A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fatal interaction betweeu yacht and ferry.
From: Lu Abel
Date: 2007 May 08, 07:24 -0700
From: Lu Abel
Date: 2007 May 08, 07:24 -0700
George: You are very likely right (again, without either of us having sources to cite) -- I thought of the wavelength issue right after I pushed the "send" button. For those who wonder what we are babbling about: Even the smallest mirror has dimensions a billion (thousand million) times greater than the wavelength of light. But radar reflectors are only a few times larger than the size of a wave of radar energy. Whenever relative dimensions are so small, "reflection" is a lot less clean. An analogy we can all see with our own eyes is to watch waves hit a seawall (or, even better, a large rock). Yes, the waves bounce off the seawall (or rock), but it's far from a clean "ray" ala light reflection. Lu Abel George Huxtable wrote: > Lu Abel wrote, responding to Gary- > > | You seem to know a lot about radar reflectors, so I'm surprised you > | didn't mention what many consider the most crucial aspect of radar > | reflector construction: that the angles between the plates must be an > | accurate 90 degrees. Since the radar signal bounces off one plate and > | then another, only by having a very accurate 90 degree corner is it > | guaranteed that the incoming radar pulse will be returned to the radar > | that emitted it! For this reason many experts advise against home-brew > | radar reflectors (I can't tell you the number of times I've read letters > | to the editor of various sailing publications with people bragging about > | constructing a reflector out of bits of cardboard covered with aluminum > | foil). > | > | Comments welcome. > > I suspect that Lu, and the "many experts", are misinformed about this > matter. > > It's not a bit like light being reflected from a corner reflector, such as > that of a "cat's eye" used for road lane markings. (do those exist in the > US?). Well it is a BIT, but you have to take the differences into account. > And the main difference is due to the very different wavelengths being used. > > In the case of a mirror reflecting light, the dimensions of any mirror are > millions of times greater than the wavelength of the light, so the reflected > light behaves in a geometrical way; just like a mirror, indeed. > > In the case of a radar reflector, with wavelengths of a few cm., and the > dimensions of any mirror being only a few wavelengths, then "diffraction", > which some of you may recall from schooldays, plays a major part. The result > is that even the best reflector, unless is dimensions are absolutely > immense, reflects its energy in a spread-out, diffuse, maner, with a > beam-width of many degrees. It's for a similar reason that to get a narrow > beamwidth, the swinging radar scanners on ships are so immensely wide. > > Because the beam from any such radar reflector is so spread-out in angle, > that's why its efficiency in reflecting energy back to its sending point is > so small; much of the reflected energy missing the target. And that's one > reason why you get such an enormous increase in effective cross-section by > increasing the size of the reflector, as Gary has pointed out. > > But the benefit, the only benefit, that you will get from the situation of > reflectors being small(ish) measured in wavelengths, is that because the > reflection is so diffuse, there's no call, at all, to get any high precision > in the relative angling of the corner faces. I would go so far as to say > that if it looks, by eye, to be about 90 degrees between the faces, that's > good enough. And so, I suggest that the cardboard and foil solution, that Lu > derides, might well be just as good as an expensive reflector. > > Let me say that my contrary view to Lu's is not based on any experience, or > reading of authoritative texts, but just on pondering about the physical > principles involved. So it could be wrong; but I doubt if it is. Neither > physical optics nor radio propagation are "my subjects". > > If Lu can provide good authority for his claim that only a very accurate 90 > degree corner is good enough, then I will take it seriously. > > George > > contact George Huxtable at firstname.lastname@example.org > or at +44 1865 820222 (from UK, 01865 820222) > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > > > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListemail@example.com -~----------~----~----~----~------~----~------~--~---