Radar reflectors

[claire]

Is it true that the best way to be seen is to switch-on your own radar?

[Wotname]

Originally Posted by Sparohok

...

I wouldn't assume that curved surfaces necessarily have a higher radar return than flat ones.

I would go further and say curved surfaces are very bad at reflecting radar waves. Radar waves reflect in a very similiar manner as light. The radar pulse hits the curved mast and is reflected in almost every direction except where it came from!

IMO an active radar detector and reflector is far better idea; essentially a radar transponder or RACON as mentioned by David M.

http://www.safetyatsea.co.nz/card.asp

I see that this model is being phased out with something new coming in 2008 - wonder what it will be

[Maren]

Originally Posted by Alan Wheeler

I have a Boating mag buried here somewhere, that gave a trial of variuose reflectors.

I think I know the article you are talking about but I can’t seem to find it. It was more recent that the 1995 article CaptainJeff linked to but did not cover the different bands of radar. I think it was printed between 2005 and 2007 if I recall. The material was excerpted from a book the author was writing or had written on the same topic.

If we are talking about the same thing, the meta-analysis goes something like this:

Because some reflectors do very well perpendicular to the surface but poorly while heeled, and others do generally well overall (but not quite as well), you need to take into account what type of boat you have.

There are both passive and active systems. Passive systems reflect a portion of the energy sent out. Active senses it and retransmits. Passive costs less; active works better. Passive tends to have a problem with angles of heel; active requires power.


I think Aramanta has an important point as most large ships are using AIS. And if you are going to smashed on the high seas, it is less likely to be by a 40 cruiser or charter boat than by a gigantic freighter who will leave your boat in bits and keep going because they won’t even know they hit you.

Another point is the height of the reflector, which I think if fairly common sense but worth alluding to anyway. A supertank might be able to make a slight change of course several miles out but if you have the reflector low and they don’t see you till the third blip and you then are too close to pick up or only able to be picked up on the X band (but since they are at sea they are looking at the S band)… well, that’s like jumping in front of bus. This is, I think, another good argument for AIS.

[lannen]

I read somewhere that mast steps produce more of a return than the commercial reflectors. I have fold out mast steps (which remain in the up position unless being used by the Admiral) and have received favorable radar contact reports.

[panthablue]

I heard somewhere that you if you crunch up a long length of aluminium foil and suspend it lengthwise down inside the mast, it's very effective.

How you get it in there is another matter of course.

[Wotname]

Yes, if its a hollow wooden mast - radar waves don't penetrate a Al mast - if they did, they would also penetrate an Al reflector rather than be reflected.

[panthablue]

Der. Yea of course. Now you mention it. that's where I heard it

[Maren]

Ah, found it. The article is from Sept 2006 issue of Sail Magazine (pages 96 to 100) the author was Phil Gallman. From the by line of the aritcle, may I present his bona fides:

Phil Gallman has a PhD in engineering and applied science and spent 27 years working as a defense contractor in radar and communications systems. He is the author of Radar Reflectors for Cruising Sailboats.

He also has a website.

His remarks on a few of the reflectors mentioned above:

If he had an octahedral (Davis echomaster) he would mount it so it was immoblilzed because at certain angles it fails to provide much return, although at others is works quite well.

The mobri S-4 seems to be a very good choice for catamaran as is provides a strong cross section but looses almost efficacy once they are tilted beyond a few degrees.

He chose a Tri-Lens from Rosendal.

... I gotta stop drinking so much coffee

[David M]

Originally Posted by Wotname

I would go further and say curved surfaces are very bad at reflecting radar waves. Radar waves reflect in a very similiar manner as light. The radar pulse hits the curved mast and is reflected in almost every direction except where it came from!

IMO an active radar detector and reflector is far better idea; essentially a radar transponder or RACON as mentioned by David M.

Safety@Sea (NZ) CARD

I see that this model is being phased out with something new coming in 2008 - wonder what it will be

Radar waves reflect exactly like light with an equal angle off the reflecting surface as the angle in. Radar waves refract slightly different from visible light. Both are the same thing...EM waves but at different frequencies. The difference being that a radar is at a lower frequency than visible light. A radar wave can bend more than visible light. Radar can actually see over the horizon slightly on the 10cm band (S-band). The 3cm band (X-Band, smaller wavelength) is use for closer in scanning because it has better resolution.

Your right Wotname, the best thing are three polished metal right angles that reflect the entire EM wave back, the next best thing is a curved surface which does reflect a portion back, the worst reflector would be a flat surface, such as surfaces on a stealth aircraft, where the chances are practically nill that the EM wave would be reflected back towards the emitter/receiving antenna.

[David M]

Originally Posted by claire

Is it true that the best way to be seen is to switch-on your own radar?

No, radars, although they work at near the same frequencies, do not operate at exactly the same frequency bands of 3cm (typically for yachts) or 10cm. Radars have the ability to sense other radar units and change their frequency if the other radar is working on the exact same frequency, so as to not interfere with each other.

The closest related thing to do would be to have a directional radar detector, but then all you would be able to do is detect them to some degree but then they still would not be able to detect you since it is a passive device.

[David M]

Originally Posted by Maren

I think I know the article you are talking about but I can’t seem to find it. It was more recent that the 1995 article CaptainJeff linked to but did not cover the different bands of radar. I think it was printed between 2005 and 2007 if I recall. The material was excerpted from a book the author was writing or had written on the same topic.

If we are talking about the same thing, the meta-analysis goes something like this:

Because some reflectors do very well perpendicular to the surface but poorly while heeled, and others do generally well overall (but not quite as well), you need to take into account what type of boat you have.

There are both passive and active systems. Passive systems reflect a portion of the energy sent out. Active senses it and retransmits. Passive costs less; active works better. Passive tends to have a problem with angles of heel; active requires power.


I think Aramanta has an important point as most large ships are using AIS. And if you are going to smashed on the high seas, it is less likely to be by a 40 cruiser or charter boat than by a gigantic freighter who will leave your boat in bits and keep going because they won’t even know they hit you.

Another point is the height of the reflector, which I think if fairly common sense but worth alluding to anyway. A supertank might be able to make a slight change of course several miles out but if you have the reflector low and they don’t see you till the third blip and you then are too close to pick up or only able to be picked up on the X band (but since they are at sea they are looking at the S band)… well, that’s like jumping in front of bus. This is, I think, another good argument for AIS.

They are supposed to be using both the X and S bands. Its in the COLREGS under long range and short range scanning. Ships have more than one radar.

I agree that if you are going to be out there then get AIS. I think that having AIS is quickly becoming common sense if you want to improve the odds of not getting in a collision.

[Maren]

Originally Posted by David M

They are supposed to be using both the X and S bands. Its in the COLREGS under long range and short range scanning. Ships have more than one radar.

I agree that if you are going to be out there then get AIS. I think that having AIS is quickly becoming common sense if you want to improve the odds of not getting in a collision.

Good catch and thanks. I didn’t communicate my intent properly and, as the sentence reads, you're exactly right; point taken.


(for those not exactly certain what specific point of concern is)

77COLREGS Rule 7 Risk of Collision

(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist.

(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.

…and, upon detection, rule 6 and possibly others

The point I intended to make was we have to do everything we can in order to mitigate risk (such as getting that reflector as high as reasonable) because it gives the best chance of avoiding trouble. I think this prudent because there’ve several instances where I’ve been in the right and others in the wrong and yet, have still ended up with the short end of the stick.

But in this case, it makes a lot of sense to attribute large commercial vessels at sea to be thinking: “I’m close to the middle of nowhere but I can see all these vessels on my AIS and most of those on my (S band) radar too. Occasionally I get this junk that shows up on my X band but it usually isn’t anything (i.e., sea return) and besides we’ve the lookout outside.”


Two last no-so-unrelated things:

1. I would argue many if not most cruisers violate Rule 5 and most with radar Rules 5 and 7.

2. What happens when boats drop off radar

[Adaero]

I don't know if you have all seen this report: -

http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf

I have also heard that the German made Rasmus 1 reflector is better than the See-Me unit.

[windsaloft]

1. So, someone asked this a bit ago, but to re-query --- if I turn MY radar on, is that radar signal picked up by another radar at all, or does it only see its own radar's reflection?

2. Regarding the AIS deal --- do big ships run this system on in Vessel Traffic Service areas, or all the time out on the open ocean? If the latter, def. interested in investigating it more

[David M]

I answered question one. The answer is no. Turning on your own radar will not make you show up on someone elses radar. It only sees its own EM transmissions.

AIS is left on by default. For a ship, there is no reason to turn it off. As a useful aid to navigation safety, it would make no sense to turn it off.