Radar-reflective SMB or Safety Sausage

[reefduffer]

Introduction

This idea grew out of a thread on scubaboard started 12/1/07
http://www.scubaboard.com/forums/ge...ignalling-equipment-searchers-point-view.html

In a follow-on post #9 the OP (Navy OnStar) mentioned radar reflection, suggesting a piece of a space blanket. The idea of an inflatable corner reflector was first mentioned by member bleeb, suggesting, correctly, that they exist, in post #19. The first suggestion of building a radar corner reflector _into_ an SMB was member weaponeer in post #43.

In this discussion, SMB (Surface Marker Buoy), and signal tube/sausage are used interchangeably. Scuba divers appreciate they are not quite the same things, but they are both inflatable fabric cylinders of similar size used for surface visibility. The problem of making them radar reflective is the same. The market can sort out which one or both it makes sense to add it to.

This is intended as an exploration of the feasibility and effectiveness of doing that. The hope is that someone might pick it up and make a product out of it. I'd probably buy one if the cost was reasonable. I'm not really interested in this in an entrepreneurial sense, but I might participate to some degree if it meant it might get done. Anyone who wants to make a product out of this owes me nothing. (Well, a couple of samples would be appreciated.) If there's anything original here, it's now in the public domain.

Caveat: I'm not a radar, antenna, or maritime safety professional; just a recreational diver and interested amateur in these subjects attempting to distill what I've read and apply it to the current discussion. I am a retired design engineer with 30+ years experience, but in a different discipline. Some of the statements in this discussion could be wrong. Do your own due diligence.

I mean this to start a discussion, not end one. Anyone with views on technical, intellectual property, manufacturing, business issues, market feasibility, or anything else relevant, is invited to express them.


References:

1995 Radar Reflector Test
This is a survey of available small boat radar reflectors in 1995, and has a great deal of tutorial information about maritime radar, radar reflectors, and radar visibility requirements, as well as practical observations of real-world implementations. This might be the best place to start.

EchoMax Radar Reflectors
The website of this manufacturer of radar reflectors marketed to the maritime and boating industry, including inflatables, has quite a bit of tutorial information. See in particular the various items under the tab "Echomax in Action!", the "Forward", and the description of the 230I.

Avmar Limited :: Suppliers of Marine Safety Equipment - Inflatable Radar Reflector
Another inflatable, the Avmar AV-1. Packed is 24 x 22 x 5 cm = 9.4 x 8.7 x 2 inches, weighs 1.1 lb.. Looks very similar to the Echomax A03I, a slightly larger unit than the 230I. Might actually be the same, OEMed.

U.S. Patent 4,352,106, Sept 1982
( Use "Quick Search" at Patent Full-Text and Full-Page Image Databases )
"Radar reflectors". John Firth.
This seems to be the basis for the current Echomax products. However, it's possible that the inflatable 230I is covered by other patents. If so, I haven't turned them up in the US, at least.

The Rescue Wing: Design of a Marine Distress Signaling Device.
Tomas Melin and Sandi Sefi
Royal Institute of Technology, Stockholm Sweden 2005
Proceedings of IEEE OCEAN2005, Washington DC, USA, September 2005.
PDF downloaded from OutBox page for Sandy Sefi
Sort of a combination balloon, kite, visual signal, and radar reflector. Sounds like a great concept, and very applicable to scuba diver rescue, but looks to be so far just just an academic exercise, no product. But the discussion has some more useful discussion of requirements, and some useful references.

There are other patents and references to kite-based designs I've found for getting a small radar reflector up high, but that's outside the current discussion. Interesting idea, though.

U.S. Patent 4,901,081, Feb 1990
"Elliptical inflatable radar reflector", Bain, John and Gentry, Gene.
Assignee: Lifeball International Corporation
Lifeball International Corporation
Seems to be a product, but available only through the Federal Stock System. Used by USN?
The illustration suggests it might be about 12" diameter x 18" long.
See also U.S. Patent 4,673,934, June 1987 "inflatable radar reflector", same inventors.

U.S. Patent 6,300,893, March 2000
"Emergency passive radar locating device", Schaff, James and Ball, Steven.
Assignee = US Dept of the Navy.
References the Bain patent (Lifeball) as prior art. Claims to be simpler and cheaper. Looks to be one octahedral corner reflector array supported by three inflated intersecting toruses.

There seem to be numerous patents on inflatable radar reflectors, most now expired (i.e., in the public domain). Note that US patent life is 20 years from application. Looking at the prior art citations in the few listed above will find many of them.

Radar, Reflectors and Sea Kayaks: A Visibility Study
University of Maine, 2004.
PDF downloaded from Maine Sea Grant - Radar Reflectors & Sea Kayak Visibility
About kayaks being seen by fishermen, which is not exactly our problem. Has some interesting stuff though. Note in particular the conclusions about the effectiveness of getting any reflector at least a few feet above the water, and the lack of effectiveness of things like lining a vest with a space blanket.

Corner Reflectors - Microwave Encyclopedia - Microwaves101.com
Basic tutorial on radar corner reflectors.

Aerospaceweb.org | Ask Us - Radar Cross Section
Nice basic tutorial on Radar Cross Section.

www.earth2.net/parts/mugu/rcs.pdf
Another tutorial on RCS, a little more advanced and detailed. Some discussion of what happens when reflector length is near the wavelength.

The 3-D Corner Reflector and Corner Reflectors Revisited
Discussion of corner reflectors as radio antennas. Useful for discussion of minimum lengths, variously in the order of 1.5 - 2.0 wavelengths. It isn't absolutely obvious that these are minimums as a radar reflector, but it adds some background to the discussion.


Preliminary Discussion

There seem to be two radar frequencies of interest. X-Band (wavelength = 3.2 cm = 1.3 inches) and S-band (wavelength = 10 cm = 3.9 inches). X-band is more universal, particularly for smaller boats, and has higher resolution, but shorter range. S-band is additionally used by larger ships, and has greater range and interference immunity. If the Coast Guard, Police, or other agencies that would be of interest to searching for a stranded diver use other radar frequencies, I haven't found reference to those yet. Most of the references seem to emphasize visibility in X-band.

Corner reflectors to improve radar visibility go back to the beginning of radar in WWII. There is an academic and patent literature that is enormous, and a very large number of commercial and military products incorporating sophisticated arrays of corner reflectors in complex spatial arrangements, novel construction methods, etc. But the core of all of them is the trihedral corner (3 adjacent faces of a cube, perhaps rounded or truncated), because it has the largest radar cross section (RCS) for a given physical size.

All radar reflector arrays available have some degree of variation in effective RCS as a function of their position relative to the radar, both rotationally in the horizontal plane, and as a function of heel angle (vertical axis tilt). They can be characterized by a max Radar Cross Section (RCS), but the variability is also important. The actual heel angles can be expected to vary more in a scuba SMB application than any boating application, and visibility to air search in this application makes RCS at high angles important.

An interesting standard for radar visibility for boats is ISO 8729 and SOLAS. See Echomax Radar Reflectors - Marine Regulations & Rules They recommend carrying reflectors with a minimum X-band RCS of 10 m2 (meter squared) peak carried 4 m (13 feet) above water level. There are various limits on the minimum RCS given some limits on heel angles etc. There are also allowances that smaller boats possibly can't meet these standards, and should do the best they can.

Translating the recommendations for RCS for boats trying to avoid collisions with other boats that don't know they're there, to a benchmark for what RCS would be useful for a stranded scuba diver trying to be found by searchers, is not obvious. But a common-sense reading says that about 1.0 m2 ought to be a sensible guess for the threshold for what RCS would be worth the expense of incorporating a reflector into an SMB, pending someone doing some experiments. Clearly we can never meet the boat recommendation, and the bigger the better.

The 1995 test study gives 1.0 - 3.0 m2 as the minimum RCS required to be seen by radar, with 2.5 m2 also mentioned. Less than 1.0 m2 is clearly of marginal value, 3.0 m2 would be a goal that might be worth added stowed size and weight to achieve. 3.0 m2 is also mentioned as a goal for RCS in the "Rescue Wing" paper.

As an aside, the evaluation of radar reflectors at 1995 Radar Reflector Test says that a typical duck has an RCS of about 0.1 m2, so this would translate to our SMB having at least 10 - 30 ducks worth of radar visibility. "Everyone understands that, when it comes to radar reflectors, more ducks is better.".


Is it enough to just make a signal tube radar-reflective?

For starters, suppose we took the simple approach of coating the inside of an SMB with aluminum film. The 1995 test paper provides a formula for the RCS of a cylinder (Fig 1): Radius r, length h, wavelength L, RCS = 2 * pi * r * h^2 / L. [Yes, it's the length that gets squared, not the radius, this is confirmed by other references, see www.earth2.net/parts/mugu/rcs.pdf for one.] For a tube of length 6 feet = 183 cm, for X-band radar at L = 3.2 cm, we have RCS (in m2) = 6.575 * r (in cm).
Radius RCS, h=72"
2" = 5.1 cm 33.5
3" = 7.6 cm 50
4" = 10.2 cm 67

Whoa! Is this going to be easier than we think? And why all the complicated corner reflectors in commercial products? Cylinders work great!

The rub is two-fold. The great RCS is because it's proportional to length squared, and we have 6 feet. But a lot of that will be hidden by ocean waves in our application. Suppose we actually get half of it high enough to be visible to a search radar. Now we've cut it by a factor of 4. If only the top two feet are visible, by a factor of 9, so our 4" tube now has an RCS of 7.44. Still, that's way more than our target RCS, and all we've done is coat a 4" tube on the inside with aluminum film!

The killer is angular response, in the sense that would be important to air search of a tube being held vertically, or to a lesser degree to the heel attitude of the tube with respect to radar on a boat. Per the same reference Figure 1, the RCS is reduced by half at an angle (in degrees) of +/- 13 * L / h. For our 6-foot tube, this is 0.22 degrees, and it's still less than a degree at 18 inches length. It will fall off more rapidly at larger angles, although a formula isn't available to me. In other words, it acts a lot like a mirror or flat plate with respect to cylinder axis tilt; point it perfectly perpendicular to the radar and it's extremely effective; point it just a little off and it's almost useless.

For another take on this, confirming, see the chart "Radar cross section of a cylinder" about 3/4 down the page at
Aerospaceweb.org | Ask Us - Radar Cross Section

So, maybe all the effort going into designing, patenting, and building these sophisticated corner reflector arrays isn't that foolish...

[ Continued in next post due to size limitations ]

 

[reefduffer]

Reflector Array in a Signal Tube.

In terms of what's actually practical for an inflatable reflector array, it makes sense to start with an existing product. The Echomax 230I Inflatable is chosen because it exists and is readily commercially available at a price that is at least in the ballpark for our application ($160), and has published dimensions and RCS. It also is designed to be mounted with the cylinder axis vertical, and claims better-than-competitor performance at high heel angles, both of which are important in our application. The intent in this discussion isn't to design a radar reflector from first principles, but to investigate feasibility of adding one to an SMB / safety tube.

So I'm making the following assumptions:
1) The 230I dimensions and RCS claimed by the manufacturer are correct.
2) Either:
A) EchoMax would be willing to adapt their design to an SMB product, or
license the design to someone who wants to.
B) Someone could independently build an inflatable design with equivalent
dimensions and RCS.

There are a couple of values given for the 230I. The manufacturer's website says it has an X-band RCS of 20+ m2, and is 30 cm (11.8 in) diameter x 75 cm (30 in) long. It weighs just under a pound. Some of the sites offering it for sale list a "peak" RCS of 28, but a 13" diameter. The following analysis uses the most pessimistic for both, RCS of 20, and 13" diameter. Maybe that results in slightly better RCS for our SMB product than calculated here, maybe not.

13" (or 12") diameter is clearly bigger than any current SMBs or safety sausages. If we were to scale the 230I to various diameters, using the relationship that RCS is proportional to the fourth power of linear dimension, we have some possibilities of interest. In the following, width is the uninflated measurement, computed as half the circumference plus two 5/16" seam flanges, which is the way my current safety sausage (XS-Scuba AC190) is constructed. Diameter, length, and width in inches, RCS in m2:

Scale Dia Width Length RCS
1.0000 13.0 21.05 30.00 20.0 EchoMax 230I
0.4615 6.00 10.05 13.85 0.91
0.4730 6.15 10.28 14.19 1.00
0.5000 6.50 10.84 15.00 1.25
0.5620 7.31 12.10 16.86 2.00
0.6150 8.00 13.18 18.45 2.86
0.6225 8.09 13.34 18.68 3.00
0.6920 9.00 14.76 20.76 4.59
0.7500 9.75 15.94 22.50 6.33

A prospective manufacturer can choose anywhere on this curve for a safety tube product, but it seems the 3.0 m2 RCS target can be met with an inflated diameter of just over 8". If this were stowed folded once before rolling, it would be less than 7" wide. It would make for a larger diameter roll than most SMBs and safety tubes, but probably acceptable.


Additional thoughts on construction:

There is room in a 6 foot tube to repeat the reflective array; the nominal 3.0 RCS scale results in an array length of less than 19 inches. The RCS could be multiplied by replicating the array along the tube two or three times. For example, we could get the same nominal 3.0 RCS by replicating the array in a 6.15 inch diameter tube three times for a length of about 42". My opinion is that this is the wrong tradeoff. In this application, wave clutter is a real problem, getting all of the array as high as possible is important. See the kayak study. Also, I think the end result is just as bulky in terms of total amount of fabric that has to be stowed, maybe more. And it almost certainly is more expensive to manufacture, since there are more internal reflector pieces to be sewn/glued in place.

A better tradeoff might be to construct it with a smaller diameter non-reflecting "mast". Half the folded width makes for a sensible design. Using our nominal 3.0 RCS scaling, we have a 8.1" diameter array 18.5" long, atop a 3.85" diameter inflated mast 53.5" high. Uninflated, the array folds in to the center to the same 6.7" width as the mast, and then rolls up. Now we're approaching the stowed dimensions of current SMBs / safety tubes. More complicated to manufacture than a simple tube, but with considerable savings in size and weight, and no loss in performance. I imagine this as still just one inflated balloon.

Another reason to constrain the radar reflector array to the top of the tube has to do with visual detection. Many higher-end SMBs / safety tubes add some SOLAS reflective tape for improved visibility to searchers at night. Oops! You can't do that over the radar reflector array, it will ruin the radar reflective performance! [This isn't in the SOLAS tape data sheet, but 3M engineering verified that there is an aluminum layer in the construction. They haven't looked at radar reflective properties and wouldn't comment, but it's hard to imagine it's not radar opaque. As an aside, I wonder how many boat owners have added SOLAS tape or equivalent to their expensive radar reflectors?].

You can, however, put some SOLAS tape on the mast below the array. 53" high for that is better than nothing.


A couple of other design issues worth mentioning:

The literature mentions that the desirable properties of a corner reflector assume a side length of at least "several" wavelengths. The 1995 test study says "as the smallest dimension of a reflector gets down to a few wavelengths of the radar signal, it quits acting as a reflector and starts to act as a lump of metal. Remember that a wavelength is 3.2 cm (1.25") for X-band, and 10 cm (4") for S-band. So small detectors [sic] must be looked at with a great deal of suspicion, as there really is no substitute for size." The literature on antennas pushes this down to 1-2 wavelengths, but I'm not sure if that's applicable to untuned reflectors. www.earth2.net/parts/mugu/rcs.pdf suggests that at dimensions near 1 wavelength the gain actually oscillates around the properties calculated assuming length >> wavelength.

Our 8.1 inch diameter strawman probably still is at least 3 - 4 X-band wavelengths depending on internal orientation of the corners. There's probably good enough confidence in that to build it and try it. I note that several commercial reflectors are this size or even smaller; the Echomax 180 is 7.8 inches diameter (peak RCS is 8 m2). But it's another reason not to try to push it much smaller without getting some expert advice on this issue.

The literature repeatedly emphasizes the importance of having the angles of corner reflectors be accurately 90 degrees, and that reflector performance seriously degrades if it's not. Many of the patents on inflatable reflectors in particular emphasize how they are eliminating warps and distortions in the reflector surfaces. In the absence of existing proofs-by-existence, e.g., the EchoMax 230I, I'd be concerned whether any theoretical inflatable design would be effective in practice. Note that the 230I is specified with lower RCS/size ratios than EchoMax's rigid reflectors. Assuming that the 230I does actually perform as claimed (and they claim certification through independent tests by QuinetiQ, a very serious engineering firm, see QinetiQ science and technology that makes a difference ), this is either easier than it sounds, or EchoMax has some design technology worth leveraging. My guess is the latter.

Weight: Assume that it's made of the same material as the 230I, our scaled design's weight would be proportional to area, which is to say the square of the scale factor. For our strawman scale of 0.6225, this would be 0.387. The 230I weighs 15 oz, although it's not clear whether that includes a pouch. Assuming not, and assuming the same weight/sq inch for the envelope material, the scaled reflector array weighs 5.8 oz.


Amateur Thoughts on the Business Issues

I think it's technically feasible to adapt the Echomax 230I technology to an SMB/signal sausage such that it will be of a stowed size acceptable to divers, and have an RCS large enough to be worth the trouble. I think the sales volumes are probably such that it can be sold into the scuba gear market for no more than the 230I is sold into the boating market, $160. Some larger SMBs sell for around $100 now, and radar reflection is certainly a differentiator. That's based on relatively uninformed guesses about 230I volumes and the market for scuba safety gear at or near this price point. That's where I'm happy to turn things over to equipment makers who know their markets.

Given the relative lack of competition for inflatable detectors, and the 230I performance, I think the first effort should attempt to involve Echomax as a design partner or even OEM. Their web page says they accept custom designs. I have not contacted them. Since they have no sales channels in the scuba gear business, partnering with someone who does probably makes more sense for them as well.

If Echomax is not interested, I note that the Echomax 1982 patent is expired, and presumably a 230I can be reverse-engineered. This assumes it's not covered by other patents. I didn't find any, but do your own due diligence. Alternatively, partner with someone else (Lifeball?) or roll your own reflector design. All of these of course has more risk and upfront cost, but cuts out partnering costs.

I hope someone does this and makes it work.


Appendix: A non-exhaustive list of some current SMBs / safety tubes:

The DAN Surface signal kit
https://www.diversalertnetwork.org/catalog/products/641-1000.html doesn't spec diameter but folded is 7.5" wide x 72".
Allowing for seam flanges, sounds like 13.75" circumference and 4.38" diameter.

XS-Scuba Safety Tube AC190
XS Scuba AC190 Safety Tube
6 feet long, 2.75" diameter. Rolls up 5" x 3".

XS Scuba SMB
XS Scuba Safety Marker Buoy - Safety Market Buoy - SMB
Also available from Leisurepro.
Discontinued product? Not on XS-Scuba site.
7 feet long x 22" circumference = 7" diameter
Rolls to 9" x 3.5 inches, weighs 15 oz.

Surface Marker WDS
Home*-*Scuba Diving Equipment - Diving Safety
145 cm x 15.5 cm (57 inch x 6.1 inch). It isn't clear if the 6.1 inch is diameter or flat width.

Dive Rite SMB
Dive Rite Lift/Marking/Safety Bags...Attention-Getting, Inflatable...Easy Retrievals
60" x 8" wide. Probably about 4.7" diameter

Carter Personal Floats
Personal Floats Safety Sausage Tubes
Five sizes from 3.5" to 5" diameter, 72" to 120" (10 feet!) long.

Zeagle Signal Tube
Safety & Signal Devices Zeagle Systems - Scuba Diving Equipment Manufacturer
60 long, 6" and 7" wide. Whether that's diameter or flat width is unspecified.

Oceanic PSD
Oceanic Worldwide - Scuba Diving Safety Gear - Personal Signaling Device (P.S.D.)
6 feet long. Diameter unspecified.

OMS SMBs
OMS Lift Bags & Surface Marker Buoys http://www.OMSdive.com
62" and 115". Diameters unspecified.

 

[Divin' Dave]

RD, I agree that the Echomax 230I, re-engineered for diving, would have a place in the market. Seems like DAN would be interested. Have you contacted them directly? And thanks for all the work, very informative.

 

[TequilaBob]

I faced the same problem with my ocean going sailboat a number of years ago. I needed something that would really create a visible "blip" on radar. A good friend, John McCready, who owns and skippers the live aboard, Eclipse, in Palau came up with the answer. We stuffed the hollow mast with tinfoil. It took the the radar waves and scattered them in every direction creating a huge "blip." Without it, a fiberglass sailboat is almost invisible. What can be done for a diver with crumpled tinfoil? I'm sure something quite easy.

Just my dos centavos.

 

[DeepSeaExplorer]

I faced the same problem with my ocean going sailboat a number of years ago. I needed something that would really create a visible "blip" on radar. A good friend, John McCready, who owns and skippers the live aboard, Eclipse, in Palau came up with the answer. We stuffed the hollow mast with tinfoil. It took the the radar waves and scattered them in every direction creating a huge "blip." Without it, a fiberglass sailboat is almost invisible. What can be done for a diver with crumpled tinfoil? I'm sure something quite easy.

Just my dos centavos.

I have some indirect experience. Years ago, I helped make an underwater reflector for for use with sidescan sonar. The purpose was to create a marker buoy that would show up on sonar so you could compare what the sonar was seeing in relation to the buoy's position.

I started with a standard radar reflector shape, fairly large, which was surprisingly almost invisible to sonar. I gave up when the chain on the buoy was more visible on sonar than the reflector was.

I think there are a few insurmountable issues. You can't carry a buoy large enough to reflect enough energy. And, on a normal basis most radars will not be tuned accurately enough to distinguish the buoy from sea conditions. It would need altitude which a diver on the surface just doesn't have.

You would probably be better served by carrying a flare and some dye.

 

[reefduffer]

RD, I agree that the Echomax 230I, re-engineered for diving, would have a place in the market. Seems like DAN would be interested. Have you contacted them directly? ...

I figured I'd just let the analysis sit in front of the community awhile and see if
someone would add something on the technical side, and or if some gear manufacturer
expressed interest, or might enlighten us all with an informed
view of this with respect to the marketplace. There are a couple of dozen
gear companies with presence on this board, as is DAN.

I faced the same problem with my ocean going sailboat .... We stuffed the hollow mast with tinfoil. It took the the radar waves and scattered them in every direction creating a huge "blip." Without it, a fiberglass sailboat is almost invisible. What can be done for a diver with crumpled tinfoil? I'm sure something quite easy.

I do believe that with a volume as big and high as that, crumpled foil, essentially
a random collection of angled surfaces, would be quite effective. But I think a
carefully engineered corner reflective array will have a larger RCS per volume
than a random array of crumpled foil. And for the diver, stowing the array in
a small volume and then deploying it under stress without tools is required,
or it won't get carried, making its RCS irrelevant. I'm sure crumpled foil works
to a degree, and the kayak study mentioned it.

I just don't see much of an application for foil to this problem. Having said that,
I am thinking of adding half or a third of a space blanket to my sausage kit,
following up on some comments in the original thread, and what I learned in my reading.
Between sunshade & windscreen, it might be welcome on an unplanned wet SI
even if not radar reflective, and if tied to the top grommet and crumpled a bit,
I'd certainly have a bigger RCS than without it. I still expect it's well below
the threshold 1.0 m2 referenced in the literature I cited.

[...]
I think there are a few insurmountable issues. You can't carry a buoy large enough to reflect enough energy. And, on a normal basis most radars will not be tuned accurately enough to distinguish the buoy from sea conditions. It would need altitude which a diver on the surface just doesn't have.

You would probably be better served by carrying a flare and some dye.

You may be correct, but that's not the conclusion I came to, and which I described
in posts #1 and #2. There are inflatable reflector arrays being purchased by, and
even patented by, the USN, for individual man-overboard types of applications.
The literature seems to say that an RCS of 1.0 - 3.0 m2 is visible to X-band radar.
The kayak study demonstrated visibility in cases that were probably at the lower
edge of that, and also severely constrained wrt height above surface.
I described a design based on scaling an existing characterized product that
would put an RCS of 3.0 m2 at 3 to 4 feet above the surface.

Would that design actually work to increase the likelihood of rescue?
I think so. I don't know so. You seem to know it won't.

I'm sure flares and dye work, but flares are a problem for air travel, and both
are of limited duration in use. Flares last for minutes. Dye for an hour or two.
I already carry a safety sausage. If it can be made significantly radar reflective
without getting a whole lot larger or expensive, I think it's a very attractive
option. However radar reflective it is, it will stay that way until it no longer matters.

Nothing says you can't have flares and dye too.

 

[muddiver]

Dude, they make work vests and life vests with radar reflecting fabric or panels. Just sew or adhear one of those to an SMB/safety sosage.

Umh, DeepSeaExplorer, sonar is a totally different type of wave than radar. Sonar will pass through a thin object like a radar reflector where radar will bounce off of a piece of sheet metal. You would have been better off with a thick metal disc supported by a bouy as a sonar reflector.

 

[reefduffer]

Dude, they make work vests and life vests with radar reflecting fabric or panels. Just sew or adhear one of those to an SMB/safety sosage.

I don't think it's that simple. Yes, any metal or metalized sheet, like foil,
a space blanket, or SOLAS reflective tape, is radar reflective. In the original
thread, back in post #49 http://www.scubaboard.com/forums/3254198-post49.html
I myself proposed this for the sausage, and then again in post #55
http://www.scubaboard.com/forums/3257021-post55.html I proposed the
same thing for BCs.

That's what started me on this investigation, and what my reading convinced me
of is that this doesn't work well enough to be anywhere near useful. The tape is
radar reflective, but believing that it is sufficient to allow you to be seen by a
search radar is like believing one of those 99 cent keychain lights is all you
need to be seen by a search boat in the dark. There will just not be enough
energy reflected back to the radar to be distinguished from noise, if even that
much. Size matters. Radar Cross Section, that is. And response over a wide
angular range to radars whose position you don't know is important as well.

The people who stick a piece of tape on an SMB or vest and advertise it as
radar reflective are selling snake oil, IMO. Think of it as an un-aimed and
un-aimable signal mirror. What's the chance of it catching the sun and reflecting
in the searcher's eyes?

Either that, or meaningful radar reflectivity is easier to achieve than all these
experts are letting on, and it's the boating and commercial shipping communities,
and the military, wasting money on reflector arrays, and setting standards for
RCS, who are buying the snake oil. Not what I'd bet on.

Please read the analysis. Among other things, I think I demonstrated specifically
that making the whole SMB out of foil doesn't accomplish anything useful.
Also, I suggest you read the kayak study from the University of Maine with
respect to foil vests.

 

[Charlie99]

Please read the analysis. Among other things, I think I demonstrated specifically
that making the whole SMB out of foil doesn't accomplish anything useful.
Also, I suggest you read the kayak study from the University of Maine with
respect to foil vests.

The thing that caught my attention in the kayak study was that the "tinfoil hat" was the best reflector, beating out the commercial corner cubes. All those conspiracy nuts were right after all....... now where did I put my tinfoil hat ...?

If the analysis you refer to is the one on cylindrical objects, I'm pretty sure that the equations were for a cylinder that is a perfect surface and has a specular (mirror-like) reflection. A crumpled foil or otherwise textured surface would give a diffuse reflection and would actually work better than a perfect surface.

You've done lots of research, although I haven't had a chance yet to absorb it. THANKS !!

In another post you commented on the problem of making perfect corners to avoid having a tightly focussed retroreflective beam that misses the originating radar. Spreading about the beam a bit by slightly degrading the surface will make geometric shape accuracy less of a problem, at the expense of having a bit lower maximum RCS.

As for tall masts, I'd prefer to avoid them, since the conditions during a surface search will often be stormy and it would be difficult for a diver to hold a large reflector aloft in a strong wind. Radar returns are distinguished from sea clutter not by having the target held high, but by being more consistent than the occasional sparkle or glitter sort of random specular return you get from the ocean. Of course, the disadvantage of having the reflector right at sea level is that it will not be radar visible when it is in the trough. Although reflector height would increase the horizon distance a bit, the maximum detection distance is going to be dominated by other factors such as signal strength of the return and the horizon distance of the radar antenna.

I don't think somebody can come up with anything significantly better than the 230I unless they use a different approach, such as a reflector tuned to the 9.4GHz/3.2cm/X band radar; or uses a different geometry that trades off size for the requirement that the user orient the reflector towards the search vessel.

Charlie Allen

p.s. At some point, the weight and size of the radar reflector makes carrying an EPIRB, ELT, or VHF radio more attractive. This is particularly true when you take into account that virtually all diveboats have VHF radio, while only a minority have radar.

A SART would make an extremely visible radar return, but they are way down on the priority list of what I'd take.

 

[jimbo2006]

Funny to have stumbled on this discussion after thinking of similar issues (less deeply) a few months ago. My concept centered around corner reflectors glued inside a balloon inflated from a small cartridge of helium (akin to a CO2 cartridge). I know early BCDs used cartridges so I'm sure the pressure isn't an issue (though corrosion could be a concern over time). In a similar vein I stumbled on a related concept that used inflated 'stays' to deploy the reflector array:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700030747_1970030747.pdf
However I think a balloon that was transparent to radar but contained the array internally might work better in any sort of wind. No doubt testing various designs would be helpful.

Any of these ideas seem better than the 4' signal tube I've got presently. Let me know when and where I can buy one ;-)