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Old 01-10-2017, 11:01   #1
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Radar Cross Section

I understand the electronics and physics of Radar, but having never operated a marine unit on a boat, my question is to those folks who have that practical experience.

There is much talk about fiberglass sailboats needing the radar reflector ball hoisted to be visible to radar. The ball has small 90 degree surfaces which will reflect x-band signals right back in the same direction as arrival. Most buoys have something similar on top.

But I would expect that a sailboats' mast and shrouds would be pretty good reflectors too.

So, how true is it that frp sailboats are hard to see on radar without the reflector ball up?

Chris
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Old 01-10-2017, 11:13   #2
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Re: Radar Cross Section

It's quite common for sailboats to be difficult to see on radar on big ships, as they tune their radars so that waves of a certain size don't show up on their screens as appearing to be boats. And at times these settings are such that sailboats don't get picked up on their radar either. Thus the wisdom of adding radar reflectors. Especially as most masts don't reflect radar signatures all that well for some radar units.

There are other things you can do as well, to improve your visibility. AIS is one, there are also electronic signal enhancers, which put out a signal on the frequencies that radars are tuned to, that make your boat more visible, electronically.

Some people even include radar reflective kites in with their life rafts to aid in their being spotted by rescuers.
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Old 01-10-2017, 11:29   #3
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Re: Radar Cross Section

From the side, a mast reflects radar quite well. From dead ahead or astern they can be very difficult to pick up as most of the wave gets bounced off to either side rather than reflected back. Same goes for some powerboats with raked superstructure of fiberglass which in itself absorbs most of the wave rather than reflect.
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Old 01-10-2017, 11:33   #4
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Re: Radar Cross Section

Quote:
Originally Posted by boatpoker View Post
From the side, a mast reflects radar quite well. From dead ahead or astern they can be very difficult to pick up as most of the wave gets bounced off to either side rather than reflected back. Same goes for some powerboats.

I presume this effect is due to the angled fore and aft stays "intercepting" much of the pulse energy and reflecting it upward instead of back to the radar?

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Old 01-10-2017, 11:38   #5
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Re: Radar Cross Section

Collision Regulations Rule 40
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Old 01-10-2017, 11:41   #6
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Re: Radar Cross Section

No, it's more to do with the radius of curvature of the mast being much smaller fore and aft. The radar signal is reflected off through a greater range of angles if the mast section is more curved. Commercial ship radars are not sensitive enough to pick up the reduced signal levels.
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Old 01-10-2017, 12:04   #7
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Re: Radar Cross Section

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No, it's more to do with the radius of curvature of the mast being much smaller fore and aft. The radar signal is reflected off through a greater range of angles if the mast section is more curved. Commercial ship radars are not sensitive enough to pick up the reduced signal levels.
Agreed.
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Old 01-10-2017, 12:09   #8
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Re: Radar Cross Section

Radar is complicated stuff, much more complex than the manufacturers would have you believe in their glossy sales brochures. My radar certification course at the Centre For Marine Training & Research at Georgian College was 11 weeks.
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Old 01-10-2017, 12:38   #9
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Re: Radar Cross Section

A marine radar radio pulse is transmitted from a special antenna that focuses the RF (radio frequency) energy into a very narrow beam.
Consider it the same as a beam of light from a narrow beam spotlight.

If this pulsed RF beam intercepts a flat surface that is vertically & horizontally at 90deg to the beam,then a strong reflection is sent back toward the transmitting/receiving antenna,just as would happen if the light beam hit a perfectly vert/horiz mirror.

The real world is not flat 90deg surfaces(except for a slab sided big ship)
The pulsed RF beam mostly will hit rounded or slanted surfaces. Some of the energy will be reflected back to the T/R antenna,but much of the energy will be reflected off vertically & horizontally & never arrive at the T/R antenna.

A proper radar reflector has specific dimension that "fit" the wavelength/frequency of marine radar transmitters-ie it is a tuned receiving antenna & is much better at "grabbing" radar transmissions than a random sized reflective surface.
Secondly,a radar reflector is built of cones with 90deg angles to each other & is supposed to be mounted such that the focal point of these cones is horiz. to the water. As shown in the attd.,the cone captures the RF beams,concentrates them into a stronger beam & reflects this concentrated beam back to the T/R antenna.
The same thing is being done in the new LED spotlight reflectors.

When a reflected pulse is received back at the radar antenna,a voltmeter circuit measures the strength of the pulse & commands the computer in the display to print a pixel whose brightness/size/color is relative to the strength of that echo.(in simple terms)

GRP is not a good electrical conductor,so it makes a poor RF reflector.
I daresay wet sails are better
Rounded or non vertical shapes,regardless of material,are poor RF reflectors.
Add to this that both vessels are "rocking & rolling" not in sync.

Luckily,your radar is spitting out & waiting for returns from "several hundreds" of pulses per second. The antenna,turning @ 24-48 RPM is basically "stopped" while several thousand pulses happen at each bearing.

I live in fog country & you would not believe the difference that a $50 radar reflector makes in any vessel's visibility,clear or foggy,night or day.
That even includes dorys & other small craft.

Don't leave home without it IMHO.
Should be a requirement under safety regs IMHO.

Len

Radar Basics - corner reflectors
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Old 01-10-2017, 12:43   #10
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Re: Radar Cross Section

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Originally Posted by deblen View Post
Don't leave home without it IMHO.
Should be a requirement under safety regs IMHO.

Len

Radar Basics - corner reflectors
See Collision Regulations Rule 40 posted above.
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Old 01-10-2017, 14:52   #11
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Re: Radar Cross Section

Quote:
Originally Posted by deblen View Post
A marine radar radio pulse is transmitted from a special antenna that focuses the RF (radio frequency) energy into a very narrow beam.
Consider it the same as a beam of light from a narrow beam spotlight
...........

Luckily,your radar is spitting out & waiting for returns from "several hundreds" of pulses per second. The antenna,turning @ 24-48 RPM is basically "stopped" while several thousand pulses happen at each bearing

.........
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Radar Basics - corner reflectors
No, there's only a few pulses on each bearing. It's at most about 20 for a typical marine radar.
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Old 01-10-2017, 15:11   #12
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Re: Radar Cross Section

Thanks folks,

So I gather that having a reflector ball up can make a real difference in a sailboats radar visibility.

Chris
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Old 01-10-2017, 15:12   #13
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Re: Radar Cross Section

Quote:
Originally Posted by deblen View Post
A marine radar radio pulse is transmitted from a special antenna that focuses the RF (radio frequency) energy into a very narrow beam.
Consider it the same as a beam of light from a narrow beam spotlight.

If this pulsed RF beam intercepts a flat surface that is vertically & horizontally at 90deg to the beam,then a strong reflection is sent back toward the transmitting/receiving antenna,just as would happen if the light beam hit a perfectly vert/horiz mirror.

The real world is not flat 90deg surfaces(except for a slab sided big ship)
The pulsed RF beam mostly will hit rounded or slanted surfaces. Some of the energy will be reflected back to the T/R antenna,but much of the energy will be reflected off vertically & horizontally & never arrive at the T/R antenna.

A proper radar reflector has specific dimension that "fit" the wavelength/frequency of marine radar transmitters-ie it is a tuned receiving antenna & is much better at "grabbing" radar transmissions than a random sized reflective surface.
Secondly,a radar reflector is built of cones with 90deg angles to each other & is supposed to be mounted such that the focal point of these cones is horiz. to the water. As shown in the attd.,the cone captures the RF beams,concentrates them into a stronger beam & reflects this concentrated beam back to the T/R antenna.
The same thing is being done in the new LED spotlight reflectors.

When a reflected pulse is received back at the radar antenna,a voltmeter circuit measures the strength of the pulse & commands the computer in the display to print a pixel whose brightness/size/color is relative to the strength of that echo.(in simple terms)

GRP is not a good electrical conductor,so it makes a poor RF reflector.
I daresay wet sails are better
Rounded or non vertical shapes,regardless of material,are poor RF reflectors.
Add to this that both vessels are "rocking & rolling" not in sync.

Luckily,your radar is spitting out & waiting for returns from "several hundreds" of pulses per second. The antenna,turning @ 24-48 RPM is basically "stopped" while several thousand pulses happen at each bearing.

I live in fog country & you would not believe the difference that a $50 radar reflector makes in any vessel's visibility,clear or foggy,night or day.
That even includes dorys & other small craft.

Don't leave home without it IMHO.
Should be a requirement under safety regs IMHO.

Len

Radar Basics - corner reflectors
This pretty much nails it (and I concur with everything Len has posted here ).
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Old 01-10-2017, 15:22   #14
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Re: Radar Cross Section

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Originally Posted by TreblePlink View Post
Thanks folks,

So I gather that having a reflector ball up can make a real difference in a sailboats radar visibility.

Chris


Not nearly as much as people would like to think, see the tests, most reflectors are pure junk. It has a lot to do with cross sectional area, and most sailboat reflectors are just too small to be effective.
If you want to increase your signature you need a transponder, an active reflector if you will.
Think of a Radar reflector as a light reflector and your searching to see something with your spotlight, a reflector helps, but someone shining a spot light back at you will be seen for sure, that is what a transponder does.

I went the active AIS route under the belief that anything likely to be enough to run me over, is likely to have an AIS themselves, and we will see each other.
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Old 01-10-2017, 15:41   #15
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Re: Radar Cross Section

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Collision Regulations Rule 40
Once again. That's not International Regulations for Preventing Collisions at Sea 1972 (COLREGs) Rule 40, which is:

Rule 40 Application
Contracting Parties shall use the provisions of the Code for Implementation in the execution of their obligations and responsibilities contained in the present Convention.


You forgot to include this bit in your copy:

"PART F — ADDITIONAL CANADIAN PROVISIONS"
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