radar reflectors and metal masts

[AngusMcDoon]

as I understand it the phone signal reaches out 40 miles to his gizmo

It doesn't. There are quite a few places along the Welsh coast where you could throw a stone onto the beach from your boat and still there's no phone signal.

Crossing the Irish Sea from, for example, Holyhead to Dublin you will have a phone signal decent enough to use internet about 5 miles out from each coast. Going from Holyhead to say Belfast 80% of it will be beyond phone coverage.

I think my AIS receiver is great. Nothing ran me down for all the years I sailed without it, but I like having it. It's combined with my VHF radio.

 

[lw395]

Re rain catching position
It would be a good idea if the suppliers of the aluminium reflectors were sold with a way of suspending them in the most efficient way. I found it quite tricky to rig mine properly.

Wouldn't it just.
I have a phot somewhere of one which had been suspended in an odd way then apparently winched tight.
It was an interesting shape.
I was going to post it on an RF forum but never got around to it.
I have seen octahedrals with a plastic stick through the middle, but not in a chandler's.

 

[Lon nan Gruagach]

Sorry, but all this "imagine your mast is chrome plated...." is nonsense.
Light will have a gazillion wavelengths across the width of a mast, radar maybe one or 2, it wont reflect anything due to its width. What you will get, however, is a vertical ish aerial that will receive and re transmit the signal (vastly diferent kettle of fish than reflection) in a 360 pattern and not constrained to the plane perpendicular to the mast.
Sure it wont beam a signal right down the neck of the radar system, but it does not need to, radar will look for a gnats fart in a hurricane, and see it if it is the right frequency.

 

[dylanwinter]

Sorry, but all this "imagine your mast is chrome plated...." is nonsense.
Light will have a gazillion wavelengths across the width of a mast, radar maybe one or 2, it wont reflect anything due to its width. What you will get, however, is a vertical ish aerial that will receive and re transmit the signal (vastly diferent kettle of fish than reflection) in a 360 pattern and not constrained to the plane perpendicular to the mast.
Sure it wont beam a signal right down the neck of the radar system, but it does not need to, radar will look for a gnats fart in a hurricane, and see it if it is the right frequency.

the interesting bit of information I am missing is if a Centaur without a reflector rates as a ten what does adding a reflector do to it
what is the signal from an average navigation mark?

does adding a reflector make me more visible than an average nav mark

does that matter if the Centaur signal is big enough to get the attention of the watchkeeper

D

 

[jac]

It doesn't. There are quite a few places along the Welsh coast where you could throw a stone onto the beach from your boat and still there's no phone signal.

Crossing the Irish Sea from, for example, Holyhead to Dublin you will have a phone signal decent enough to use internet about 5 miles out from each coast. Going from Holyhead to say Belfast 80% of it will be beyond phone coverage.

I think my AIS receiver is great. Nothing ran me down for all the years I sailed without it, but I like having it. It's combined with my VHF radio.

Another factor to consider is that this iPad is being provided by an ex MICHIGAN state trooper. Is it a US IPad on a contract with a US network. Will he have the international roaming to be able to constantly download data as AIS updates.

 

[BrianH]

as I understand it the phone signal reaches out 40 miles to his gizmo

so there will not be many times when we are further out than that

AIS reporting sites such as Marinetraffic, Vesselfinder, et al, are very hit-or-miss and no substitute for an on-board receiver. They rely largely on voluntary, amateur receivers for their data feed and they can be widely and randomly distributed, with delayed reporting ... or unserviceable.

I have a Marinetraffic station local to my marina that is more out of service than in, subsequently my signal is only intermittently reported when picked up by a station 28nm distant. Vesselfinder, which does not disclose its reporting station contributors, is fairly consistent though and was the best in plotting a mate's passage from NE Italy to Gibraltar late last year.

 

[KellysEye]

The cylinder radar reflectors outperform the octahedral reflectors.

 

[lw395]

the interesting bit of information I am missing is if a Centaur without a reflector rates as a ten what does adding a reflector do to it
what is the signal from an average navigation mark?

does adding a reflector make me more visible than an average nav mark

does that matter if the Centaur signal is big enough to get the attention of the watchkeeper

D

The radar cross section of a fairly tall narrow cylinder is roughly its height x width when it's perpendicular to the radar.
So a centaur mast is roughly 0.15m by 12m? 1.8 sqm?
This will drop off at any significant angle of heel when viewed from the side.
But it's significant. 10sqm is a good reflector.
Whether 1.8 is visible would depend on sea state/clutter.
Your typical nav mark will have an octo or similar.

 

[ghostlymoron]

"Barry, a retired michigan state trooper is beinging his I pad and that gives him a read oout of AIS."
The readout from ais online is not much use in practice even if you get a signal. It won't show your position like on a plotter and so you won't be able to tell where the targets are and how far away they are. It's not live, real-time info.

 

[dylanwinter]

AIS reporting sites such as Marinetraffic, Vesselfinder, et al, are very hit-or-miss and no substitute for an on-board receiver. They rely largely on voluntary, amateur receivers for their data feed and they can be widely and randomly distributed, with delayed reporting ... or unserviceable.

I have a Marinetraffic station local to my marina that is more out of service than in, subsequently my signal is only intermittently reported when picked up by a station 28nm distant. Vesselfinder, which does not disclose its reporting station contributors, is fairly consistent though and was the best in plotting a mate's passage from NE Italy to Gibraltar late last year.

ah well - back to old school number one eyeball then

D

 

[dylanwinter]

The radar cross section of a fairly tall narrow cylinder is roughly its height x width when it's perpendicular to the radar.
So a centaur mast is roughly 0.15m by 12m? 1.8 sqm?
This will drop off at any significant angle of heel when viewed from the side.
But it's significant. 10sqm is a good reflector.
Whether 1.8 is visible would depend on sea state/clutter.
Your typical nav mark will have an octo or similar.

jolly good

so does that mean I need ten centaur masts to equal one reflector

this is persuasive

I am still abit confused about the figure for the radar reflectors = the folding ones seem to claim a higher bounce back than their surface area presented to the radar

D

 

[dom]

The "Radar Reflectors" tested by QinetiQ were tested in an anechoic chamber, in isolation, so that their responses could be accurately assessed. These are effectively enhancements to the radar signature of the vessel to which they are attached.

All reflectors other than the Visible, the Cyclops and the Tri-lens are variations of a corner reflector, even the carpet plastic tubes.

The tube reflectors have bi-fold corners and will only return any echo when the incident waves are at 90 degrees to the corner, which is why they are so ineffective.

Other corner reflectors have either "stacked arrays" inside a cover (like the Blipper) where the corners are facing different directions, or planar like the folding ones that have to be arranged in the "catch water" position in order to give a good return. These are corners in three dimensions and the principle of these is to reflect as much of any incident wave back in the direction from which it came. The maximum return is when the incident wave is co-planar to the axis of symmetry of the corner. If you study the polar plots of the strength of the echo, they form a teardrop along this axis, fading as the direction varies.

Lens reflectors are designed to focus incident radar waves and reflect them back in the direction from which they came, which is why the give such consistent echoes. These are maximised across polar arcs of around 120 degrees, which is why the Tri-lens has three.

When a radar pulse strikes a target, how much of the incident radiation is reflected back to the transmitting antenna depends on the nature of the target. Ships are largely made of steel which give good consistent echoes and local effects can generally be ignored. No operator is going to miss a Hapag -Lloyd container ship because it's return varies between 1000 and 1200 square meters due to the various lumps of oddly shaped metal strewn around it's decks. Leisure craft are made largely from Fibre reinforced plastic, which is relatively transparent to radar frequencies and tend to absorb radiation rather than reflect it. Furthermore, any radar return from, say, a Westerly Centaur will vary due to it's relative instability, and the scattering effect of the metal items such as masts, anchors, booms and rigging. In calm conditions the variation will be slight, so there will be some consistency regarding the return, but in confused conditions there will be little consistency from the return and it will likely be lost in general clutter at the receiver, particularly if the sensitivity control of the display has been used to eliminate spurious echoes.

All reflectors have different performances depending on the frequency band used by the radar being tested. One that presents a good radar cross section to an X-Band radar will present a different one at S-Band.

What an exceptionally interesting and informative post - should be put straight into the next Yachting Monthly / Practical Boatowner IMHO :encouragement:

 

[skyflyer]

jolly good

so does that mean I need ten centaur masts to equal one reflector

this is persuasive

I am still abit confused about the figure for the radar reflectors = the folding ones seem to claim a higher bounce back than their surface area presented to the radar

D

Dylan

Although not strictly accurate (as has been stated above) using the light analogy then no reflector is like looking at your chrome mast from behind a torch, whereas a reflector is like a car's rear reflector applied to the mast. because the mast on its own will only reflect light that falls on it perpendicularly whereas the reflective material will reflect light back the way it came no matter what angle

this explains it in light terms

http://www.schoolphysics.co.uk/age14-16/Light/text/Bicycle_reflector/index.html

 

[Topcat47]

The problem with assessing the radar cross section of small leisure craft and the contribution of adding a specific radar enhancement is complex and not helped by spurious analogies with different wavelengths within the electromagnetic spectrum. Cars don't rock and roll in the same manner and to the same extent as small leisure craft. Furthermore car reflectors are reacting to white light and are selective in the frequencies that they reflect. They also operate over relatively tiny angles with respect to the incident radiation (in this case light waves).

It's not simple, there are no clear and evident similarities with common phenomena in every day life. Light will not penetrate the gel coat of that GRP hull, unless it is particularly thin, higher frequencies of EMR will (Radio, Radar, Alpha Gamma and X-rays will). If there is any doubt in your mind about this, simply take a commercial radio receiver into the saloon of your boat and switch it on.

The QinetiQ report is available on-line. this enables a clear comparison between all the enhancements tested. If you wish to understand how these items interact with the signature of your craft, then you need to do the maths to assess the Radar Cross Section of the vessel concerned and beware THEY ARE HARD SUMS and there are no shortcuts. There are people (many of whom work in QinetiQ) who have dedicated their lives to sorting out these problems. There is no shortcut.

Passive reflective devices work 24/7/365 and require no electricity. Transponders only work when the batteries are charged or the engine is running. YOU pays your money and YOU make your choice. Fit or do not fit. If you need your decision validated by someone else, perhaps you aren't making the right decision.

 

[dylanwinter]

The problem with assessing the radar cross section of small leisure craft and the contribution of adding a specific radar enhancement is complex and not helped by spurious analogies with different wavelengths within the electromagnetic spectrum. Cars don't rock and roll in the same manner and to the same extent as small leisure craft. Furthermore car reflectors are reacting to white light and are selective in the frequencies that they reflect. They also operate over relatively tiny angles with respect to the incident radiation (in this case light waves).

It's not simple, there are no clear and evident similarities with common phenomena in every day life. Light will not penetrate the gel coat of that GRP hull, unless it is particularly thin, higher frequencies of EMR will (Radio, Radar, Alpha Gamma and X-rays will). If there is any doubt in your mind about this, simply take a commercial radio receiver into the saloon of your boat and switch it on.

The QinetiQ report is available on-line. this enables a clear comparison between all the enhancements tested. If you wish to understand how these items interact with the signature of your craft, then you need to do the maths to assess the Radar Cross Section of the vessel concerned and beware THEY ARE HARD SUMS and there are no shortcuts. There are people (many of whom work in QinetiQ) who have dedicated their lives to sorting out these problems. There is no shortcut.

Passive reflective devices work 24/7/365 and require no electricity. Transponders only work when the batteries are charged or the engine is running. YOU pays your money and YOU make your choice. Fit or do not fit. If you need your decision validated by someone else, perhaps you aren't making the right decision.

I am merely seeking the facts

it seems to me thjat the tests done by the magazine were utterly pointless without some sort of naked yacht bounce based measuring stick

my decision is to act on the side of caution and hang the octahedral from the cross trees at night and keep a top notch mark one eyeball going around the clock

but if the centaur is already kicking off a radar bounce that no-one can miss then it would be good to know if I am wasting my time

 

[skyflyer]

not helped by spurious analogies with different wavelengths within the electromagnetic spectrum. Cars don't rock and roll in the same manner and to the same extent as small leisure craft.

my analogy was designed to answer Dylan's question as to why a reflector with 90 deg internal angles performs better than a flat sheet of metal of the same cross sectional area. Presumably they DO perform better on account of finding them on buoys, airfields (radar range calibration markers) and so on.

Rocking and rolling has no significance beyond changing angles - the bike reflector example in the link i gave is all about changing angles and explains why such a reflector works no matter how the angle changes.

I would venture to suggest that for the non radar scientist the analogy is very helpful indeed?

Of course not all light is reflected either - a surface we see as green absorbs all but the green wavelengths, and so on. A mirror is designed to reflect a much greater percentage of lights wavelengths. An angular reflector is designed to reflect whatever wavelength it reflects back in the same direction as the incident light. A flat surface radiates the incident light in many different directions.

Of course the analogy isn't perfect, but to say it is not at all helpful in explaining whats happening is somewhat disingenuous too perhaps?

 

[johnalison]

I am merely seeking the facts

it seems to me thjat the tests done by the magazine were utterly pointless without some sort of naked yacht bounce based measuring stick

my decision is to act on the side of caution and hang the octahedral from the cross trees at night and keep a top notch mark one eyeball going around the clock

but if the centaur is already kicking off a radar bounce that no-one can miss then it would be good to know if I am wasting my time

There are all sorts of theoretical reasons why certain objects/reflectors might work in such and such a way, but what matters is how they work in real life. Even more, it matters how they work in adverse conditions, such as in rain. As far as I can see, only the active "reflectors" can achieve reliable performance in all conditions, but that doesn't mean that other reflectors can't make a significant contribution.

My experience with my average yacht radar over some 15 years is that a yacht with no reflector can usually be seen at about 1.5 miles and one with a reflector at about 2.5, in good conditions and without adjustment. The most disappointing targets are fishing boats with rounded sections, which are often no easier to see than small yachts. For coastal cruising, I can easily see that basic precautions are all that are needed, especially for small craft and those with a limited budget.

 

[ghostlymoron]

"my decision is to act on the side of caution and hang the octahedral from the cross trees at night and keep a top notch mark one eyeball going around the clock"
I've been along the route you're planning with only a small octohedral reflector and at a similar time of year. The mark one eyeball was not a lot of use as the visibility deteriorated to the point that we couldn't see the bow from the cockpit. We were left with the mark one ear and expecting to see an enormous bow appearing through the fog at any time. At times we could hear engines but it's very difficult if not impossible to tell direction or distance.
However, the octohedral must have worked as nothing hit us and we arrived safely at Milford Haven.
Soon after that I fitted a fixed passive radar reflector which is claimed to present a much larger reflective area. I understand that it has a multitude of mini octos inside and will reflect a signal back in the direction from whence it came regardless of attitude. My budget wouldn't stretch to radar which is probably the best thing to have in fog. I didn't get an active type because I don't like to rely on powered equipment. AIS would also have been useful especially as you can see and contact any possible threating vessels.
I think you were very cavalier in removing your fixed reflector for very marginal gains in sailing performance. (I'd ditch the behind mast furling though). I hope you live to regret your decision.

 

[dylanwinter]

"my decision is to act on the side of caution and hang the octahedral from the cross trees at night and keep a top notch mark one eyeball going around the clock"
I've been along the route you're planning with only a small octohedral reflector and at a similar time of year. The mark one eyeball was not a lot of use as the visibility deteriorated to the point that we couldn't see the bow from the cockpit. We were left with the mark one ear and expecting to see an enormous bow appearing through the fog at any time. At times we could hear engines but it's very difficult if not impossible to tell direction or distance.
However, the octohedral must have worked as nothing hit us and we arrived safely at Milford Haven.
Soon after that I fitted a fixed passive radar reflector which is claimed to present a much larger reflective area. I understand that it has a multitude of mini octos inside and will reflect a signal back in the direction from whence it came regardless of attitude. My budget wouldn't stretch to radar which is probably the best thing to have in fog. I didn't get an active type because I don't like to rely on powered equipment. AIS would also have been useful especially as you can see and contact any possible threating vessels.
I think you were very cavalier in removing your fixed reflector for very marginal gains in sailing performance. (I'd ditch the behind mast furling though). I hope you live to regret your decision.

cavalier - I will go with that

D

 

[rob2]

Caution suggests that (having read the QinetiQ report) one would have little faith in any passive device as the sole strategy. The radar response from a "naked" vessel may be expected to vary greatly depending on its aspect to the radar and the ambient conditions. The analogy I understand for the octohedral reflector is from the way a snooker ball played onto the cushion close to a corner, bounces on two cushions (which are perpendicular to each other) and returns along a close parallel track. With an octohedral, each corner is similar but 3D, It produces lobes of response aligned with each corner reflector.

I inherited a football type reflector like Fylan's with my boat and it remains onboard to be hoisted to the spreaders if required, but I also fitted an RTE to switch on when vis reduces.

Rob.