RAI
Well-known member
Who said the hole has a big RCS? But it has other properties than waves. Without a radar reflector, on a grp yacht, what else does a ship's radar have to go on? The rounded rigging reflects very little back.
Passive radar reflector - anything better than Echomax?
- Thread starter JumbleDuck
- Start date
TernVI
Well-known member
The rounded rigging has plenty of RCS.
Reflection from things like masts and rigging has been well documented since the 50s.
Much has been written about the scattering from waves.
The 'holes in the water' concept probably has some validity if you've got a GRP minesweeper and an airborne radar.
WIth the right signal processing, you can track a ship's wake on radar.
But I don't think P&O or Wightlink are going to do that.
Reflection from things like masts and rigging has been well documented since the 50s.
Much has been written about the scattering from waves.
The 'holes in the water' concept probably has some validity if you've got a GRP minesweeper and an airborne radar.
WIth the right signal processing, you can track a ship's wake on radar.
But I don't think P&O or Wightlink are going to do that.
Slowboat35
Well-known member
Luneberg lenses as in the TriLens reflector seem to be the strongest signal reflectors.
Tri-Lens Radar Reflector | Practical Sailor
Tri-Lens Radar Reflector | Practical Sailor
sarabande
Well-known member
Can you give links to that please ? Preferably technical evidence rather than hearsay.
JumbleDuck
Well-known member
Thanks, folks, I have decided to leave the Echomax passive up there for a bit longer and save up for a dual-band active one.
Refueler
Well-known member
Wouldn't actual events be worthy of note ?
This boat in middle of Baltic in F4 - 5 moderate seas with no reflector was plotted on a ships ACAS at over 10nm ...
Sunrider 25, GRP hull.
The whole incident actually was :
I was sailing back from Gotland to Ventspils ... we noted a ship on port bow which appeared to be on a steady bearing. Discretion being the better part of staying alive - I took a bold and obvious round turn to ensure we were not run over ... I am not one to risk quoting ColRegs when 10,000 ton of feeder ship is bearing down on us. BOLD and OBVIOUS is the key here.
As he passed ahead - having never altered course - I called him .. and the OOW stated he had me plotted for over 10nm on his ACAS and was expecting to pass 1 cable astern of me ... as I replied back to him ... 1 cable ???? Do you think that's sufficient with your wash and my unable to gauge such close 1/4's ?? He apologised and sailed on.
I was left with the carnage down below his wash had caused ...
I have to quote one of the Masters I sailed with when I was starting out at sea :
Why pass close to another when you have the whole ocean to use ?
Regardless of the actions he and I took .. the fact is he said clearly that my boat was plotted on his ACAS at over 10nm - which I found surprising given the wave height etc. At the time I had sails up and would consider they were quite likely reasonably damp but but not wet. I would consider that the large Perkins in the boat was mostly behind waves ... so suggest that mast / sails probably were the biggest RCS factor ...
RAI
Well-known member
So is there sufficient rigging RCS to obviate the need for radar reflectors on an average AWB?
Like 10 m2 of RCS?
RAI
Well-known member
Wise choice. Nothing like being seen.
Poignard
Well-known member
sarabande
Well-known member
Out of curiosity, does anyone know the size (not power) of the radar signal that is sent from a decent sized ship ? I am imagining a radar particle bouncing off an oval mast that is not at right angles in both x and z coordinates relative to the radar emitter
I can find the wavelength of X and S band but I am trying to discover the particle size and hence related energy that needs to be returned in order to make a good paint.
I can find the wavelength of X and S band but I am trying to discover the particle size and hence related energy that needs to be returned in order to make a good paint.
RAI
Well-known member
It's a wonder what a skilled radar operator can squeeze out of his kit
and a horror as to what might be missed by a watch keeper with his radar on auto settings.
RAI
Well-known member
I would turn to Skolnik for an answer, but there may be better books on radar.
AntarcticPilot
Well-known member
It's the length of a mast that makes it a pretty good reflector in the plane perpendicular to it. It's diameter increases the bandwidth over which it reflects. The curved surface is irrelevant to the reflecting properties; it isn't specular reflection from the surface of the mast, it's an induced signal in the mast. Rigging wires are also good reflectors, but over a much narrower bandwidth because of their small diameter, so overall not as good as the mast. We don't rely on the reflection from the mast because it isn't directional; the "reflected" energy is spread in all directions, with the exact pattern modified by rigging etc. The idea of a corner reflector is that they return power back to the source. However, they have to be a) accurate and b) large to be useful. The former is unlikely after a while up a mast, and the latter is difficult in smaller yachts.
RAI
Well-known member
Right, do you have an estimate of the RCS of a typical sloop's mast boom and rigging?
AntarcticPilot
Well-known member
No; but it won't be insignificant. I doubt if it can be calculated very precisely, as every combination of mast and rigging will give different polar responses. The problem is just that - in some directions it will be excellent; in others non-existent. Also, it will be strongly directional in the plane perpendicular to the mast, so any heel will make it less visible.
Disclaimer - I'm not a radar expert. I've worked alongside radar experts quite a lot, though, and have a good qualitative feel for radar, but I certainly don't know everything in Skolnik's well-known "Introduction to Radar Systems"! I worked on satellite radar altimetry and on ice-sounding radars, but my area was the software for digital recording, control and post-processing, not the actual RF side of things. But my colleagues delighted in explaining their design decisions, so quite a bit rubbed off! But one thing that made me immensely distrustful of passive reflectors was the enormous difficulty my colleagues had in setting up a corner reflector to test aspects of the operation of the ERS-1 radar altimeter. To be effective, it had to have dimensions of metres across, and the angles and dimensions had to be maintained with very high precision - I recall that it was accuracies of millimetres over several metres. The problem with corner reflectors is that if they aren't precisely made and maintained to the same precision, they can actually be worse than a length of wire.
D
Deleted member 36384
Guest
Minimal comment on interference on Page 80 of this Yachting Monthly article that tested reflectors. It suggests that the reflection from masts, hulls etc may enhance or reduce the returns from a radar reflector; no values are given.
https://keyassets.timeincuk.net/ins...20/filebank/reflector_performance_ym_june.pdf
For information, a USA article that comes to the same conclusions that are generally accepted: the best are marginally better than nothing, the worst are not worth having fitted, beware being overly confident of being seen on radar using a reflector: -
https://cdn.ussailing.org/wp-content/uploads/2018/03/radar-reflector-tests.pdf
https://keyassets.timeincuk.net/ins...20/filebank/reflector_performance_ym_june.pdf
For information, a USA article that comes to the same conclusions that are generally accepted: the best are marginally better than nothing, the worst are not worth having fitted, beware being overly confident of being seen on radar using a reflector: -
https://cdn.ussailing.org/wp-content/uploads/2018/03/radar-reflector-tests.pdf
TernVI
Well-known member
RobbieW
Well-known member
Is that the speed of fright ? ?
TernVI
Well-known member
RobbieW
Well-known member
186000 is one of those numbers that sticks in my mind, a bit like 3.142, as an approximation (in miles/sec) of the speed of light. Google tells me the number you were looking for is 62,400
