poorly.

Certainly in most circumstances the active reflector will provide superior performance, and this seems to be the conclusion reached in the above article (the problem discussed only occurs at short ranges).

[s/v Jedi]

I just read the test of that west-marine link posted earlier in this thread. They tested in a small room and the SeaMe did just fine. Actually, without heel, it did about 50 times as good (!!!) as the average of all the rest incl. tri-lens (showing 500 m2 surface instead of 10 m2). They conclude it would be the only reflector that conforms to ISO requirements.

But I also read that indeed the tri-lens does well under heel. I am confused and now think it must have been the weight that they concluded it not so good for mono-hulls... can I get away with that? ;-)

I'll read the other test too but like you already wrote, the SeaMe always wins every test.

cheers,
Nick.

[s/v Jedi]

So, I just read the other test too (small document). This effect on short range that they write about is pure speculation, they did not test this at all.

In theory, they are right: As the ship comes closer, your echo becomes bigger. But at some point, the Sea-Me (or any other active system) can't transmit a bigger pulse because it has reached maximum output. As they come closer still, your echo can even become smaller.

This is true but not relevant because you start off as a 500 m2 surface which is like a mammoth oil tanker. IMO it does not matter if you look a bit smaller when they come closer because they can see that looking out the window anyway.

Remember that passive reflectors give an echo equal to a surface of 10 m2 (a good one) so the active system is just so much better that it puts any passive system incl. a tri-lens to shame.

This test too shows the Sea-Me to be the winner. This units just always wins and I can't believe that so many cruisers reject it, especially when they buy the expensive tri-lens instead. In Europe much more boats have the Sea-Me, may be because there's more ship traffic?

ciao!
Nick.

[chala]

Originally Posted by s/v Jedi

Pls. post the link as all the tests that I am aware of praise the Sea-Me.
cheers,
Nick.

Sorry no link it was in a British sailing magazine some month ago.

[Paul Elliott]

Originally Posted by s/v Jedi

This test too shows the Sea-Me to be the winner. This units just always wins and I can't believe that so many cruisers reject it, especially when they buy the expensive tri-lens instead. In Europe much more boats have the Sea-Me, may be because there's more ship traffic?

Here's my reasoning (actually, my fuzzy thinking) about this:

The Tri-Lens, while expensive and heavy, is probably the best-performing passive reflector available, by a wide margin in some cases. It requires no wiring, and doesn't consume power. It also has a good sales/distribution network.

The Sea-Me is usually shown at the top of the mast. where it's going to get in the way of many styles of VHF antenna, anemometer, tricolor, etc. You've got to run wires up to it, and it consumes power (not much -- 150 mA most of the time -- but some).

My own decision to get the Davis octahedral was made before AIS or the Tri-Lens were introduced, but today it might go like this:

• I really ought to have a radar reflector.

• I do occasional ocean races, and they require a reflector. The smallest one available is the Mobri, and while that complies (?) with the race requirement, in my opinion it is essentially useless as a reflector

• The Davis performs reasonably well, it's inexpensive and light (although a bit of a pain to mount), and has that "I cross oceans!" look to it (don't underestimate the style factor when it comes to boat gear)

At this point I have (for the sake of argument) crossed the threshold of dimishing returns in my personal pain/gain equation. Other gear may provide improved performance, but the actual effect on my real-world safety is difficult to quantify.

If I were singlehanding (or shorthanding), and thus less likely to be keeping an adequate watch, the benefit of a better reflector might be more compelling. As it is, the combination of having a crew of four or more, the areas where I sail, the fact that I have a radar and an AIS transponder, all make it easier to decide that I've done enough in this regard.

When it comes to safety, there are many factors at play, and reflector effectivity is only one of them.

[Paul Elliott]

And here are links to the reflectors I mentioned:

Sea-Me: Sea-me Active Radar Reflector. Safety at Sea. Drastically Improves Radar Visibility. Amplifies Radar Signal. Automatic Ship Detection. Marine safety equipment and Marine electronics.
Tri-Lens: trilensweb12002 - Page: 1 of 5
Davis: Marine and Boating Products by Davis

Mobri: Radar Reflector 1 (this is a reflector study done by West Marine -- worth reading!)
LensRef: Radar Reflector 1

The data on all these different reflectors (active and passive) isn't that hard to measure or understand. What's tough is deciding what to do once you've got the data!

[s/v Jedi]

Well, according to figure 23 the Davis scores pretty bad. The large tri-lens scores best with an average RCS of just under 5 m2; The Davis is only around 2 m2; an octahedral in catch-rain position outperforms all but tri-lens and Sea-Me at 2 m2 pretty flat for any elevation but the Sea-Me starts out at 100 m2 and even at it's worst (20 degree elevation) is still 8 m2 so between 4 and 50 times better.

cheers,
Nick.

[Paul Elliott]

the Davis scores pretty bad

It's all relative, isn't it? The Davis scores much better than some, and worse than others.
Thr problem is that this stuff is odorless, colorless, tasteless, etc. You can't numerically calculate the effect that a particular reflector will have on your chances of survival. In the very few published cases where a reflector might have been a factor in a collision, there are so many confounding factors that it is nearly impossible to reach a rational decision.

So, we go on emotion and gut-feel, on cost and convenience, and are guided by our own priorities and enthusiasms. There are so many things that I could make "better" on my boat, but each involves a tradeoff somewhere else.

That said, my Davis reflector got banged up recently, and I need something to replace it. Perhaps that is why I sound so philosophical about this -- I'm in the middle of "analysis paralysis". I usually have to make an emotionally-driven decision to break out of the rut, but at least the analysis has eliminated the poorer choices.