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Old 05-05-2008, 14:16   #61
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I go along 100% with what Talbot has said.

An example of a small target that gives a good radar return is an aircraft's propellor. I have wondered if a metal bladed wind turbine on a yacht would be equally effective (while spinning) - anyone know?
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Old 05-05-2008, 16:41   #62
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Something interesting... Most fiberglass boats are made of E-glass. Same stuff used for radar domes, because its largely transparent to radio waves. I wonder if a thin skin of something reflective inside the hull... and deck would give a better image on a radar screen?

Edit: About the luneberg lens, and others that have limited angles of operation. What prevents one from mounting them on a gimble of some sort? It'd be mostly perpendicular to the horizon. (Most monos nose down a few inches as they heel.)
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Old 05-05-2008, 18:32   #63
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Something interesting... Most fiberglass boats are made of E-glass. Same stuff used for radar domes, because its largely transparent to radio waves. I wonder if a thin skin of something reflective inside the hull... and deck would give a better image on a radar screen?

Edit: About the luneberg lens, and others that have limited angles of operation. What prevents one from mounting them on a gimble of some sort? It'd be mostly perpendicular to the horizon. (Most monos nose down a few inches as they heel.)
There are two types of Luenberg lens being used for radar reflectors. One (the Lensref) has a "belt" at the equator of the sphere. This reflects well over a 360deg radius, but not at large tilt angles. The Lensref is OK for power boats and I suppose catamarans. This one might benefit from a gimballed mounting, as long as you could dampen the swinging.

The TriLens uses three Luenberg lenses, each having a reflective surface covering almost a complete hemisphere. This reflection covers about 120 degrees in a conical vertical and horizontal pattern. Placing three of these Luenberg lenses oriented at 120deg (each pointing in a different direction), gives good reflection coverage for just about any reasonable angle of heel. The disadvantage is size, weight, and cost.
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Old 06-05-2008, 04:03   #64
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Quote:
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Something interesting... Most fiberglass boats are made of E-glass. Same stuff used for radar domes, because its largely transparent to radio waves. I wonder if a thin skin of something reflective inside the hull... and deck would give a better image on a radar screen?
That's an interesting point. Maybe you could glue in a roll of tin foil inside the hull running the whole length of the boat behind the cabin furniture just below deck level, and on the inside of cabin roof for when the boat is heeled. Also, if you keep it wrinkly, this would increase the signal response.


???????????????????
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Old 06-05-2008, 10:55   #65
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That's an interesting point. Maybe you could glue in a roll of tin foil inside the hull running the whole length of the boat behind the cabin furniture just below deck level, and on the inside of cabin roof for when the boat is heeled. Also, if you keep it wrinkly, this would increase the signal response. ???????????????????
This might work (it would definitely work in some cases), but a flat plate is a good reflector in only one direction. Consider a mirror: When you look into it you can only see your face when the angle is fairly perpendicular. A flat metal plate is a mirror at radar frequencies, and the radar's "eyes" will only see what gets reflected back to it.

So, as the boat heels, pitches, and yaws, the aluminum may or may not reflect back a signal. Since the hull is (usually) curved the reflection will be closer to that from a hemispherical mirror, but will still be fairly directional.

As for wrinkling the foil, unless the wrinkles are greater than a wavelength they will have little effect. The wavelength of an X-band radar signal is about 3.2cm -- they would have to be pretty big wrinkles.

The S-band radar (used on the high seas) operates at a wavelength of 10cm. This is why most of the reflectors we use are such poor performers at S-band: the wrinkles (corner dimensions) are too small.
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Old 06-05-2008, 11:08   #66
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Hmm...

3.2cm is a touch over 1.25 inches. It wouldn't be a huge deal with a press brake to make a corrugated sheet. May have to call up my cousin with the sheet metal fab shop and try this out... anyone have a radar they want to donate to science?
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Old 06-05-2008, 20:56   #67
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I realize that many of you are quite familiar with all this, but sometimes it helps me get more comfortable with stuff like radar and reflectors if I can compare them to things I use in everyday life.

For example, when I was helping green engineers get up to speed on very fast digital designs (picoseconds and nanoseconds, sub-micron integrated circuits, 10 Gbit/s and faster data rates) I would sometimes make them a "nanosecond ruler", that they could lay across a circuit-board trace and see how far a signal would travel in a given time. In this case, on a typical circuit board, a signal goes about one-half inch in one nanosecond. This helped give them a feeling for the what dimensions might be critical, in a way that a pure mathematical analysis would never convey.

In dealing with radar signals, I like to think of optical mirrors for an analogy. We all see and use mirrors daily, and are quite familiar with what they do and how they function. The difference is the wavelength of the signal: X-Band radar has a wavelength of about 3.2cm, and optical light has a wavelength of around 500nm (nanometers).

This is a ratio of 64,000 : 1, so the optical equivalent to my 12" diameter Davis octahedral reflector would be about 188-millionths of an inch in diameter. Or, scaled up for radar, your 2"x3" emergency signalling mirror would be about 2 miles x 3 miles in size.

So, based on our familiarity with optics and mirrors, what we might think is a significant feature in a radar reflector may not be significant at all. Don't underestimate the importance of wavelength.

Also, the angles on a corner-reflector are very important, and provide the extremely useful property of reflecting a signal directly back to it's source. If the angles are off by a few degrees, the signal is reflected somewhere else, and not towards the source (radar receiver).

Take two mirrors and place them at right angles (you can sometimes see mirrors like this in clothing stores). Shine a flashlight into them, and see how the beam is reflected. A corner-shot on a pool table demonstrates the same principle. If the angle is not 90 degrees, the beam won't hit you on the rebound. Note that this really only retro-reflects in a plane, and if the mirrors are not perpendicular to the source, the beam goes up or down.

The corner reflectors we use place a third surface at right angles to the other two (think of two walls and a ceiling, meeting at a corner). This confguration reflects back to the source no matter the angle. Throw a tennis ball at a wall-wall-ceiling corner, and see what happens (you have to compensate for gravity).

When the angles on a wrinkled surface are random, then the reflections are random. Some will return to the radar, but most will not. You need a very big "random-wrinkle" surface to compete with a true corner-reflector, which sends all the signal that hits it back to the source. Also, if the wrinkles are less than a wavelength (actually, several wavelengths), they don't act as small reflectors, but instead blend into an "average" surface.

As usual, I'm short on useful answers (I wish I had some!), but I thought I'd share my thoughts on how to analyze the situation.
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Old 06-05-2008, 22:53   #68
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Paul,
Excellent post.
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Old 07-05-2008, 04:28   #69
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Paul Elliott:
I echo Steve's kudos. Very illuminating.
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Old 07-05-2008, 09:15   #70
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Practical Sailor did a test of different materials. Aluminum foil was close to worthless.
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Old 07-05-2008, 09:21   #71
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Once again, excellently described Paul.

It seems to me that there is no real solution to this problem, the precision required for a reflection is so fine that in the randomness of the ocean there can never be any guarantee of a small boat being seen by a ships radar, and even if you are seen by the radar, there is no guarantee that the watchkeeper will see your blip on the screen.

Maybe we should look at the alternatives more closely, AIS etc.

In the meantime, my suggesttion would be to make better use of VHF. I can see no reason why a small boat shouldn't broadcast its position, direction and speed over a limited range, particularly in fog, at night, and when crossing shipping lanes. At least you can feel you have done something posative instead of just hoping that there isn't a supertanker heading straight for you.

And I'll keep my trihedral hanging up the backstay just in case it will do some good.
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Old 07-05-2008, 09:58   #72
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It seems to me that there is no real solution to this problem, the precision required for a reflection is so fine that in the randomness of the ocean there can never be any guarantee of a small boat being seen by a ships radar, and even if you are seen by the radar, there is no guarantee that the watchkeeper will see your blip on the screen.
Years ago a friend recalled his experience while on a very foggy crossing of the Atlantic.

He found himself on a collision course with a ship, so he contacted via VHF to inquire if he was visible on the ship's radar.

The ship replied, "Hold on for a minute, let me turn it on..."


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Old 07-05-2008, 11:04   #73
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And that just about sums it up!
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Old 07-05-2008, 12:00   #74
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Once again, excellently described Paul.
Maybe we should look at the alternatives more closely, AIS etc.
Crossing the English Channel in fog without aids such as radar and or AIS is an experience best avoided.

Radar is good in that it gives you all the contacts and also navigation information. Unfortunately unless you have a good gyro and a tracking system like MARPA, or you have good knowledge of radar plotting, it can be as much use as a chocolate frog in a heatwave.

AIS provides more information, such as vessel identity and position course and speed (sometimes incorrectly) for most big merchant ships (should be all, but .....) If you have an AIS engine (which just collects the data and provides it to something else to display) and connect to a laptop, and use a bit of software such as that supplied by ShipPlotter it will also provide CPA and timings plus an aaudible warning of anything crossing definable specs (such as CPA distance within a certain time). If you are really concerned about a contact, you can obtain MMSI info and contact the ship directly. This system is very cheap <$300 US and also provides confirmation of radar contacts so that it becomes easier for the inexperienced to understand the radar display.

I would not be without an AIS.
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Old 07-05-2008, 15:24   #75
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Radar is good in that it gives you all the contacts and also navigation information. Unfortunately unless you have a good gyro and a tracking system like MARPA, or you have good knowledge of radar plotting, it can be as much use as a chocolate frog in a heatwave.

I would not be without an AIS.
Agreed. Small cost for greatly increased safety with respect to large ships in shipping lanes. On the other hand I like radar for weather which AIS won't do.

Paul: Wonderful summary. With respect to the nulls in the probability of detection, I would rather not have a null at a distance so close that when I did come back into view, that there would be no time to respond.

4 miles at 4 knots vs 4 miles at 20 knots.
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