Mike2309
Well-Known Member
at 2.1.3 is an illustration of an octahedral mounted in the recommended "catch-rain" orientation. It looks to me to be the same ( wrong) way up as the Plastimo job pictured above it.
Aligning radar reflectors
- Thread starter peter@
- Start date
at 2.1.3 is an illustration of an octahedral mounted in the recommended "catch-rain" orientation. It looks to me to be the same ( wrong) way up as the Plastimo job pictured above it.
LittleSister
Well-Known Member
Except that the convex surface of the mast will largely deflect and scatter, not reflect, radar signals, whereas the radar reflector surface is arranged to reflect it.
uxb
Well-Known Member
Beadle
Well-Known Member
Yes
Thats the usual argument, but I'm sceptical about it
To start with a radar signal doesn't "bounce" like a ball off a wall - its a much more complex process than that.
bedouin
Well-Known Member
Except in this case the measurements back up the theory.
Note that with the classic octahedral design the signal needs to bounce off 2 surfaces to be reflected back the way it came - that is why it is so direction sensitive
Beadle
Well-Known Member
Yes but
If you have a single vertical wire that happens to be 1/2 wavelength long and another slightly longer vertical wire 1/4 wavelength behind it the signal (or at least most of it) will bounce back the way it came.
In fact if you are very very good at very very hard sums you can make it go any direction you like
If you have a single vertical wire that happens to be 1/2 wavelength long and another slightly longer vertical wire 1/4 wavelength behind it the signal (or at least most of it) will bounce back the way it came.
In fact if you are very very good at very very hard sums you can make it go any direction you like
l'escargot
Well-Known Member
With the movement of the boat, where will it reflect it back too, to where the the receiver was perhaps? A scattered signal at least has some chance of being picked up by someone. It is no good reflecting a narrow signal back to where the receiver once was.
Piers
Well-Known Member
Robert Avis was a radar guru. I say was, because a few years ago he sadly died. At the time, there were may tributes to him on these forums. Deleted User said of him,
"Robert was a Royal Naval Reserve Commander, Yachtmaster Examiner, MCA examiner, seaschool and radar instructor, MBY contributor, author and all round good bloke and gentleman."
The point I wanted to make is that I remember him telling us on one of his Radar courses that the best reflector a yacht has it the hole it makes in the water, and that the only reflector better than this would be an active reflectors such as the Sea-Me.
"Robert was a Royal Naval Reserve Commander, Yachtmaster Examiner, MCA examiner, seaschool and radar instructor, MBY contributor, author and all round good bloke and gentleman."
The point I wanted to make is that I remember him telling us on one of his Radar courses that the best reflector a yacht has it the hole it makes in the water, and that the only reflector better than this would be an active reflectors such as the Sea-Me.
D
Deleted member 36384
Guest
I don't follow this due to my ignorance. Did he mean that the hole in the water reflected the rays better than the boat i.e. a boat with no Radar Reflector was as useless as hole in water? Or is a hole in the water caused by the hull an actual reflector of the rays? Maybe I am over analysing!
vyv_cox
Well-Known Member
I believe it's your second. When on a boat with no radar reflector whatsoever, we were advised by Liverpool Coastguard that they had us clearly on radar from at least 12 miles out. The hole in the water effect is lost in heavy seas when it is overwhelmed by wave reflections.
Robin
Well-Known Member
We had a similar report that we were seen clearly by Ushant Traffic Control when 15 miles off, conditions were calm sea and thick fog. I had called them to ask if they knew what the visibility was like in Chenal Du Four and they helpfully replied with a report from the Pte St Matthieu Signal Station and an offer by them to talk us through if needed using their radar. We had radar and a trusted and many times tested GPS/plotter route so didn't need outside assistance, but it does show we can be visible. We had a fixed Firdell Blipper on the mast at the time.
onesea
Well-Known Member
Yes but allot depends on the radar operator, and the settings they apply...
Some are better than others radars and operators that is
A 1/8th turn assorted controls on some radars can be the difference between invisible and invisible. Where the setting should be varies upon the conditions range scale etc...
bedouin
Well-Known Member
Not to mention the conditions - with any form of sea then the waves are going to prevent there being any radar return from the hull (including hole in the water), engine and so on.
So information that a CG can see you on a calm day says very little about whether a ship can see you in any form of sea
D
Deleted member 36384
Guest
Thanks for that (and Robin's example). It makes sense in light of the wave reflection ("sea clutter" I noticed it was called on one Radar set) that in conditions where this is low, a large water hole would reflect back signals. The first new thing I have learned in 2012.
AntarcticPilot
Well-Known Member
Not relevant. The corner cube reflectors left on the moon were extremely precise pieces of kit, designed to reflect laser beams, not radar. Yes, theoretically a corner cube reflector is very efficient - BUT it has to be impossibly precisely made to maintain that effectiveness. At radar frequencies, deviations of a few mm from perfectly square will degrade the performance vastly. How many octahedral reflectors do you think maintain the angles with the requisite precision? Enclosed ones may do better - for a while. But they are usually much smaller, and so intrinsically less effective.
I speak from being part of a team that used corner reflectors to evaluate the performance of orbital radar altimeters - the design criteria for the corner reflectors (at a frequency lower than radar and thus less demanding) were very difficult to meet, even with the resources of the Rutherford Appleton Laboratory!
As the Ouzo report makes clear, passive reflectors are of very little use.
D
Deleted member 36384
Guest
I wouldn't say they are of very little use. Most fog conditions tend to be in lighter wind conditions (although the North Sea is the only place I have witnessed a gale and fog simultaneously). It would be possible to sail upright (slowing down, good seamanship) and thus optimise the operating conditions for an Echomax or Firdell Blipper in poor visibility. As a strategy, for leisure sailors, it offers a possibility of increasing detection, without the high cost of an active device.
Most of my fog experience (all around the UK coast) involved motoring in calms , of course night sailing in good weather, when one is healed is also a good time to have a decent radar signature. However, a good lookout may overcome the angle issues.
So a rigidly mounted cylindrical reflector could be an appropriate solution if used within its limitations.
CreakyDecks
Well-Known Member
It's fairly obvious that a reflector designed to operate over a quarter of a million miles is going to need to be an "extremely precise piece of kit". One being used to send a signal ten miles or so isn't going to need to be quite so precise. The other factor is that a typical radar reflector is only about ten wavelengths across at X band, so the reflected radiation will be quite widely spread. I doubt that a few dents will affect the reflected signal much. The point is that it is better than something cylindrical like a mast.
If they don't work, then why are they used on marker buoys? I suspect that they work perfectly well when people are actually looking for them.
Robin
Well-Known Member
My personal view is that a good octahedral (mounted correctly) or a Firdell or Gillie Firth type does work in practical conditions, possibly despite the theoretical and scientific tests. Perhaps it is not the actual strength of the echo return that is enhanced which is important (although it certainly helps) so much as the consistency of a return, however weak. This is because I believe an intermittent echo is easily missed, especially if the radar operator is not using some kind of 'track', 'wake' or 'plot' setting that keeps a shadow trace on screen, even if the main return vanishes regularly.
We have experienced fog and wind together many times, when the fog is the kind that arrives as cold water comes up from the deeps after a tide change, meeting warm damp air. The wind blows away the surface mist, only for it to be replaced by more so more condenses and the mist thickens quickly into fog, more wind now means thicker fog. We have sailed in F6 with radar and still been able (with constant monitoring and adjustment) to pick up small boats, even the odd French one with no reflector, a Mobri flat on it's spreaders or a titchy octahedral point up on the backstay. That said my own anecdotal evidence is that boats with decent reflectors were seen (on our small boat set) consistently from around 4mls and intermittently at up to 5mls. Those without were hard to pick out at 2mls.
Some people also seem to confuse a large return as a large target and want their own return to be seen as that. A large return is often merely because at the target's range the wide beam width of the typical yottie scanner 'sees' it as being wider than it is. OK so a big ship side on to the beam is big, but the same ship head on is not, except as widened by the beam width angles. I don't think I explained that very well, but if you draw lines out from the centre (your position) through the edges of a return, the farther away from the centre the bigger the target appears.
We have experienced fog and wind together many times, when the fog is the kind that arrives as cold water comes up from the deeps after a tide change, meeting warm damp air. The wind blows away the surface mist, only for it to be replaced by more so more condenses and the mist thickens quickly into fog, more wind now means thicker fog. We have sailed in F6 with radar and still been able (with constant monitoring and adjustment) to pick up small boats, even the odd French one with no reflector, a Mobri flat on it's spreaders or a titchy octahedral point up on the backstay. That said my own anecdotal evidence is that boats with decent reflectors were seen (on our small boat set) consistently from around 4mls and intermittently at up to 5mls. Those without were hard to pick out at 2mls.
Some people also seem to confuse a large return as a large target and want their own return to be seen as that. A large return is often merely because at the target's range the wide beam width of the typical yottie scanner 'sees' it as being wider than it is. OK so a big ship side on to the beam is big, but the same ship head on is not, except as widened by the beam width angles. I don't think I explained that very well, but if you draw lines out from the centre (your position) through the edges of a return, the farther away from the centre the bigger the target appears.
AntarcticPilot
Well-Known Member
The distance isn't all that material. The simple fact is that to be effective at all requires precise maintenance of the angles of the corner reflector - any deviation greater than a small fraction of a wavelength results in spreading of the signal. My direct experience is of a Ku band radar looking vertically downward from about 700km up; for a corner reflector to be effective it had to be many metres across, and maintain it's geometry to better then a centimetre. A radar reflector at best 50cm across with doubtful maintenance of the critical angles is simply not going to be very effective.
Basically a corner reflector is all or nothing - it works (and is geometrically very precise) or it is no better than an equivalent bit of tin-plate. ANY deviation from the precise 90 degree angles results in signal spreading, and could well be worse than the curved surface of the mast!
Incidentally, the mast has a fair chance of operating as an effective radiator provided the band-width of the radar is reasonably large. In this case, as long as the length is a multiple of a frequency within the band-width of the radar, then it will act as a strong reflector. The cross-section of the mast will increase the band-width over which it is an effective radiator, as well. Note that to increase the band-width of a dipole antenna, you increase the thickness of the radiating elements.
Buoys are a) probably built more robustly and at greater cost than something to be hoisted up a yacht's mast and b) can be big enough to be worth-while.
Piers
Well-Known Member
Been away for a couple of days...but in answer to your qn, Robert was saying that the hole in the water gave a better radar return than any passive reflector.
This would only really apply in weather conditions where the sea was fairly flat, typical of foggy conditions. An emotional sea would 'hide' the hole in local chop.
