Aligning radar reflectors

[Deleted member 36384]

I can understand where your coming from but you cant buck the report, facts are facts : passive reflectors on small boats are of little use. .....

I do agree. I read the report some time ago and it was actually quite shocking just how poor all types of passive reflectors are at the size carried on yachts. I certainly assumed that the canister type and octahedral type were good, now I know they are not. The key position is that one must understand the limitations of these devices and manage the vessel as appropriate.

The dual band Sea-me is currently retailing at £694 (£834 including VAT at 20%). This is a lot of money for relying on someone else to detect you, amongst a set of vessels of which some will not have the capability of detecting you anyway.

Of course, one answer is to have a radar set on a yacht and know how to use it, which puts the detection of vessels firmly in ones own control. This Raymarine SL72 is retailing at £1362 including the VAT, which to me anyway, would be worth saving that bit more for.

In the meantime, using my Echomax will provide a modicum of additional reflection capability. Yes, its is still a poor situation with regards to a enhancing a radar reflection.

 

[Simondjuk]

Does anyone in the know have any comments on the following?

From the the last page of this document...

Active radar target enhancers are discussed in Chapter 8. These receive a radar pulse, amplify it, and retransmit it. On the positive side, active RTE provide a much larger effective RCS than is possible with a passive RTE of practical size. On the negative side are the higher cost, power consumption (which is an issue for long distance cruising in sailboats), reliability of electronics in the marine environment, and the fact that the active units, at least the ones the author is aware of, operate only at X-band and not at S-band. Active units saturate if the input signal power is above some limit as will happen when the vessel gets close to the radar. This means that the effective amplification decreases as the range decreases once the saturation range is passed. The effective RCS of an active RTE actually decreases at short range and the effect is more pronounced with higher power radar. In some cases a small passive reflector provides a stronger reflection at short range than does an active unit with nominally larger RCS.

Unfortunately, Chapter 8 is not included in the chapters available to view online, so I don't know if the above is more fully explained therein, but would be interested to know if this is a well known weakness of active RTEs.

Further, if it is an issue, I wonder how many users of active RTEs are aware of this potential hole in their defences.

 

[bedouin]

Does anyone in the know have any comments on the following?

From the the last page of this document...


Unfortunately, Chapter 8 is not included in the chapters available to view online, so I don't know if the above is more fully explained therein, but would be interested to know if this is a well known weakness of active RTEs.

Further, if it is an issue, I wonder how many users of active RTEs are aware of this potential hole in their defences.

The points about power consumption and reliability are perfectly true of course. I think that there are now active RTEs that work on S band as well as X - and it is worth pointing out that passive radar reflectors only work on X band anyway.

The bit about saturation seems to be a technical issue only - I can't see how it would really matter in practice.

 

[Simondjuk]

The bit about saturation seems to be a technical issue only - I can't see how it would really matter in practice.

This was the bit I was most interested in. As I read it, the issue is that active RTEs may give little or no return at close range, which I'd say might matter quite a lot to those assuming they can rely on one in order to be seen!

 

[bedouin]

This was the bit I was most interested in. As I read it, the issue is that active RTEs may give little or no return at close range, which I'd say might matter quite a lot to those assuming they can rely on one in order to be seen!

All it is saying is that there is a maximum return they can make - so that once you get nearer than a certain distance the output of the RTE doesn't get any greater - although there will still be an increase in the signal received by the transmitting radar as it gets closer

However the return signal will still be plenty large enough to be detected, so it would only be ignored if there were some clever filtering in the receiver. It is hard to see how that would cause a problem - not least because by that time you would hope that the vessel itself is contributing significantly to the return.

 

[Piers]

On a previous post I made on these forums, I posted the following,

"Last year, I had a trip on the bridge of a Sea-Cat. I asked the skipper how effective he found his two radars with small boats. His reply included "I was taking a Sea-Cat around Ushant in foggy conditions at 35 knots. Nothing on radar. Suddenly, a 40' sailing boat passed going in the opposite direction some 50 metres off my starboard side. He had no visible radar reflector on his vessel. One day", he added, "there will be dreadful accident, but we don't have the time to slow down.""

It was listening to this that made me decide to upgrade my active radar reflector from an X band to a dual X and S band (Sea-Me or Echomax both make them).

It also rings a warning bell when you remember reading on March 28 2011, that, "fast catamaran Condor Vitesse was in collision with a French fishing vessel, just before 0800 this morning, in thick fog. Two fishermen were saved but a third died."

The report showed the Condor was doing 37kts (in thick fog) and the (French) report blamed the Condor's crew for not keeping watch.

Whatever the cause of the accident, it reminded me of the maxim 'see and be seen'.

 

[Simondjuk]

All it is saying is that there is a maximum return they can make - so that once you get nearer than a certain distance the output of the RTE doesn't get any greater - although there will still be an increase in the signal received by the transmitting radar as it gets closer

However the return signal will still be plenty large enough to be detected, so it would only be ignored if there were some clever filtering in the receiver. It is hard to see how that would cause a problem - not least because by that time you would hope that the vessel itself is contributing significantly to the return.

Thanks for the comments, bedouin.

The thing I was puzzling over was the bit saying that the return from an active RTE at close range may be smaller than that of a passive RTE. Many passive RTEs being next to useless, I made the seemingly erroneous assumption that they may be prone to all but vanishing from a ship's radar when at close range (whatever the writer's definition of that may have been).

 

[bedouin]

Thanks for the comments, bedouin.

The thing I was puzzling over was the bit saying that the return from an active RTE at close range may be smaller than that of a passive RTE. Many passive RTEs being next to useless, I made the seemingly erroneous assumption that they may be prone to all but vanishing from a ship's radar when at close range (whatever the writer's definition of that may have been).

If you discount the nonsense ones, passive reflectors aren't really "next to useless" - the problem is their performance is inconsistent.

I have the EchoMax, which is probably good enough when the boat is upright, but is not so hot when the boat is healed over.

The danger of knocking the passive reflectors is that it might discourage people from fitting them - which is downright dangerous. The better reflectors are reasonable pieces of kit and certainly much better than nothing.

 

[peterb]

About 40 years ago I was developing an equipment using a He-Ne laser beam reflected by an optical corner cube. Knowing my interest a friend from the USA gave me one of the reject corners from the Moon array. Of course, this is far more accurately made than the normal radar reflector, but it does show the behaviour common to both.

The pic above is taken looking in to the open face of the corner. The three lines, apparently at 60 deg to each other, are the junctions of the three faces. The centre of the cube is where the three lines meet. Any ray of light entering the cube will be reflected three times, then exit the cube at 180 deg to its original path, apparently emerging from a point diametrically opposite its entry point. If you look at the pic you can see the camera, but it appears upside down. The centre of the lens appears to be at the centre of the cube. Provided that the cube can 'see' the lens, then the alignment is not significant; the lens always appears to be at the centre.

The same effects happen with an accurately made radar reflector. The beam from the normal small boat radar diverges quite quickly, so that it more than covers the reflector. The bit that enters the reflector is turned through 180 deg and displaced, but still diverges, so that the further you are from the reflector the smaller the signal that you recieve.

If you turn the reflector at an angle to the radar beam, then the 'window' will appear smaller. The further you turn the reflector away from the beam, the smaller the effective window and hence the smaller the amount of energy returned. To give uniform coverage in all directions you need lots of reflectors; the usual octohedral gives the effect of of 6 corners. Cylindrical reflectors like the Firdell and Gillie-Firth reflectors have a multiplicity of corners inside their cylinder and thus have a more uniform reflection.

 

[Robin]

Not so, the danger is that by fitting one you think you will be seen, which is down right dangerous because the chances are you wont be, the MAB report makes it clear that they are not fit for purpose, the best of the bunch was pretty dire. Of course they didn't test test them on the water on a boat but my own experience tells me they don't enhance the radar signature of yacht so what do they do? make you feel safer perhaps? a placebo effect.

The only reason for fitting one is to comply with a regulation/recommendation.

I'm sorry but I disagree.

My own practical experience of using boring motor trips to hone my radar skills makes me believe that a good reflector is a definite advantage. We sailed a lot in French waters where they were more casual in their attitude to such things as navigation lights and radar reflectors, despite having many more rules to obey in theory. Many French boats have no permanent radar reflector, have a tiny octahedral threaded onto the backstay point upwards or a silly little Mobri the size of a Smartie tube. Motoring on a popular route gave opportunities to check radar versus visual, check at what ranges a good return was received and with approaching traffic often getting to see what reflector if any was used.

As I said in an earlier reply my own practical tests showed that, using a basic small boat radar, a boat with one of the usual Brit type Firdell/Gillie reflectors would show consistently at 4mls and intermittently beyond that, yet one without would not become consistent until 2mls. These were not scientifically controlled tests and were in calm conditions (which is why we were motoring and bored enough to do it) but at least they were real life afloat, on water from a boat and using a small boat radar.

Another indication to me is the difference between the radar returns from a power boat and from a yacht. Many power boats do not have reflectors (but have large chunks of iron in the engine room) whereas most yachts do have them and have smaller chunks of engine room iron. Yachts typically showed consistent returns at much greater ranges than power boats back to the 4mls versus 2mls again. Note when I say consistent returns I mean when the echo is constant on the screen and not just an occasional spot that could be the back of a wave or nothing at all.

I don't believe at all that a good reflector is merely a placebo.

 

[Talbot]

Don't waste your time and money....Well, if you have an aluminium mast and any kind of engine. They will give a much bigger reflection than some stupid bit of tin bolted onto yer mast.
................

Obviously a response by someone who has no real concept of Radar Echo Area, and the enhancements derived from 90 degree reflctors. The best performance from a decent reflector such as Echomax (the best) or firdell (nearly as good) is maximised when the reflector is vertical, but can cope with heel albeit with degraded effect. The thin tubes seen on a lot of boats are cheap, and worth what you pay for them! The old fashioned octagonal reflector is not bad provided it is mounted correctly!

The best effect is achieved by use of an active system such as the see Me or the echoMax version (and preferably operating on the two different frequency bands used by merchant ships) - but this uses power!

This is covered under SOLAS V regulation 19

2.1 All ships irrespective of size shall have a radar reflector:

2.1.7 if less than 150 gross tonnage and if practicable, a radar reflector or other means, to enable detection by ships navigating by radar at both 9 and 3 GHz;

RYA Note: 'If practicable' means if it is possible to use a radar reflector on your boat then you should use one. Maritime and Coastguard Agency (MCA) guidance says that if your boat is more than 15m in length, it should be practicable to fit a radar reflector that complies with the current IMO test standard 8729:1997 (i.e. the reflector should have a radar cross section (RCS) of 10m²). If your boat is less than 15m in length, you should fit the largest radar reflector (in terms of RCS) that you can. Whatever size your boat is, you should fit the reflector according to the manufacturer's instructions and as high as possible for maximum detection range.

 

[uxb]

Obviously a response by someone who has no real concept of Radar Echo Area, and the enhancements derived from 90 degree reflctors.

This is covered under SOLAS V regulation 19

Obviously a response from someone how forgets how much a mast moves on a small boat at sea and knows everything from a book and nothing from experience.

Regulations? Yes, good things aren't they? That RCD is a work of art....


Regulations are for people too stupid to think for themselves. Or commercial operators to greedy to care.

 

[Talbot]

Obviously a response from someone how forgets how much a mast moves on a small boat at sea and knows everything from a book and nothing from experience.

Obviously I know nothing about this subject. I had 14 years seagoing in bigger ships followed by specialist training and user application in a field that includes Radar Echo area as a major part of the subject. Plus nearly 30 years of boat ownership.

So what would I know.

 

[uxb]

So what would I know.

As much as the rest of us on radar reflectors - only I'm obviously correct........