radar reflectors and rules

[haulback]

Discussion somewhere about radar reflectors led me to thinking....

Ends up that the one I have put so much trust in for last 8 years has a RCS (radar cross section) of, at the very best - as stated by manufacturer - 4 sq metres. SHTP rules want one that has an RCS of 10 sq. metres.

Problem being, I can not find ANY that come close to this. Best I can see is one that has RCS of 7 sq metres. Now what???

This seems to be the most thorough report I have stumbled across on the matter, although I am sure there are others, and mostly what I am basing opinion on.

http://www.ussailing.org/safety/Studies/radar_reflector_test.htm#INDEX

Jim/Haulback

 

[Alchera]

Interesting.... you seem to be right. The Davis reflector I've been using doesn't come close to 10 meters squared in RCS, nor do most of the others. I wonder where the original SSS requirement came from? I looked at the requirements back to 1996 in the archives and it was in there as well.

At any rate, it would seem that most reflectors on the market today do not meet the SSS requirement as currently written!

 

[tiger beetle]

Interesting.... you seem to be right. The Davis reflector I've been using doesn't come close to 10 meters squared in RCS, nor do most of the others. I wonder where the original SSS requirement came from? I looked at the requirements back to 1996 in the archives and it was in there as well.

At any rate, it would seem that most reflectors on the market today do not meet the SSS requirement as currently written!

The Davis Echomaster meets the requirements of the SSS TransPac NOR by being an octahedral shape of 12" mininum diameter; the rules do not require an RCS of 10 square meters for this design. Note that Davis also documents their Echomaster as having an RCS of 13.2 square meters, which also meets the rules even if we don't believe the manufacturer.
http://www.davisnet.com/product_documents/marine/manuals/152_EchoRdrRfr_INS.PDF

The largest Mobri radar reflectors (S4, M4) have a documents RCS of 4 square meters - you would need three of these to meet the TransPac requirements.

So all is not lost.

The Sea-Me that Adrian distributes is a powered active radar responder (the manufacturer calls it a 'Target Enhancer') - when the unit is painted by an X band radar it amplifies and re-transmits that radar signal.
http://www.sea-me.co.uk/

- rob/beetle

 

[Alchera]

The Davis Echomaster meets the requirements of the SSS TransPac NOR by being an octahedral shape of 12" mininum diameter; the rules do not require an RCS of 10 square meters for this design.

What I don't understand is where that arbitrary number of 10 square meters came from, since we are clearly OK with a design that doesn't meet that requirement. Why not pick a number that more closely reflects the performance of the Davis if that is what is acceptable?

- Mark

 

[sleddog]

What I don't understand is where that arbitrary number of 10 square meters came from, since we are clearly OK with a design that doesn't meet that requirement. Why not pick a number that more closely reflects the performance of the Davis if that is what is acceptable?

- Mark

The "arbitrary number" comes from a long time ISAF Safety at Sea requirement for passive radar reflectors, now ISAF #4.10.1 that requires an RCS not less than 10 sq.meters and minimum digonal of 18", flown at least 13 feet off the water.

Curiously, US SAILING has modified and reduced this requirement, and refers to "minimum documented" 'equivalent echoing area' of 6 sq.meters and minimum diameter of 12". No height off the water is apparently mentioned by US SAILING in their ambiguous modification...

Somewhere along the line, SHTP rules writers chose to combine ISAF with US SAILING, leading us all to a certain level of misplaced confidence, that our little reflectors help us to be seen by big ship's radar, if we don't see them first.

The Pardey's were recently in town, and very confident in their all wooden boat's radar signature aboard TALIESIN. Larry had filled their 40 foot hollow spruce mast with aluminum foil. On subsequent testing here locally, they were disappointed to learn they could only be seen on radar to 2 miles, not as far as a local bell buoy and about the same as an Olson 30 signature.

~sleddog

 

[haulback]

(quote)
The largest Mobri radar reflectors (S4, M4) have a documents RCS of 4 square meters - you would need three of these to meet the TransPac requirements.

Woundn't this mean you just have 3 that cannot be (theoreticaly) seen, or at least not as well??? To my way of thinking, at least in this case, 4+4+4 would NOT equal 12 - they are just 3 smaller, individual targets.

Is there any other reflector - apart from the Davis - that meets the rules??? if not, why not just tell everyone not to bother to show up unless they have a Davis Echomaster????

Jim

 

[tiger beetle]

(quote)
Woundn't this mean you just have 3 that cannot be (theoreticaly) seen, or at least not as well??? To my way of thinking, at least in this case, 4+4+4 would NOT equal 12 - they are just 3 smaller, individual targets.

Good point - I don't know if radar targets are additive.

- beetle

 

[dogbark]

I think Jim is right that reflectors are not cumulative for the same reason that five handheld VHFs broadcasting at five watts doesn't give as much range as one 25 watt radio. But it is interesting reading the differences between the ISAF standard and the USSailing standard and the SHTP rule regarding reflectors. See http://www.ussailing.org/safety/ISAF/cat4_0607mono.asp
ISAF uses an 18" min w 10sqm, USSailing seems to allow 12" min and 6 sqm and SHTP a little of both. Even though the Davis site says the Echomaster has 12 sqm, the performance diagrams they have provided(courtesy of Admiralty surface weapons test) don't even show peaks of 10 sqm.
Given the current availability of passive reflectors I think Jim is correct, one needs to show up with an Echomaster or have some darn good documentation. Al

 

[Alchera]

It appears this rule would be a good candidate for modification at some point, since the way it is written right now just forces skippers to buy the Davis Echomaster. The RCS requirement should be reduced to 6 sq m to bring it in line with the US Sailing requirements, and open up the race to other reflectors that work just as well as the Davis.

- Mark

 

[tiger beetle]

It appears this rule would be a good candidate for modification at some point, since the way it is written right now just forces skippers to buy the Davis Echomaster. The RCS requirement should be reduced to 6 sq m to bring it in line with the US Sailing requirements, and open up the race to other reflectors that work just as well as the Davis.

- Mark

That would work.

Alternatively, drop the octahedral dimension and utilize the 10m2 requirement. I have not read through the radar studies to see how well the echmax reflectors worked, nor do I particularly want to mount something this large on the boat:
http://www.echomax.co.uk/Echomax_Products.htm

Would the TransPac allow an actively powered radar transponder/responder such as the Sea-Me in lieu of a passive radar reflector?

- rob/beetle

 

[AlanH]

There is a commercially-sold passive radar reflector that is up there at 10m2. It's called th Tri-Lens, Luneberg lens reflector. The smaller model is nowhere near 10m2, but the larger model, which weighs 13 pounds, is.

Before looking at it, though, you might read this:

http://www.tri-lens.com/practical_sailor.txt

...here's an advertisement for it at Defender... cha-chinnngggg.. $500

http://www.defender.com/product.jsp?path=-1|118|107602|275622&id=303311

West Marine sells the little model, which is about 2.5 m2, for $150



http://www.westmarine.com/webapp/wc...=true&storeNum=5003&subdeptNum=3&classNum=109

By all accounts the Mobri sucks, has all sorts of valleys in its response and only works even halfway well at very narrow angles of heel. Still and all, they're easy to get into the rigging, they don't have a lot of windage, and I've found an ebay seller that makes a knockoff for $15 instead of $70. That's cheap enough that you can put 3-4 of them around the boat, at assorted points, slung at different angles and hope that at least one of them will be "right" at any given time.

That's been my hope and prayer...three Mobri knockoffs in the rigging and an Echomaster on a stern mount about 10 feet off the water.

 

[Alchera]

Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

- Mark

 

[seabird51]

Mobri or mast??

The Davis chaffed through its halyard in 2002. I replaced it with a Mobri but since the advent of AIS I have asked every ship about my radar signature and have always been told that it was excellent with and without the Mobri. something tells me that most of the credit goes to the big wide and tall aluminum mast. Am I correct in this assumption? Or flirting with death.
Lou

 

[dogbark]

Lou, I think you are definitely flirting with death, but so are we all, regardless of radar reflectors. Back to the real question though. I believe that an elliptical aluminum mast has a radar signature that is pretty good, particularly in larger sizes when in a certain orientation to the radar signal. However in other orientations it is near nothing. Look at the radar returns of the various reflectors that are designed to be optimal. There are still areas where the returns are very weak. The Pardey's with the wrinkled aluminum foil is another example. I am sure that in some instances they have a great reflector but in others they don't. In all the independent testing I am aware of only a very few commercially available reflectors seem to pass muster. Some of these are heavy and expensive or don't work well at more than a little heel. I will carry what the committee deems acceptable but I will not depend on it to guarantee I will be seen by another radar (whether there is an observer at the screen or not).

Mark is on the right track though about the cumulative effect though I think a better analogy is a small light bulb. A thousand of them would be seen better than one but still not a far or distinct as a focused fresnel type lens makes one. Al

 

[dogbark]

Another study done in England, http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf
Very interesting conclusions and recommendations at the end. Al

 

[Ergo]

Radar reflectors

There's an interesting article in Cruising World this month about the boat that was sunk by a sperm whale at the same time some of us were returning in'06. The part that caught my attention was the writer's contention that the boat's kevlar main was a great radar reflector. Makes sense but I hadn't heard this claim before. If true, it would seem to satisfy the race requirement.

Bill Merrick

 

[Alchera]

I find it pretty difficult to believe that kevlar can reflect radar. Anyone know of any other references to support this?

The other consideration is that even if kevlar *were* reflective, that still doesn't make it an effective safety device. Radar reflectors are designed to reflect a signal directly back at the transmitter. A flat sheet of steel can be invisible to radar if you angle it such that the radar signal reflects off in a direction away from the transmitter. That's why stealth aircraft have all those flat surfaces. So even if a sail were reflective, the odds of it reflecting a signal back to the radar transmitter would be slim.

-Mark

 

[Paul Elliott]

As I recall, the sails were carbon fiber. Even so, carbon fiber shouldn't be a particularly good reflector, since I believe that it is quite lossy. Some forms of carbon are used in dissipative elements for radio-frequency anechoic rooms, although I admit that I don't know if CF has the same properties.

Even if the sails were reflective at radar frequencies, the orientation of the sails would have to be correct to send back a signal. I wouldn't trust that this would be the case.

So, how to explain the report of a strong radar return? I don't know, but it sounds iffy to me.

- Paul
- S/V VALIS

 

[Chesapeake]

Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

- Mark

I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.

 

[Alchera]

I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.

While this may be true in a carefully controlled laboratory environment using perfectly coherent radar pulses and perfectly designed and arranged radar reflectors, I don't think this effect is significant in the real world. Radar pulses are not perfectly coherent frequencies, and reflections distort and modify the waveform and polarization, even from a radar reflector. If the destructive interference effect were a serious factor, then it would be the same problem with large targets even without radar reflectors, since you have many surfaces all reflecting back to the transmitter in that case as well. But we all know that large radar targets in general have larger returns than small radar targets.

- Mark