Radar reflector testreports
Radar reflectors report.pdf (2.13 MB)
Great info. I just had my radar reflector snap off and am in the market for a new one. Thanks!
– Geoff
Here’s a text version of the Practical Sailor report with the line wraps (kinda) removed.
Practical Sailor, August 15, 2001
Tri-Lens Radar Reflector
In the September 1995 issue, we reported results of radar reflector testing done at the Stanford Research Institute in Palo Alto, California. This was made possible thanks to Dick Honey, then a semi-retired senior principal scientist at SRI, uncle to former SRI research engineer, well-known navigator and sailor Stan Honey (also the guy who puts the yellow first-down band across the screen of your TV during pro football games). Stan also assisted, as did West Marine’s Chuck Hawley and a third member of this Bay area brain trust, Jim Corenman.
Two Types
Marine radar is of two types: X-band and S-band. The former is the type commonly carried aboard small boats. It operates at a frequency of about 9.4 GHz (9400 MHz) and has a wavelength of 3.2 centimeters (cm). S-band radar is used aboard large ships and operates at 3 GHz with a wavelength of 10 cm. Ship operators generally use their X-band radar near shore because it has better resolution and can pick up smaller targets better than S-band, which is commonly preferred offshore due to its longer range and superior resistance to sea clutter.
Terms
Radar reflectors are described in terms of their Radar Cross Section (RCS) and their performance is measured in decibels (dB) relative to some reference, usually a 1 m² sphere. In our 1995 test, a recommended minimum average RCS of 2.5 m² was adopted at the suggestion of GEC Marconi, maker of the Firdell Blipper reflector. Radar reflector performance is a direct function of size, proportional to the fourth power of its linear size.
Doubling a reflector’s size increases its effective area 16 times. In the 1995 report, we wrote: “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 (1-1/4”) for X-band, and 10 cm (4") for S-band. So, small reflectors must be looked at with a great deal of suspicion, as there really is no substitute for size."
Besides strength of return, consistency also is critical. Many radar units are not continually monitored but set to automatically sound an alarm if a target registers three consecutive “hits.” One big hit won’t alert the ship’s crew that a smaller vessel is in the vicinity.
Reflectors tested in the 1995 report included the octahedral Davis Echomaster and Emergency; the trihedral and dihedral-based Holland Yacht Equipment, Firdell Blipper, Mobri, High Gain Rotation and Cyclops; and the Luneberg lens-type Lensref. Oh, and we also tested the Radar Flag, which was practically invisible.
1995 Results
The top-performing reflector six years ago was the Davis Echomaster in the vertex up position with 0° heel, which registered 63% of returns above 2.5 m². In the double-catch rain position and 20° heel, just 43% of retuns were above 2.5 m². The Lensref has virtually no nulls; its polar plot is a virtual circle. Had the minimum average RCS been just 2.0 m², the Lensref would have easily won with 100% of returns greater than the threshold. At 2.5 m², however, just 30% of the returns exceeded the threshold. And this was at less than 18° heel, over which its performance fell off
dramatically. A gimbaled mount was recommended.
The Tri-Lens
Last year, we were contacted by Tim Rozendal of Rozendal Associates in Santee, California, asking us to test his new reflector. Tim said he has supplied more than 20,000 Luneberg lenses to the US Navy, Army, Air Force and Coast Guard. When he became interested in developing a product suitable for the recreational boating industry, he studied our 1995 SRI report and came up with two sizes of the Tri-Lens Radar Reflectors. Due to computer problems at SRI, we were unable to have Rozendal’s Tri-Lens tested in the same anechoic chamber as the others. Instead, we directed Rozendal to Ohio State University’s Electro Science Laboratory Compact Range. A copy of the 1995 report was supplied to OSU so that the same test protocol be followed, and a Davis Echomaster was also tested as a control.
The Tri-Lens has a novel configuration of three modified Luneberg lens reflectors oriented about a vertical axis. It was measured only at X-band. We asked Dick Honey to review the OSU report and interpret the data. In his summation to PS, he said the Tri-Lens shows “the uniform azimuthal response expected of Luneberg lenses, as well as the excellent tilt (heel) response expected from the design, i.e., each lens looks only into its 120° azimuthal sector so that the spherical reflector on the back of each lens can be quite large, with no aperture blocking as occurs with most single Lunebergs designed for 360° coverage. In other words there are very few narrow angular gaps
in the coverage between each lens.”
At 0° heel, 90% of returns exceeded the 2.5 m² threshold; at 20° heel 70% exceeded the threshold and at 25° heel 69%. The Tri-Lens easily out performed all other reflectors in the 1995 test. Honey nevertheless had criticism of the Tri-Lens: “Our reservations,” he wrote, "are not with the angular coverage, but with the magnitude of the reflection. (This same reservation applies to virtually all passive radar reflectors on the market today!)
The Rozendal reflectors use 5.25-inch diameter spheres inside a somewhat larger radome, the spheres being approximately 4.2 wavelengths across at X-band, but only 1.4 wavelengths across at S-band. This means that they may not be much better at scattering S-band radar signals back towards the radar than the average winch, and S-band may be the only radar in use on the high seas (if any).
"As mentioned in our earlier test, the radar cross-section (RCS) of similar reflectors increases as the fourth power of the size, so it would take a relatively small increase in the size of the Luneberg lens to bring the X-band RCS up to 10 square meters (m²) the minimum echoing area requirement in the latest International Standard (ISO 8729, Marine radar reflectors, sec. 5.1.1). That is, a perfect Luneberg lens 5.25 inches in diameter should have an RCS of about 2 m² at X-band, but a lens only 40% larger or 7 inches in diameter should have an RCS of 10 m². In addition, it would have a 2 wavelength diameter at S-band, where it might begin to reflect better than a winch. Unfortunately, this would add significantly to the weight of a Lens-based reflector, and weight is already a concern with these reflectors, relative to an octahedral reflector like a Davis Echomaster.
"Rozendal also offers a Mini-Lens reflector, which as expected given its small size, cannot compete with any reflector on the market except for the Mobri. “Wouldn’t it be great if a radar reflector could achieve an RCS of at least 10 m² at X-band over 360° of azimuth with trivial nulls and wide tolerance for heel? Of course, it should be less than a foot in diameter, weigh little, and be “almost free,” as they say in Mexico.”
In response to these criticisms, Rozendal admits that the Mini Tri-Lens “does not perform well suing the 2.5 m² threshold. It was,” he said, “intended for small boats and coastal sailing. It should be mounted as high as possible, preferably on top of the mast. The mini Tri-Lens does not have a large RCS but gives a consistent return. It has three narrow dead spots. The Mobri has 16 dead spots and any octahedral will have eight dead spots. A premium must not only be put on RCS but on the consistency of the return as well. Anything that will help improve a vessel’s radar visibility is worthwhile, particularly short-handed vessels and those without radar.”
The International Standard for Marine Radar Reflectors (ISO 8729) was revised in 1997 and comes two years after our 1995 test. In addition to the section 5.1.1 noted above, there are other relevant sections. 5.1.2 requires that a reflector’s azimuthal polar diagram “shall be such that its response over a total of 240° is not less than 2.5 m2². The response shall not remain below this level over any single angle of more than 10°.”
And section 5.2 states, “The performance of the reflector up to at least +/- 15¡ from the horizontal (the degree of heel) shall be such that its response at any inclination remains above .625 m² over a total angle of 240°.”
Rozendal asserts that "This specification was written around the performance spec of the largest Cyclops unit (it weighs approx. 22 Ibs.). They claim their unit is the only passive reflector that meets ISO 8729. No aluminum octahedral reflector is technically able to meet this specification.
The Tri-Lens exceeds this specification. In a future issue, we may engage the debate about how best a sailboat may avoid collision with another vessel, recreational or commercial: Besides passive reflectors, other options include electronic radar target enhancers (RTE), masthead strobes and installation of a small radar, the idea being that the best safeguard is for you to “see” the other guy, rather than hoping he sees you.
The standard Tri-Lens measures about 12" across, is 6" high and weighs about 5.5 lbs. West Marine now carries them. Price includes a Mast Mount Bracket and a coupler that can be adapted to any marine antenna mount.
As for Honey’s wish for a 10 m2 RCS/360° reflector, Rozendal said he does make a large Tri-Lens using 8" diameter lenses. It has a RCS of 10 m² for 300° and minimal degradation when in elevation. It weighs around 15 Ibs. and sells for $699.
While bigger is better when it comes to radar reflectors, there are practical limits aboard small boats. Installing a radar unit may be a skipper’s best defense against collision at sea, but not everyone wants or can afford one. And even then, hanging a relatively inexpensive passive reflector certainly doesn’t hurt. Compared to the high cost of the Lensref (about $425), the Tri-Lens is a bargain. It costs more than the Davis Echomaster, but out performs it. If you’re in the market for a passive radar reflector, we recommend the Tri-Lens.
Rozendal Associates, Inc., 9530 Pathway St., Santee, CA 92071; 619/562-5596
email: rozendal@pacbell.net, www. tri-lens.com.
Table Ranking of Reflectors Based on a Minimum Return of 2.5 m²
Reflector >2.5 m² X-band RCS
Gap
Tri-Lens @ 0° 90% 10
10°
Tri-Lens @20° 70% 4
10°
Tri-Lens @ 25° 69% 4
10°
Davis Echo Vert-Up @ 0° 63% 7 17°
Davis Double C.R. @ 0° 48% 5
31°
Davis Double C.R. @ 20° 43% 4.5 36°
Lensref @ 0° 30% 2.4
103°
I removed the PDF version of the report you worked up so nicely. Which reflector are you buying?
I must admit that I haven’t figured that out as of yet. I think that’s an expense that I’m holding off until the spring when the bills from Christmas are a thing of the past. Also, I wish that the report contained more info such as dimensions, weight and in the case of the Sea-Me, power requirements. Right now the Sea-Me is very interesting, but I don’t know about power and/or how I’d get power to it. But at least I know where to come and find the info now!
– Geoff
Power supply 12v DC
Current consumption in quiescent state 150mA
Current consumption when transmitting 350mA
Control box fuze 0.5A Quickblow 20mm
I think I read somewhere that the FCC had not yet approved them for use here.
George
I recall a conversation wither a tanker Captain, in a bar of all places, and I asked him about small craft radar visability on large ships. He felt that a AIS system was the better way to go (he also sailed 30 footers to Bermuda). The thinking was that the large ships have a 3 mile avoidance circle and their AIS will track your AIS and if your course intersects the circle they will change course/speed, if they can. A small radar return may not be noticed and may not be plotted. They may also start a fuel change over to a fuel grade that provides max power for manuvering.
AIS is an expensive soultion but does provide positive feedback to both parties and keeping out of the 3 mile circle is a lot easier for the small guy.
Anyone have any experience with the AIS on small craft?
AIS receivers are straight forward and (relatively) inexpensive. So much so that it would be foolhardy to make an ocean passage without one. AIS transmitters are expensive. We will purchase an AIS receiver which will communicate and display on our Furuno radar via a NMEA 0183 Connection. I don’t have the brand at my fingertips but as I remember it was a less than $200 solution including a dedicated antenna.
George
I think I read somewhere that the FCC had not yet approved them for use here.
Here in Europe that are more widely used. For the Faraday Ostar Transatlantic race (the former OSTAR, but now for amateurs only), you must have a SeaMe or equivalent actice responder to enter.
See also the Kannad:
kannad radar transponder.pdf (183 KB)
Anyone have any experience with the AIS on small craft?
I have a NASA (2nd generation, which is two channel) AIS engine (receiver only) connected to a laptop and Seaclear II since this season. I have a special antenna signal splitter to be able to use the VHF antenna for the AIS as well. It all works fine and is simple to setup. Haven’t used it much yet because I’m usually far away from busy shipping areas. Many sailors in the NL’s now have an AIS receiver, some have a transmitter.
About visibility of the yacht’s AIS transmitters from ships there’s a lot of discussion going on around here. The small craft AIS is class B and can be recognised as such by the big ships’ AIS. Ships’ captains have already admitted that in busy areas (approach of ports of London, Antwerp, Zeebrugge, Rotterdam and Hamburg), they will filter out the AIS class B signals to prevent clutter of their screen. Yachts have to give way anyway, is their simple and correct motivation.
Long ago I decided to invest only minimally in BEING seen. I’d rather invest in SEEING myself. I don’t want to depend on other people’s alertness to stay alive. Therefore, I have an AIS receiver only, plus a radar with home made levelling system.
An investment in an active radar transponder like the SeaMe I would only consider because it gives you an alarm and global direction on an incoming radar signal. Mind you, these active transponders only work on one of the two radar bands used by ships.
Remember that AIS class B is not approved in the US. You can buy a class A transponder, but those are quite expensive.
I have a Milltech Marine SR-161 http://www.milltechmarine.com/SR161.htm single channel AIS receiver that I’ve had installed for about 3-4 years. It interfaces to a RayMarine C80 display which is at the helm. Although this is only a single channel receiver, I’ve never perceived that I’ve missed seeing any ships. Typically I can see ships at 30-50 miles out, so there’s plenty of time to obtain all of the AIS data.
I think that one of the key issues of AIS is being able to call a ship by name and say “Princess of the Sea, Princess of the Sea…this is BlueJacket, the sailboat 2 miles on your port bow. I just wanted to make sure that you saw me since it’s rather dark out here.” That works much better than “Cruise ship and lat/long…”
Personally I want an AIS transponder as I want them to be able to see me as well as I see them. I done lots of off-shore passages and am amazed at how fast these ships can approach you. I want to give both of us every chance to see one another. I don’t find AIS very useful in crowded harbors, but I see it as extremely usefull when off-shore.
Oh, I did find that the RayMarine chartplotters have a limit of 50 targets…Here’s an image from NY Harbor.
AIS Class B transponders were approved by the FCC about a month ago–after a long and perplexing delay. I don’t know if any individual transponders have been approved. See here:
http://www.panbo.com/archives/2009/01/ais_2009_forward_in_all_directions.html
for some interesting possible additions to the AIS repertoire.
We have a Sea Me radar target enhancer, mounted at the top of the mizzen mast. It’s light, small, and shows up well on other people’s radar. Tests suggest it works well when heeled, too. I have crossed the North Sea to Norway with it on several occasions and never noticed a period of more than a minute without the unit being triggered, even when 100 miles offshore, but then, there are a lot of oil rigs and support vessels around there. Since the tested apparent cross sectional area is about the same as small ship, I presume I can be seen by anyone using ship’s radar (both X & S band required on commercial vessels). I would not wish to rely only on passive avoidance of collision whilst at sea, but I feel more confident with the Sea-Me than the Firdell Blipper I carry; this tends to swing around a lot if hoisted on a spare halliard, unless taken to the masthead, where it chafes the headboard.
I’m still struggling with AIS reception/chart plotter integration, but I hope it will prove more useful and understandable than radar. I would not wish to use class B AIS transmission in busy waters; it is effectively telling large ships that you are a vessel of no account. At least, if they have an anomalous radar signal it may stir them enough to look out of the bridge window!
Hey Gerald,
The busiest bit of water I’ve ever sailed was over the top of Norway and into the North Sea. I loaded up a raymarine radar screen using every MARPA slot available and wishing I had more! That was a scary night indeed and as far as I could tell the big guys were busy trying not to hit one another and didn’t much care about little ol’ us. I think next in line would be down along the Mexican coast near Cozumel. Again the Marpa came in very handy. Trying to figure out the lighting on a cruise ship is crazy; is that a green running light or a Chinese lantern? heh heh
George
George,
Exciting times in Norway! The Norwegian policy in inshore waters is for all commercial vessels to have precedence over pleasure boats, apparently regardless of the IRPCS. Possession of Class B AIS would just be proof that it was your fault they ran you down!
I think the Sea-Me is being developed to work on both X and S band radar, and that will be my next buy in sailing electronics. Improvement in passive reflectors seems to involve an unacceptable weight penalty. Any substantial increase in mass at the top of the mast is undesirable, especially if said mast is flexible and the sea rough. Although the figures and polar diagram for the Tri-Lens seem the answer to a small boat owners prayer, mounting one on a Freedom masthead might be impractical, if not downright dangerous.
Gerald, only if you have wraparound sails, there’s no other place for a radarreflector then the masthead. If no wraparounds, you could mount a reflector say two thirds up the mizzen, some 12 meters above sea, and visibility to the horizon would be 13 kilometers. With a radar equipped ship approaching you (assume radar height also 12 meters) they would spot you at 26 kilometers. Quite enough, I’d say.
I must say that I haven’t found MAPRA to be very useful on a sailboat underway in anything less than flat seas. On a RayMarine C series chartplotter, the calculated course/speed/CPA bounce around way too much to be useful. AIS on the other hand is rock solid, not relying calculations from a moving platform.
– Geoff
Michel,
I take your point, but I do have wrap-around sails, and the Sea-Me sits at the masthead, weighs less than half a kilo, and has an effective cross sectional area an order of magnitude above the minimum recommended. The power drain is so low that I keep it on all the time when at sea. Installation was a breeze, as well: for the first two seasons, I fitted it with the mast in-situ, from a bosun’s chair. That put it 13m above the waterline: hence the continual triggering on a 200 mile crossing.
If I change the mizzen to a conventional sail, I’ll fit a radar, too. Meanwhile, the various poles and towers I have seen strike me as cumbersome and vulnerable on a 10m boat.
By the way, there was a comparative test of reflectors in ‘Yachting Monthly’, June 2005, with similar results to that included in this thread.
Kind regards,
Gerald,
How did you get the wiring down the mast when installing the SeaMe with your mast standing? I’m always interested to learn new tricks!
Gravity… it’s not just a good idea, it’s the law!
Ducking now! heh heh 
Really I’m waiting for a legit answer myself because just when I thought I knew every trick for a tough wire run someone comes along with a new idea. I have used a vacuum cleaner and feather tied to a piece of light mono-filament line to get a pilot fish though a complicated tuna tower once.
George