Active Radar Reflectors and SOLAS V Implications

[RAI]

Only in the sense that sail area - displacement ratio is "the measure" of a yacht's performance. Radar cross section is certainly important, but it is not the only important thing. For example, it depends on the angle at which radar waves come in so in real life the motion of the boat, and the response of a radar set to a target with rapidly changing cross section need to be taken into account. The Qinetic report also predicts that all radar reflectors provide no return whatsoever at certain distances due to Brillouin zone effects. Well, that's nice, as long as you only ever sail on a completely flat sea.

I think it would be a tall order to do a scientific comparison at sea. The number of variable leaps and the conditions are not consistent over time. The factors that you quote are likely to make detection more difficult and dependent on the settings of the radar and of course the reflective area of the target vessel itself.
QinetiQ did provide their measured average values for the reflectors, which are perhaps the best comparison between reflectors.

 

[prv]

A bit of a dumb question but is there any kind of legal requirement to carry a radar reflector or maybe insurance implications ?

Errm, yes. That's exactly what this thread is about - what devices meet the legal requirement.

A few years ago, Chapter V of the SOLAS regulations was incorporated into UK law (sorry, I don't have to hand the actual UK act or SI that implemented this). The vast majority of this chapter does not apply to vessels under certain sizes (in hundreds or thousands of tons) and therefore is irrelevant to yachts. But a few choice items have no small-vessel exemption, so they apply to us just as much as any other UK law. In summary, these are:

- You must fit a radar reflector (or "other means") unless impractical to do so
- You must have a passage plan (though RYA opinions is that for leisure vessels it need not be written down)
- You must have a copy of a certain table of distress signals (lots of publications - almanacs, leisure charts, etc - now include this, so you may comply even without realising)
- You must report any dangers to navigation you encounter
- You must respond to distress signals from others
- You must not make false distress signals.

Most of these are common sense or not a big deal, so it's only the requirements for radar reflectors and passage planning that excite any interest (and discussion of the latter seems to have died down now as well).

Pete

 

[RAI]

I'm not answering the OP's question but I'm wondering just how relevant previous reports on radar reflector testing are.
A few years ago and everyone was using analogue radar. The swing is now towards digital. So the radar reflectors that were totally inadequate a few years ago may well give sufficient return to make a blip with todays digital radar.
Another dilema: do you spend the money on active radar or AIS transponder? As boats get modernised I suspect the later may be the better option.

"Digital" (CW) X-band radar may or may not be better at detecting and discriminating targets than the older pulse radars but I think larger target RCA is always better. I wonder if the new CW radars work as well at long range as the old pulse radars. I wonder also it the existing RTE's respond to CW radar?
Personally, I think AIS is a much better tool for observing AIS equipped vessels, the trouble is all the others.

 

[Storyline]

Yikes ! Thanks for the comprehensive reply. I was under the impression that is acceptable to carry one of the foldable types that can be hoisted on a spare halyard when you are in poor viz.

 

[prv]

I can understand anyone who's boat is fitted only with a 4" Plastimo tube would feel unhappy with that conclusion.

Kindred Spirit had a 2" Plastimo tube, but I wasn't unhappy with that conclusion. I already assumed that the thing was almost entirely useless, but having it made me legal .

It would have been awful to bolt a huge white plastic excrescence to the top of my beautifully-varnished spruce mast. I never sailed her in fog, and rarely anywhere that ships would have needed to see me.

Pete

 

[prv]

"Digital" (CW) X-band radar may or may not be better at detecting and discriminating targets than the older pulse radars but I think larger target RCA is always better.

Agreed.

I wonder if the new CW radars work as well at long range as the old pulse radars.

No, they don't; the manufacturers are quite open about this. They point out that extreme range detection is not usually that important to leisure vessels, but that if you do need it then you would be better with a pulse system.

(Range with the "broadband" sets is not terrible by any means, just not quite as good as pulse.)

I wonder also it the existing RTE's respond to CW radar?

I believe they don't. Not a problem at the moment as ship radar is all pulse AFAIK, but I guess that might change.

Pete

 

[RAI]

I believe they don't. Not a problem at the moment as ship radar is all pulse AFAIK, but I guess that might change.

If I had an RTE I would like it to warn approaching yachties as well as fishermen of my presence on radar, they tend to be the ones that my radar doesn't see. There are more of them too. Back to a big reflector, or several.

 

[Marsupial]

[/I][/COLOR]So a passive reflector for S-Band would be desirable. QinetiQ only tested at X-Band. S-Band results would probably be worse for all the reflectors tested as size matters for the longer wavelength. Further, I believe the SOLAS ship's watch probably prefers the S-Band as it is less rain sensitive compared to X-band.

The QinteQ report does mention S band reflectors it goes something like, most leisure yachts aren't big enough to carry one.

Its all about band width, X band will/should/could see objects 100 mm across on a good day, S band on the other hand will have great difficulty seeing a small object like a yacht on any day at any range, container ship no problem, block of flats no problem. Hence any passive reflector that will be seen by S band will be very large. Even the X band passive reference target is a pretty big object (from memory) about 600 mm in dia so where passive reflectors are concerned big is beautiful. An effective S band reflector will be very big.

As for the rules - does it say reflectors must be carried or that reflectors must be deployed? I always thought it was the former allowing the skipper to deploy the device when conditions dictate.

 

[RAI]

The QinteQ report does mention S band reflectors it goes something like, most leisure yachts aren't big enough to carry one.

I only found: "It should be noted however that passive reflectors will offer some performance at other frequencies including S Band."

Its all about band width, X band will/should/could see objects 100 mm across on a good day, S band on the other hand will have great difficulty seeing a small object like a yacht on any day at any range, container ship no problem, block of flats no problem. Hence any passive reflector that will be seen by S band will be very large. Even the X band passive reference target is a pretty big object (from memory) about 600 mm in dia so where passive reflectors are concerned big is beautiful. An effective S band reflector will be very big.

I think its all about wavelength, not bandwidth. X-band is about 9 GHz and S-Band at about 3 GHz. So nominally I assume an S-Band reflector needs to be about three times larger than an X-Band reflector for the same RCA. A bit like antenna dipole lengths. Conversely, that would imply that the passive reflectors tested might have about one third the RCA in S-Band than in X-Band, where they are based on corner reflectors. I'm not sure about the Luneberg lenses, which might be tuneable (?). Anyway, one third of very little isn't much use.
I read somewhere that a lot of a GRP boat's RCA comes from the whole in the water that it makes. One up for the metal hulls maybe.

 

[Storyline]

As regards the stowable radar reflectors, Solas V seems to be clear that ships constructed after 1st July 2002 should have (if practical) a radar reflector 'fitted' (Regulation 19 2.7) but if is not so clear for ships built before that date fall as they fall under the International Convention of safety of life at sea. That appears to suggest that you have to have one but not necessarily fitted. I have not read that fully so cannot be certain.

 

[lw395]

A reflector does not have to be very big to work, although bigger is generally better.
Radar cross section of objects such as yachts tends to vary hugely with angle of arrival and particularly heel.
A typical aluminium mast will reflect quite well, but if it is not vertical, the reflection will mostly go into the sea or high in the air, not back toward the source.
The dominant term is what the radar receives is usually range. The radar equation contains the term R^-4, because the energy spreads in two dimensions on the way out and on the way back. Therefore a change of radar cross section from 10sqm to 1 sqm is only a 45% reduction in range.
So just because your cheap radar reflector does not meet the standards that none of the others really meet either, does not mean it is useless.
Most of us are not going to sail around with a 2ft octahedral at the top of the mast. A mickey mouse reflector is way better than nothing.

 

[RAI]

A mickey mouse reflector is way better than nothing.

Is that the one with big round ears?

 

[Roberto]

Is that the one with big round ears?

a poor mickey mouse with metal ears plus a twisted neck and cronic otitis because of the rain-catcher position

 

[RAI]

a poor mickey mouse with metal ears plus a twisted neck and cronic otitis because of the rain-catcher position

Dang! Now the NSA and Google know that I've looked up cronic otitis.

 

[Roberto]

Dang! Now the NSA and Google know that I've looked up cronic otitis.

gosh they also know I am unable to spell cHronic..



Anyway, apart from relying in laboratory tests (often a lot simpler than real life diversity), there is the more practical method of actually asking radar stations around oneself what type of radar echo they have of our boat, in a variety of conditions. Plus, it makes a convenient radio check too.
I do it from time to time, to a passing ship, a land signal station, a fellow boat, with flat sea, while beating, etc. Just ask "what kind of radar echo do I display?".

Now I know I am similar to a mid-size fishing boat

 

[jfm]

An active reflector picks up a signal, amplifies it and sends it back.
A CARD picks up a signal, amplifies it and sounds an alarm.
The majority of the problems are the same. If CARD type gadgets cannot be relied upon, it requires a leap of faith to rely on active repeaters.

lw395, it is arguably a leap of LED watching rather than faith. Modern RTE have LEDs (and bleepers too, if you want) that tell you they have detected incoming radar. Mine has different colour LEDs depending on whether it has detected incoming X or an S. In busy waters, those LEDs are flashing lots, so I think you can conclude that the thing works better than the CARD you mention

Of course, normally the LEDs are useless because they are permanently lit by your own radar, but from time to time you can switch your own radar off for a minute to see that the thing is detecting other radars.

 

[JumbleDuck]

I think it would be a tall order to do a scientific comparison at sea.

Perhaps, but that doesn't automatically make a mathematical model satisfactory. As I said, just testing a couple of reflectors at sea would have given at least some validation to the model.

QinetiQ did provide their measured average values for the reflectors, which are perhaps the best comparison between reflectors.

I respectfully disagree. Without knowing the polar diagram, and in 3D (or at least 360 degrees round and 45 degrees above and below horizontal), average radar cross section is a very crude measure.

 

[JumbleDuck]

Some of you may be interested to know that there is a BBC film in production about the development of the Chain Home radar system in the 1930s. Eddie Izzard stars as Robert Watson-Watt. Should be on screen next summer, for the 75th anniversary of the Battle of Britain.

 

[Marsupial]

I only found: "It should be noted however that passive reflectors will offer some performance at other frequencies including S Band."

I think its all about wavelength, not bandwidth. X-band is about 9 GHz and S-Band at about 3 GHz. So nominally I assume an S-Band reflector needs to be about three times larger than an X-Band reflector for the same RCA. A bit like antenna dipole lengths. Conversely, that would imply that the passive reflectors tested might have about one third the RCA in S-Band than in X-Band, where they are based on corner reflectors. I'm not sure about the Luneberg lenses, which might be tuneable (?). Anyway, one third of very little isn't much use.
I read somewhere that a lot of a GRP boat's RCA comes from the whole in the water that it makes. One up for the metal hulls maybe.

This from Echomax "On S (3GHz) band a passive reflector offers about 1/10 the response obtained on the X (9GHz) band." so the S Band reflector needs to be 10 times larger!

But lets not forget that most boats do have a Radar Cross Section of sorts and a good radar with a good operator will see them but the operator has to be looking - if you see what I mean.

Its in conditions that make Radar a marginal device that we need something that "helps" hence all this discussion.

 

[Jean]

Some interesting comments. I have now found the answer to my original question here: http://www.theradarreflectorsite.org/StandardsAndRequirements.htm , and as long as it covers the relevant S and X band frequencies, I guess it comes under "by other means" and hence suggest it is OK.