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 ]
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 ]