Ouzo - radar reflector report published

[Seven Spades]

I am pleased my tube reflector fell apart as it made me buy a See Me, but the real cost is around £900 with installation...

It is a shame we can't buy shares in See Me.

 

[fisherman]

Another thing: I brought my radar scanner down from the top of the trawl gantry and remounted it in a lower position. In the high position I suspect I was getting more clutter because the beam was looking down into, and could 'see' more of the sea surface. A large ship radar may give the same results, especially when the target is small, and I'm not sure that the tests took this into account.

 

[misterg]

[ QUOTE ]
What it does not say is what the RCS of a typical 30ft GRP yacht is. What for example is the RCS of a typical mast and rigging

[/ QUOTE ]

There is some discussion of this in the annexe to the original Ouzo report. (here) but you have to wade about halfway through a random collection of documents (page 32 of 65).

This is quoted without permission (sorry):

"It is estimated that the RCS of a small <30' yacht will be in the order of 1m^2 to 10 m^2 depending on its aspect angle to the radar - this is demonstrated in figure 2 [Above - Andy]. It is believed that the Pride of Bilbao would have viewed the Ouzo from its port aft quarter. This is at an angle of -135 [degrees] in figure 1(sic), therefore an estimated RCS value of 1m^2 has been used to quantify the yacht in the later modelling."

(There is no figure 1, but a reference to figure 2 makes sense - Andy)

In the later modelling, they assume a height for the yacht reflection of 1m ASL.

(There are other bits that I could quote here, but this part of the file is a scanned document, so not easy to cut & paste)

Andy

 

[Bergman]

I agree.

I am buying a second Octahedral, riveting it out of phase with the existing and hanging the resultant construction from the port cross trees on a short length of thick spring so that it always hangs verically or nearly so.

Not fully worked out the damping necessary, but I'll think of something.

Add that to radar and I think its good enough.

 

[RivalRedwing]

Anybody thinking about swapping out their present passive reflector for a tri-lens?

 

[Bergman]

I thought the tri lens was a passive reflector?

 

[Seven Spades]

There was no mention of the mast shadow effect. With a See Me it is normal to mount it at the pinnacle of the mast giving it a 360 degree field of coverage.

The way mopst reflectors are mounted partically those in plastic wrappers they are attached to the front of the mast blocking the view from the rear. This seems to be a most dangerous practice as there must be a higher risk of being run down from behind than ahead.

 

[RivalRedwing]

[ QUOTE ]
I thought the tri lens was a passive reflector?

[/ QUOTE ]

It is, but would also appear to be the reports perferred passive option

 

[Marsupial]

Agreed, but its got way to go, it needs to be lighter and cheaper and in performance terms although its the best of the passives it still doesnt match the sea-me. Lets face it most sailors rely on GPS why not spare another 350ma for a decent radar reflector?

How about spending £50 on a gimbal mounting for a sea-me, or shaving £1500 off the Polar - no contest!

The shame of all this is that it has taken the loss of a yacht and crew to get proper objective testing of these devices into the public domain, let's not forget that.

We can pontificate on how these devices perform in the field and how the tests dont take this into consideration but I think we can be sure that they will all perform worse in the real world than they did in the test, and that is a worry.

 

[Oldhand]

We don't all sail fat arsed flat bottomed boats that are pigs when heeled. Many yachts are designed to sail to windward at 28 to 30 degrees of heel and will be under powered in waves if sail area is limited to reduce heel. Have you seen an Americas Cupper sailing at 20 degrees or less heel in a breeze recently?

 

[AntarcticPilot]

One point that never seems to get mentioned with passive reflectors is that they have to be made very accurately to work at all. Many years ago, I was involved in a project to calibrate a satellite born radar altimeter system (ERS-1) using corner reflectors. The required precision of construction was very challenging even for a workshop used to that kind of fabrication, and maintaining the accuracy under field conditions in the Arctic (colder but otherwise much less demanding than up the mast of a yacht) was almost impossible. It is very likely that the average octahedral reflector is only just good enough when bought and probably worthless after a week or so up a mast, subject to wind and weather - especially if someone has riveted another one onto it. The required precision of the right angles is much less than a degree; I forget exactly what (I am not a radar engineer) but certainly small enough to be difficult to attain.

 

[misterg]

[ QUOTE ]
One point that never seems to get mentioned with passive reflectors is that they have to be made very accurately to work at all...

[/ QUOTE ]

Hear! Hear!

I strongly suspect that the width of the 'nulls' in the response of the octahedral reflectors is related to the accuracy of the angles. I'm not a radar engineer, either, but the geometry of corner cube reflectors gives an 'error' in the reflected beam of about 4x the 'error' in the angles (3 surface reflection).

For the radar to 'see' it, the reflector has to send as much energy as possible back to the radar antenna. If this has a 2° beam-width, we should probably aim for the return being within 1°, so the octahedron needs to be accurate to ~0.25° over all surfaces - If the sheets were perfectly flat, this would mean about 0.5mm error for the Plastimo under test.

They're just not that accurate. It would be interesting to see how a *precision made* octahedron would perform.

Riveting two together runs the risk of them cancelling each other out. We need a way of testing these things! I'm not convinced that the only solution is an active device.

Re-reading the annexes to the Ouzo report again, It strikes me that the issue of PoB's X-band radar being out of calibration didn't receive the attention it deserved (showed about the same as S band, so judged as insignificant - erm, hang on, Ouzo would have been ~10x as visible on X as S). I've never used radar, so I don't understand the significance of the radar showing only one of its 4 calibration rings.

Andy

 

[Andrew_Fanner]

>>>
Riveting two together runs the risk of them cancelling each other out. We need a way of testing these things!
>>>
It seems sensible to me that this solution, still cheap enough to allow any budget to install, could benefit from the formalised testing in the Qinetiq report. Any idea how this might be possible? While the thorough test report made interesting reading there does seem to be at least one further step that needs to be taken, a better passive reflector and there were no suggestions that this could/should be done.

 

[guydea]

does anyone have any knowledge of the inflatable echomax reflectors - seem to rorc apporoved and offer advantages of being hosted when needed as per raincatchers but without smashing your rigging up