Radar Reflectors - RYA advice

[Greenwichman]

There have been several threads about radar reflectors, but the issue continues to perplex many ....... the advice just published by RYA seems balanced and helpful .........
http://www.rya.org.uk/newsevents/ne...utm_medium=Email&utm_campaign=june12-cruising

 

[RobbieW]

I read the article and, as you say, quite balanced.

April last year I went to a Royal Institute of Navigation talk at Warsash Maritime college. The subject was Integrated Navigation Systems, part of the evening led to a discussion on AIS as there were a number of yachtsmen in the audience as well as the Merchant Navy folks. The management summary is that on a current well maintained and updated merchant ship, AIS information is part of the normal display used on the bridge. However, as one goes down the shipping ladder the equipment becomes older and less integrated. Yes, anything over 300gt is required to have an AIS reciever. The impression I got was that the lowest common denominator for collision avoidance, after the Mk I eyeball, is RADAR.

Thus, I fitted a dual band RADAR transponder before going cruising last year rather than an AIS transponder. Of course I have an AIS reciever too. In terms of (no ) bang for the buck I still think it was the right first choice. I'd be interested to see this aspect of the debate discussed though.

 

[maxi77]

I think the most important thing about the article is they once again point out no system of alerting another vessel to your presence is infallible so you need to keep a good look out yourself whatever you fit.

 

[Deleted member 36384]

I think the most important thing about the article is they once again point out no system of alerting another vessel to your presence is infallible so you need to keep a good look out yourself whatever you fit.

Yes, I came to a similar conclusion and wondered if this is no longer the default position amongst leisure sailors today i.e. reliance on machines is now so common place that one assumes it to be safer than just maintaining a visual and audible watch.

Certainly in the 1980's it was known that the Octahedral reflector was very inefficient and others coming on the market were not proven either. The mantra being taught by the RYA was fairly consistent from my own experiences of practical and theory courses: take a fix, slow down, post additional lookouts, listen (engine off), head inshore / away from transit routes, be aware of other vessels in shallow waters, especially if approaching harbour entrances, etc etc. It was all easier said than done, especially the heading inshore bit if one was well offshore.

However, the main point of all this defensive stuff was that the risk of a collision was much higher, so the natural tendency was to respect the seriousness of the position.

I have said a few times on various threads that today it is all that much easier to know where you are and what is around you in any visibility. Perhaps the reliance on efficient technology naturally lessens the individuals perception to the risks. For example, it was taught to me that lifejackets were compulsory in reduced visibility, at all times (even sleeping when off watch and underway) as well as not clipping on; being attached to the boat is the last thing someone wants as a ship's bow looms directly overhead!

I am glad that the RYA has issued this to highlight that the use of our senses is just as necessary toady and are also complimentary to other aids.

It would be interesting if anyone who has attended a practical or theory course recently can advise what was taught for reduced visibility.

 

[Simondjuk]

Even given every AIS receiver and transponder, active and passive RTE and piece of radar kit there is to have, anyone who isn't compelled to peer into the fog with their ears pricked has a worrying diluted sense of mortality, if you ask me.

 

[Daedelus]

It would be interesting if anyone who has attended a practical or theory course recently can advise what was taught for reduced visibility.

Day Skipper theory:-

General for radar:

Bigger (radar x section) the better
Higher the better
Always fit one

Analysis of various types of reflector and a reference to:- http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf which provides a detailed analysis of the different types.

Comment of dual band version of see me and a pat on the back for AIS.

Plus keep a darn good lookout yourself, lifejackets slow down listen and so on.

 

[timbartlett]

Day Skipper theory:-

General for radar:

Bigger (radar x section) the better
Higher the better
Always fit one

Analysis of various types of reflector and a reference to:- http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf which provides a detailed analysis of the different types.

Comment of dual band version of see me and a pat on the back for AIS.

Plus keep a darn good lookout yourself, lifejackets slow down listen and so on.

Worth bearing in mind that the reason the RYA has to say "always fit one" is because it is a legal requirement for all small craft outside "categorised waters" (ie. outside estuaries and places such as the Solent) This is because the MCA persuaded IMO to include it as part of the revision of SOLAS a few years ago.

It must have been acutely embarrassing for all concerned when the MAIB employed Qinetiq (what used to be the Defence Research Agency) to investigate the performance of radar reflectors as part of the Ouzo enquiry. The report was delayed, as negotiations over the wording took place between the MAIB, Qinetiq, the MCA and the industry, but the long and the short of it is that none of the passive reflectors available met the performance standards the MCA had set.

Even that was based on a relatively simplistic test that ignored the effects of mounting the reflector on a real boat!

Here's the conclusion to the Quinetiq report (to which Daedalus has provided a link). (The highlighting is mine):

Conclusions
The following is concluded;
· The Sea-Me is a good example of an active reflector (RTE) exceeding the requirements of the current and future ISO 8729 at heel/elevation angles of up to 15˚, it is also very small and light. Drawbacks are that it requires power to operate (which on a yacht is at a premium), it will only operate at X-Band and will offer no performance at S-Band.
· The POLARef shows excellence is possible but at a price, technically it just fails meet current ISO8729 [1] or its replacement [2]. The main drawbacks are it is very costly at £2000 and its quite heavy at around 5kg. It is currently used as a radar measurement standard although it could possibly be re-engineering for commercial production which could reduce the price.
· The Large Tri-Lens performs well especially at larger angles of heel and elevation, it just falls short of ISO8729 [1] having a peak RCS of 8.5m2 but otherwise performs well. It is the heaviest reflector supplied for test at 5.5kg
and costs around £300.
· The Echomax 230 narrowly failed to meet SO8729 during this testing, but showed good peak and average RCS performance. The reflector is reasonably priced at £130 and weighs 2.4kg; the main drawback was a RCS drop-off above an elevation angle of 10˚.
· The Firdell Blipper 210-7 narrowly failed to meet SO8729 during this testing, but showed good peak and average RCS performance. The Blipper is priced at £130 and weighs 1.8kg; the main drawback was a RCS drop-off above an elevation angle of 10˚.
· The Standard Tri Lens does not meet ISO8729 as the peak RCS was too low at 4m2. However its consistent RCS response outperformed most of the other reflectors when heeled over beyond 10˚; it is reasonably priced at £130 and weighs 2.5kg.
· The Plastimo 16” octahedral is inexpensive at £16 and lightweight at 0.65kg but failed to meet ISO8729 in either tested position. It had reasonable peak and average performance averaging around 2m2 but had wide nulls which kept its stated performance level down. Other drawbacks are that its mounting
arrangement is by suspension only (often in an unfavourable position) and could be subject to damage.
· The Davis Echomaster failed to get close to SO8729 during this testing. Its peak RCS is too low at 7.5m2 and its average performance is only 1.75m2. This reflector is priced at £60 and is lightweight; it can be mounted on a rod as well as by suspension (in the correct catch-rain position).
· The 4” tube reflector performed very poorly.
· It is concluded that either the active Sea-Me, POLARef and the Standard or Large Tri-Lens radar reflectors are the best reflectors at heel and elevation angles of over 10˚.

I love the idea that a product that fails to meet the required standard shows that "excellence is possible"!

 

[VO5]

Joke, isn't it ?
Nothing to beat a Mark 1 eyeball I say.

 

[sailorman]

Joke, isn't it ?
Nothing to beat a Mark 1 eyeball I say.

is 50 metres far enough in fog then

 

[lw395]

It pays to remember that the Radar Equation, which gives the returned signal power at the receiver, has a 'range to the power of 4' term in it, so a reduction of 50% in the radar cross section of a target only reduces the detection range by 16%.
So I take the view that a reflector that falls a little short of the standard is still well worth having.
I think you are as likely to get run down because there is nobody looking at the screen as because your reflector is a bit poor.
I feel that an AIS receiver is the most useful aid in poor visibility, possibly even better than having radar itself, certainly better value for money.

 

[timbartlett]

It pays to remember that the Radar Equation, which gives the returned signal power at the receiver, has a 'range to the power of 4' term in it, so a reduction of 50% in the radar cross section of a target only reduces the detection range by 16%.

So I take the view that a reflector that falls a little short of the standard is still well worth having....

Having been involved in some far more sophisticated tests than the MAIB ones -- carried out for PBO at the same establishment that the MAIB used -- a few years earlier, I am acutely aware of the fact that several "radar reflectors" actually reduced the radar cross section of my Sonata.

As you say, the radar range equation does include a power 4 so in order to achieve a 1.5x increase in detection range, you'd need to increase your RCS by a factor of 5. Even if a radar reflector that met the standard actually existed, it wouldn't achieve anything close to that kind of enhancement for a cruising yacht. It would struggle to achieve a 5x increase in the RCS of a rubber dinghy with an outboard motor!

 

[Elessar]

the fact that several "radar reflectors" actually reduced the radar cross section of my Sonata.

I know it's a bit of a black art but have you any idea how???

 

[maxi77]

I know it's a bit of a black art but have you any idea how???

I suspect it may be down to the reality that accurate real world testing can be very difficult to achieve. There is a lot in the atmosphere that is in reality very difficult to measure in these tests which can impact on the results which is why so many products are sold on the basis of laboratory tests not real wold performance.

 

[timbartlett]

I know it's a bit of a black art but have you any idea how???

Yes.
Oversimplifying a bit because this is a web forum, so I have a limited number of words, and no fancy graphics etc:-
Visualise the reflector as a single compact object, and the boat as another single compact object, both equally reflective.

If they are at exactly the same distance from the radar scanner, then theirreflected ecoes will be in phase with each other, so they will reinforce eachother: i.e. boat+reflector = stronger echo than boat by itself.

If the boat heels slightly, so the reflector is a quarter of a wavelength further from the scanner than the boat, then the echo from the reflector will have to travel half a wavelength further, so it will be exactly out of phase with the echo from the boat. If the boatand reflector have identical cross sections, then they will cancel out.

In reality, of course, neither the boat nor the reflector are single point objects, both are constantly moving relative to each other, and the radar cross section of both varies depending on the angle from which the radar waves are impinging on them, so the situation is not at all easy. There are also factors such as the effect of multipaths -- eg some radar energy is reflected from the surface of the sea on its way to or from the boat and reflector, rather than travelling directly from scanner to reflector.

 

[timbartlett]

I suspect it may be down to the reality that accurate real world testing can be very difficult to achieve. There is a lot in the atmosphere that is in reality very difficult to measure in these tests which can impact on the results which is why so many products are sold on the basis of laboratory tests not real world performance.

It's quite true that real-world testing is not easy -- a point that manufacturers always use to rubbish the effect of any radar reflector test that doesn't say their product is the best!

But back in the days when magazines carried out serious product testing, I worked on radar reflector tests for two different magazines. Both used a combination of "real world "let's see how it looks on a radar screen" with anechoic chamber tests. As it happened, both lots used the same resources at the same establishment as the MAIB subsequently used for their tests in the Ouzo report. And in one case even that was just the beginning -- we went on to use some far more sophisticated kit that could plot polar diagrams of RCS for a real boat with a real reflector.

Funnily enough, I have a lot more faith in those independent tests than I do in the computer-generated creations that appear in sales literature.

 

[maxi77]

It's quite true that real-world testing is not easy -- a point that manufacturers always use to rubbish the effect of any radar reflector test that doesn't say their product is the best!

But back in the days when magazines carried out serious product testing, I worked on radar reflector tests for two different magazines. Both used a combination of "real world "let's see how it looks on a radar screen" with anechoic chamber tests. As it happened, both lots used the same resources at the same establishment as the MAIB subsequently used for their tests in the Ouzo report. And in one case even that was just the beginning -- we went on to use some far more sophisticated kit that could plot polar diagrams of RCS for a real boat with a real reflector.

Funnily enough, I have a lot more faith in those independent tests than I do in the computer-generated creations that appear in sales literature.

Having been both poacher and gamekeeper in similar events in the end it is all a minefield quite simply because in the feild you actually have so little control over factors which can influence the results. In the end real world performance is a statistical probability, not a certainty

 

[Plomong]

I know it's a bit of a black art but have you any idea how???

The radar cross-section of an article (boat, for example) is usually expressed as a polar diagram giving received signal as a function of azimuth, with the object at the centre, azimuth being measured around the object under test.

When the object reflects from several distinct parts separated by significant distance (significant in terms of the radar wavelength), as is the case with a sailing boat, the received signals from different parts will intefere, and so the polar diagram will have peaks and valleys, and maybe even some nulls. Reflective parts include the mast, the engine, the guardrails, etc, but I would expect the last to have poor reflectivity.

What happens when you add a radar reflector up the backstay, for example??

Well, the radar cross-section of the boat is modified, introducing additional peaks and valleys, and maybe neww nulls, that were not there in the original polar diagram.

HTH

Plomong