Active Radar Reflectors and SOLAS V Implications

[prv]

Some interesting comments. I have now found the answer to my original question here: http://www.theradarreflectorsite.org/StandardsAndRequirements.htm , and as long as it covers the relevant S and X band frequencies, I guess it comes under "by other means" and hence suggest it is OK.

You needn't have waited so long - that's exactly what I said back in post number 8

The link is a little out of date, as it worries about the fact that RTEs only operate on X band. That's no longer true, both of the main makers are now dual-band.

Pete

 

[Storyline]

That site also does not make it clear if you need to have one 'fitted' which is what interests me. The actual Solas wording is not as clear as it should be for the average yachtie. It is there but you also need the previous legislation as it is not all retrospective.
Edit: I know the regs were written primarily for commercial ships but small craft are very much affected by them

 

[Topcat47]

The QinetiQ tests were carried out in an anechoic chamber. The reflectors were measured in the vertical position (which gave the best returns) and at various angles of heel. Polar plots were made in each case. Stacked arrays (fender or dustbin types) depend on the plurality of corner reflectors contained in the tube to give a polar circular response. RCS is the only way to compare radar reflectors and the chamber ensured that there were no other electromagnetic signals to degrade the results. The scientific staff working at Funtington are world class in their field and are expected to extrapolate from measurements to give real world simulations of performance.

The smaller tube reflectors do not contain trihedral corners which accounts for their poor performance. QinetiQ considered the most effective reflectors were the lens types, of which the lunaberg triple lens reflectors are examples.

The tests included simple trihedral reflectors CORRECTLY MOUNTED. In a personal survey of twenty two yachts fitted with these at the masthead, 12 were fitted in the catch rain position, 10 were fitted with an apex uppermost. Of the 12 fitted in a catch rain manner only 7 had the reflector in the best orientation, the other five were skewed. in all cases the reflector was fixed and unable to swing with the heel of the vessel.

Comparative performance at sea mounted to any single design of yacht would not be comparable between other classes to designs as the yacht's ability to generate radar echoes would interfere with the performance characteristics of the reflector being tested.

The performance of stacked arrays can be degraded in a number of ways, not least by simple deformation of the stack, or ingress of water. The lens systems are so designed to be mountable without piercing the cover, unlike the stacked arrays which are commonly held in place by the bolt holes at the top and bottom of the covers. A much earlier study carried out on behalf go the West Sussex Trading Standards found one common stacked array in their sample was nearly an inch longer than the distance between the bolt holes in the cover which gave the array permanent curve to the detriment of it's performance.

The earlier study included mounting radar reflectors on a rubber dinghy at anchor and looking at the returns from a shore based radar. Only two of the reflectors could be distinguished from background sea clutter. This aspect of the trial was redacted from the final report, as no quantitative measures were made and the conclusions could easily have been seen as subjective. The triple lunaberg lens was not current and consequently not tested at that time. Perhaps Sussex Trading Standards still have a copy of the report in their archives.

The advantage of a passive rather than an active system is that it is independent of power. My reflector works 365 day of the year, 24 hours a day. My service battery wouldn't keep a transponder operating for more than a couple of hours in the channel unless I switched off all my other yacht electronics.

Once appraised of the facts, it is open to anyone to act or not on the conclusions and opinions are like ********s, no one needs any special qualifications to hold one. If you want someone else's who better than that of someone who has sent their working life studying the field of interest?

 

[prv]

My service battery wouldn't keep a transponder operating for more than a couple of hours in the channel

That's an exceptionally tiny service battery, then!

Echomax quote their consumption at 23mA quiescent, 155mA when responding to an S-band ping, and 190mA when responding to X-band. Let's assume a really busy area and you're constantly being lit up by X-band radars. If you can only run for two hours that's under 400mAh, or about a 1Ah service battery if we don't want to over-flatten it.

My cordless drill has a bigger battery than that.

Compared to the 2-amp draw of an incandescent tricolour bulb, an RTE is tiny.

Pete

 

[JumbleDuck]

The QinetiQ tests were carried out in an anechoic chamber. The reflectors were measured in the vertical position (which gave the best returns) and at various angles of heel. Polar plots were made in each case. Stacked arrays (fender or dustbin types) depend on the plurality of corner reflectors contained in the tube to give a polar circular response. RCS is the only way to compare radar reflectors and the chamber ensured that there were no other electromagnetic signals to degrade the results. The scientific staff working at Funtington are world class in their field and are expected to extrapolate from measurements to give real world simulations of performance.

OK, first question: why would an anechoic chamber be particularly suitable for testing radar reflectors. Foam wedges have little effect on electromagnetic radiation.

I have no doubt that they measured the radar cross sections accurately, but then I have no doubt that it's possible to measure the sail area of a yacht accurately. That does not necessarily mean that either measurement can be use to predict performance accurately.

Comparative performance at sea mounted to any single design of yacht would not be comparable between other classes to designs as the yacht's ability to generate radar echoes would interfere with the performance characteristics of the reflector being tested.

That does not sound like an insuperable problem. In any case, if the reflectors' effectivenesses depend on teh yacht to which they are fitted, theoretical models of performance are even less useful, no?

The earlier study included mounting radar reflectors on a rubber dinghy at anchor and looking at the returns from a shore based radar. Only two of the reflectors could be distinguished from background sea clutter. This aspect of the trial was redacted from the final report, as no quantitative measures were made and the conclusions could easily have been seen as subjective.

It's a pity that they didn't replace a bad experiment with a good one.

If you want someone else's who better than that of someone who has sent their working life studying the field of interest?

In your lifelong professional experience, so all radar reflectors, of all types, disappear completely at the distances Qinetiq predicted?

 

[prv]

OK, first question: why would an anechoic chamber be particularly suitable for testing radar reflectors. Foam wedges have little effect on electromagnetic radiation.

Presumably it's anechoic to electromagnetic radiation, not (just) sound waves. Perhaps the foam is impregnated with something that does have an effect?

Where I work (although I have no involvement with it) there is a similar chamber formerly used for testing the effects of EM on prototype computer hardware. That has exactly the same type of blue cones (though bigger, perhaps due to differing frequencies of interest). It seems unlikely that both they and DERA would both make the same bizarre mistake.

Pete

 

[RAI]

OK, first question: why would an anechoic chamber be particularly suitable for testing radar reflectors. Foam wedges have little effect on electromagnetic radiation.

The metalised foam cones are designed specifically to absorb and not to reflect the frequencies used in the chamber. Thus the only thing reflecting in the chamber is the radar reflector under test. That way, the measured return is from the reflector and not from the chamber walls. QinetiQ's tests are repeatable and very representative of the reflector's actual performance. More particularly, as all reflectors were tested the same way, the comparison between different types of reflectors is valid. The good ones are good, the bad ones are bad and no amount of field testing will prove otherwise.
Radar detection range is based on a probability.The range set at 95% IIRC, so there is a 95% probability of detecting the reflector at that range. It gets more complicated and specific when one takes into account the number of times the radar paints the target on each scan. Each paint increases the probability of the target being detected. It doesn't change the comparison of good and bad reflectors.

 

[JumbleDuck]

The metalised foam cones are designed specifically to absorb and not to reflect the frequencies used in the chamber. Thus the only thing reflecting in the chamber is the radar reflector under test. That way, the measured return is from the reflector and not from the chamber walls. QinetiQ's tests are repeatable and very representative of the reflector's actual performance. More particularly, as all reflectors were tested the same way, the comparison between different types of reflectors is valid. The good ones are good, the bad ones are bad and no amount of field testing will prove otherwise.

OK, so an EM anechoic chamber, rather than an acoustic one. That sounds fine.

Radar detection range is based on a probability.The range set at 95% IIRC, so there is a 95% probability of detecting the reflector at that range. It gets more complicated and specific when one takes into account the number of times the radar paints the target on each scan. Each paint increases the probability of the target being detected. It doesn't change the comparison of good and bad reflectors.

Exactly.

 

[lw395]

Presumably it's anechoic to electromagnetic radiation, not (just) sound waves. Perhaps the foam is impregnated with something that does have an effect?

....

Correct. I have worked in such chambers. The foam is carbon loaded, or sometimes iron loaded and absorbs radio waves. We call it RAM, for radar absorbent material. It does also tend to absorb sound, which coupled with the lack of lights, makes the chambers quite weird spooky places. Measurements in a chamber are usually more repeatable, if the chamber is good at the band in question, and big enough. Measurements on open ranges have more variables like ground reflection, external interference etc, but can be cheaper to do.