radar reflector

[LittleSister]

The other issue rarely discussed is that the tests are carried out on radar reflectors mounted alone in a test chamber. In practice you have to mount them somehwere on the boat. The standard mounting on front of a mast will be invisible, at least in theory, to a ship approaching from behind. (The Echomax website has some theoretical parameters.)

Following on from this I wonder whether the Tri-ball type (fresnel lens?) reflectors which mount on top of the mast (and which had a slightly lower performance than the Echomax type in the Ouzo lab report, IIRC) would not in practice give a better all round perfomance in practice.

Regarding the potential for two (or more) reflectors cancelling one another out, I would guess that this is unlikely within the length of a yacht, but I would be interested to hear from someone who knows about the wavelength, etc., of radar commenting on the plausibility of this.

 

[timbartlett]

I appreciate the possibility of the returns from two reflectors cancelling each other out.

Would it be any good fitting two reflectors vertically one exactly above the other so that they were always the same distance from the radar source but doubling the effective area?

If you had two identical reflectors at exactly the same distance from the transmitter as each other, then they would indeed present double the RCS of a single reflector. But if you moved one of them a quarter of a wavelength further away from the transmitter, then the signal would have to travel a quarter of a wavelength further out, and a quarter of a wavelength further back, so it would travel half a wavelength further -- i.e. it would be exactly out of phase with the echo received from the other reflector.

Given that the wavelength of X-band radar is just a tad over 3cm, a quarter of a wavelength is about 8mm. According to my mental arithmetic, if the centres of the two reflectors were 48cm apart (vertically), one degreee of heel or pitch would be enough to cause 8mm of displacement (horizontally). Another one degree would cause an extra 8mm of displacement, so the signals from the two would be back in phase and adding together again.

...Regarding the potential for two (or more) reflectors cancelling one another out, I would guess that this is unlikely within the length of a yacht, but I would be interested to hear from someone who knows about the wavelength, etc., of radar commenting on the plausibility of this.

Afraid it's not "unlikely" at all: it's inevitable!
Any practical yacht is made up of countless radar reflectors, both inside and outside the hull -- anything that conducts electricity will do. Even the crew will have an effect (I've had my own personal RCS measured at Funtingdon -- IIRC I achieved a peak of just over 2sqm! I've still got the polar diagram somewhere!

As the yacht pitches, yaws, and rolls, and as it rises and falls relative to the surrounding surface, the relative position of all those things change. As I've shown above, just a few millimetres is enough to make a difference. So the boat's RCS is constantly changing -- a fact that has been borne out by practical tests. I did a lot of work on this with Funtingdon for PBO in 1999. I would like to think that the articles are still available somewhere, but unfortunately I do not have a copy.

 

[VicS]

The other issue rarely discussed is that the tests are carried out on radar reflectors mounted alone in a test chamber.

Essential in order to eliminate variables over which you would have no control if you attempted to test them on a boat.

 

[blackbeard]

......SOLAS Regulation 19 is one of the (relatively few) SOLAS requirements that applies to yachts.
19.2.1.7 requires "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. According to MCA guidelines, any vessel over 15m in length should be able to fit a reflector that complies with ISO 8729:1997. According to them, smaller vessels should fit the largest reflector possible That "guideline", coupled with the fact that the MCA was the driving force behind 19.2.1.7 in the first place, is why it took so long for the MAIB to publish the radar reflector section of the Ouzo report: they were trying very hard to come up with a form of words that did not highlight the fact that the MCA had just made it complulsory for yachts to fit something that (a) did not exist and (b) was about as much practical use as a lucky rabbit's foot!

.......

Thanks for this.
Does, of course, raise the question of what we should actually do. A bit of a let-out in "if practicable" especially if the kit does not actually exist.
Is it reasonable to assume that fitting a good big passive reflector as high up as practicable shows a willingness to comply with the spirit, if not the letter, of SOLAS?

I appreciate that the ideal would be an active reflector, but the OP can't afford this (nor can I)! They have another problem too; preferred location is above mast head, but using the recommended bracket they blank out an unacceptable arc of all-round-white and tricolour masthead lights. Alternative is to mount on a pole at pushpit but it's difficult, on a small boat, to reach recommended height; although 3 metres or a bit more should be feasible. According to Echomax, they should NOT be mounted in front of mast or on spreaders, even though these seem to be popular locations for passive reflectors.

 

[Twister_Ken]

I appreciate the possibility of the returns from two reflectors cancelling each other out.

Would it be any good fitting two reflectors vertically one exactly above the other so that they were always the same distance from the radar source but doubling the effective area?

For the price of doing that, why not fit an active transponder?

 

[RAI]

If you had two identical reflectors at exactly the same distance from the transmitter as each other, then they would indeed present double the RCS of a single reflector. But if you moved one of them a quarter of a wavelength further away from the transmitter, then the signal would have to travel a quarter of a wavelength further out, and a quarter of a wavelength further back, so it would travel half a wavelength further -- i.e. it would be exactly out of phase with the echo received from the other reflector.

Sorry to be sceptical about this. A container vessel has billions of surface reflections cancelling each other out but still ends up with a very large radar cross section - because the reflections also add up. The likelihood of perfect cancellation must decrease with increasing reflective area. Additionally, the radar paints a target many times on each sweep, each paint increasing the probabilty of target detection. Radar would not work if multiple reflections always cancelled each other out and never added to each other.

 

[st599]

Sorry to be sceptical about this. A container vessel has billions of surface reflections cancelling each other out but still ends up with a very large radar cross section - because the reflections also add up. The likelihood of perfect cancellation must decrease with increasing reflective area. Additionally, the radar paints a target many times on each sweep, each paint increasing the probabilty of target detection. Radar would not work if multiple reflections always cancelled each other out and never added to each other.

You get constructive and destructive interference.

In Tim's example, items with a difference of (an odd multiple of) a 1/4 wavelength will totally cancel out. Similarly, at multiples of 1/2 wavelength, a constructive interference occurs. In between you get a bit of both and will end up with some signal.

Think Young's Slits.

The size of the Radar target doesn't change this.

For an example, think of your radio fading.and getting stronger when driving in town. Caused by multipath

 

[onesea]

Also sceptical about reflections cancelling each other out. I am sure it CAN happen but feel it is unlikely.

Having used radars regularly I am often surprised how radar can miss some targets and reflect form others.

For me the more metal there is up there the better chances of being "seen".

As reports say allot comes down to the diligence and skill of the radar operator. That goes for the ship OOW and any yachty operators out there!

 

[st599]

Also sceptical about reflections cancelling each other out. I am sure it CAN happen but feel it is unlikely.

Not only likely, but 100% inevitable. Do not think of your radio signal as a laser beam, it isn't colimated.

The beam width means that the transmitted signal will arrive at the target a) via a direct path and b) via a large number of reflected paths off waves, other objects, the troposphere etc. This induces a current in anything conductive it hits which causes another radio wave to scatter in all directions. Some of this induced signal will return to the Radar a) via a direct path and b) via a large number of reflected paths off waves, other objects, the troposphere etc.

Any 2 reflections which are 180 deg out of phase will cancel. Targets in real conditions will fluctuate in received signal.

However, a big target will be hit many times per sweep, on some the target will cancel and be undetectable, on some it wont. These are fed into a integrator - as long as some are detectable it will be shown.

Without going into Rayleigh and Rician fades, read:

http://books.google.co.uk/books?id=...page&q=radar destructive interference&f=false

http://books.google.co.uk/books?id=...page&q=radar destructive interference&f=false

http://books.google.co.uk/books?id=...page&q=radar destructive interference&f=false

http://en.wikipedia.org/wiki/Multipath_fading



PS you now know how stealth planes work

 

[st599]

Sorry to be sceptical about this. A container vessel has billions of surface reflections cancelling each other out but still ends up with a very large radar cross section - because the reflections also add up. The likelihood of perfect cancellation must decrease with increasing reflective area. Additionally, the radar paints a target many times on each sweep, each paint increasing the probabilty of target detection. Radar would not work if multiple reflections always cancelled each other out and never added to each other.

You don't need perfect cancellation, just enough to go below the minimum detectable signal to noise ratio.

 

[onesea]

Not only likely, but 100% inevitable. Do not think of your radio signal as a laser beam, it isn't colimated.

.............................................
PS you now know how stealth planes work

I am sure you are right, please do not take it personally, I am not interested in learning how stealth planes work (if I could do that I would off earned enough money by now to have a boat big enough not to worry about radar reflectors )

Plus if it has taken this long and much money to make near radar invisible planes, it cannot be that easy to cancel out the reflection?

You don't need perfect cancellation, just enough to go below the minimum detectable signal to noise ratio.

Same as you do not need a perfect reflectian just enough to bring it above the minimum noise ratio? Or possibly more importantly ensure the operator has the radar set up correctly? Which is the point I was also trying to make.

So the trick is making a reliably good enough reflector, on a constantly moving platform? I do not believe this has been done yet, we can improve are radar returns using reflectors but there is no guarantee.

If I had to rely on on some one detecting me on radar and I had 2 radar reflectors I would still use both.

Having viewed you argument I would place them with as much separation as possible (preferably vertically as well). As I guess this would help reduce cancellation but would also figure it would increase the chances of making a radar return above the Minimum noise ratio.

 

[timbartlett]

...If I had to rely on on some one detecting me on radar and I had 2 radar reflectors I would still use both.

Having viewed you argument I would place them with as much separation as possible (preferably vertically as well). As I guess this would help reduce cancellation but would also figure it would increase the chances of making a radar return above the Minimum noise ratio.

Two lucky rabbits' feet would be twice as lucky as one?

I'm sorry, but ye canna change the laws o' physics.Consider a typical ship's radar beam --20 degrees vertical and 1.5 degrees horizontal. In very crude terms (disregarding fresnel zones etc.,) at six miles range, most of its transmitted energy is spread over an area 0.15 miles wide and over a mile high -- roughly 500,000 square metres.

If you stick a ship in the way of that radar beam -- lets say the ship is 200m long and 25m high -- it has a physical cross ssection of 5000sqm so about 1% of the energy in the main beamwill hit it.

If, instead, you stick a radar reflector in the beam -- let's say the "reflector" is 500mm high x 250mm wide -- it has a physical cross section of about 0.125sqm. At 6 miles, 0.000025% of the energy in the main beam actually hits it.

Ships reflect lots of radar energy because they are big. Radar reflectors don't, because they are not.

-------------
There are many other issues here, but one that is often forgotten is that ships' watchkeepers do not constantly scrutinise their radars: they place a lot of trust in clutter clearance technology and ARPA. Some clutter clearance technologies will treat a contact as clutter (i.e. they will suppress it) if it does not show up on successessive pulses, while ARPA will not track a contact if the contact does not show up for at least 50% of scans. So it's no good being a strong but intermittent contact: it's far better to be a weak but consistent one.

 

[onesea]

Two lucky rabbits' feet would be twice as lucky as one?

Nah lucky feet have no value at all, very poor radar reflective area. Although marginally less than a reflector , that I do not deny I was not the one who asked about 2 radar reflectors.

I am just sceptical about 2 radar reflectors cause less returns than 1.

I'm sorry, but ye canna change the laws o' physics.Consider a typical ship's radar beam --20 degrees vertical and 1.5 degrees horizontal. In very crude terms (disregarding fresnel zones etc.,) at six miles range, most of its transmitted energy is spread over an area 0.15 miles wide and over a mile high -- roughly 500,000 square metres.

If you stick a ship in the way of that radar beam -- lets say the ship is 200m long and 25m high -- it has a physical cross ssection of 5000sqm so about 1% of the energy in the main beamwill hit it.

If, instead, you stick a radar reflector in the beam -- let's say the "reflector" is 500mm high x 250mm wide -- it has a physical cross section of about 0.125sqm. At 6 miles, 0.000025% of the energy in the main beam actually hits it.

Ships reflect lots of radar energy because they are big. Radar reflectors don't, because they are not.

I agree but at the same time on that principle 2 radar reflectors would still increase your reflective area, if that is of any value is a different matter.

-------------
There are many other issues here, but one that is often forgotten is that ships' watchkeepers do not constantly scrutinise their radars: they place a lot of trust in clutter clearance technology and ARPA. Some clutter clearance technologies will treat a contact as clutter (i.e. they will suppress it) if it does not show up on successessive pulses, while ARPA will not track a contact if the contact does not show up for at least 50% of scans. So it's no good being a strong but intermittent contact: it's far better to be a weak but consistent one.

I agree but it depends on how often this cancellation effect occurs? every other sweep? every 2nd sweep? every tenth sweep? every 20th sweep? Every thousandth Sweep?

I fear it cannot be that much as I have never passed a marina or small boat anchorage and noticed whole areas blank!

 

[Greenwichman]

Two lucky rabbits' feet would be twice as lucky as one?

I'm sorry, but ye canna change the laws o' physics.

There are many other issues here, but one that is often forgotten is that ships' watchkeepers do not constantly scrutinise their radars: they place a lot of trust in clutter clearance technology and ARPA. Some clutter clearance technologies will treat a contact as clutter (i.e. they will suppress it) if it does not show up on successessive pulses, while ARPA will not track a contact if the contact does not show up for at least 50% of scans. So it's no good being a strong but intermittent contact: it's far better to be a weak but consistent one.

Spot on, Tim. I have offered some of these observations in previous threads, from my perspective as a professional big-ship mariner and as a yachtsman:

First - yachts and craft under 50 feet loa are unlikely to be detected on radar at sufficient range, even if they are carrying radar reflectors; radar reflectors are simply not worth the money or the weight/windage aloft. As earlier forum threads have discussed, radar reflectors have a very poor technical performance. An active transponder may enhance the echo, but otherwise small craft should always assume they will not be detected.

Secondly - within 4 miles of a big ship (even further out in heavy weather, and upwind) sea returns are likely to obscure small contacts on the large vessels' radars. Decision-making by yachts within 5 or 6 miles of a large ship should therefore assume that even if detected a small grp or wooden craft may not be tracked on ARPA. The probability is that a yacht or other small craft will not be noticed by the watch-stander.

Thirdly - the best defence for a small craft is to sail defensively; avoid shipping routes. If no option and you find yourself in shipping lanes assume you won't be seen and take very early action to pass clear of large vessels. If fog or low visibility occurs use separation zones as safe havens in which to heave-to. Don't rely on "safe speed" on the part of others - too often it will be "safe" only in relation to other large vessels that paint strongly on radar. Small fry are at greater risk nowadays because watch-standers (mistakenly) believe in their radars and owners want commercial schedules maintained.

Bill Anderson's 'Question of Seamanship' (YM - April 2010) has some good advice, but even so his scenario makes an unsafe assumption that ARPA is tracking a yacht within 3 miles. His important point - with which I agree - is that the yacht skipper in that fictional case should have been taking much earlier decisions about avoiding the risk of collision.

Fog and yachting are a dire mix. Yacht radar, AIS etc are all very well, but reliance on such aids leads to technology-assisted collisions. There is loads of proof, sadly.

None of this is intended to defend high speed on the part of large vessels proceeding in fog; I don't condone it and they should slow down, but the reality is that those of us in small craft are at great risk and we should take measures to reduce the risk.

 

[Aeolus]

It improves the odds in fog

Not disagreeing with any of the science above, however I know that my boat, with an echomax mounted above the spreaders on the side of the mast, was detected at a distance of about 5 miles when I crossed the TSS in the channel because I called up one of the ships on VHF and asked then whether they had seen me on radar and, if so, at what distance. Admittedly weather was pretty good and the sea was pretty flat. But flat seas are fairly normal around the British isles when there is fog and that is the situation that worries me most because, without radar or ais, there is nothing I can do to detect and avoid possible collisions.

I think that my radar reflector does improve the chances that the other vessel will see me in fog and avoid me. It's not 100% but it improves the odds. I accept that it is probably less effective in rough weather.

 

[bedouin]

This criticism of radar reflectors is overdone - and not backed up by the very reports they cite.

Any of the "good" radar reflectors (octohedral, Firdell, EchoMax and so on) will vastly improve your radar return in most conditions. The comparison tends to be in terms of "worst" case - all of them have "nulls" where they add very little radar return. The better reflectors have fewer nulls.

I think everyone should carry one - either a firdell/echomax permanently fitted or an octohedral to be hoist. When hoisting a reflector much better performance will be obtained if it stays horizontal/vertical as the boat heals

 

[timbartlett]

...it depends on how often this cancellation effect occurs? every other sweep? every 2nd sweep? every tenth sweep? every 20th sweep? Every thousandth Sweep?

I fear it cannot be that much as I have never passed a marina or small boat anchorage and noticed whole areas blank!

100% cancellation is just as likely/unlikely as 100% reinforcement, and both will be quite rare. But partial cancellation and/or reinforcement are going on all the time, to some extent. You can't predict when they will occur, as the phenomenon is effectively random. If you had two identical reflectors, perfectly aligned with each other and half a metre apart, just one degree of pitch or roll would be enough to go from perfect reinforcement to perfect cancellation: and another degree would be enough to go from perfect cancellation to perfect reinforcement. And just 1mm of misalignment would be enough to have a measurable effect.

Prompted by this thread, I just dug out the results of the trials that were carried out by the Defence Research Agency for PBO in 1999.

What we found was that the mean RCS achieved by a 7m Sonata, motoring in circles with no radar reflector, was 41sqm. We then did exactly the same test with each of ten reflectors in turn, and found that in seven cases out of ten, the mean RCS achieved was lower than that of the bare boat.

 

[RAI]

You don't need perfect cancellation, just enough to go below the minimum detectable signal to noise ratio.

Ok. So my boat has a radar signature, but a small one. I add a classic relector and it increases my boat's radar target area, it doesn't entirely cancel out the boat's reflection. Then instead I add a multiple relector (Echomax for example), which contains several classic corner reflectors, the target area increases even further. Then I add a second one, I think the average radar refective area will about double. Some nulls in the pattern will be filled in, some peaks flattened out, but the average refective area increases with each additional reflector, especially if they are all orientated slightly differently.

 

[bedouin]

I think the average radar refective area will about double. Some nulls in the pattern will be filled in, some peaks flattened out, but the average refective area increases with each additional reflector, especially if they are all orientated slightly differently.

No - that is not correct.

If they are mounted directly above each other (to within 1 to 2 cm) then that would be correct - in fact that is how the firdell/echomax work. As soon as you get a lateral separation of more than a few centimetres very strange things begin to happen because of the interference.

The average area for the 2 is roughly the same as for one, but now you have many more peaks and nulls