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 07, 20:48 -0700
From: Lu Abel
Date: 2007 May 07, 20:48 -0700
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. Lu Abel Gary LaPook wrote: > Gary wrote: > > I read this entire report which was very interesting. What will put the > fear of god in you is the finding that even with their Davis radar > reflector mounted, that the probability of detection was still minimal. > Allot of sailors rely on those things. > > I looked at the issue of radar cross section in the past and built my > own radar reflector. The standard reflector, like the one in this > incident consists of three mutually perpendicular plates which form 8 > corners so as to reflect radar energy back in the direction that it came > from, these corners are aligned to cover the horizon. Unfortunately, > there are deep nulls in the reflective pattern so there are many gaps in > the pattern. > > The Davis reflector on this boat is 12.5 inches in diameter meaning that > the edge where the plates meet to form each corner is only 6.25 inches. > Only one of the corners is effective for any particular radar beam so > the rest of the reflector has no value, the strength of the echo is > determined by the one corner or "bucket" facing the radar transmitter. > If you do the calculation for a bucket with a 6.25 inch edge circular > edge (such as the davis) for x-band radar (9.4 ghz, lambda or wavelength > of 3.2 cm) you find the radar cross section has a maximum value of 9.7 > meters ^2. If you do the same calculation for an s-band radar (3 ghz, > 10cm wavelength), the kind used by large ships when off soundings, you > find only 1.0 meters^2 since the effectiveness of a corner reflector > varies inversely with the square of the wavelength. > > The effectiveness also varies with the shape of the plates and with the > length of edge where the plates are joined. A corner reflector made of > plates that are square rather than round, as in the davis, returns an > echo 2.42 times as strong than the round plates. But the main > determinant of strength is the length of the edge. The strength of the > echo varies with the length of the edge to the fourth power. If you > double the length of the edge you increase the echo 16 times! So if you > combine these two factors and make a radar reflector with square plates > that is twice as big you increase the strength of the echo by almost 39 > times. So if you make a reflector that has 12.5 inch buckets the RCS is > 379 meters^2 for an x-band radar and 38.4 at s-band. > > O.K. this would be too big to hoist aloft. > > So what I decided to do was to make a radar reflector with just one > bucket that I could aim at any ship that I thought might be a hazard. > This would be much more effective than shinning a flashlight on the > sails since it would set off the automatic alarm on the radar scope. I > got 3 sheets of aluminum 12 inches square and joined them with two > hinges so that it can fold flat when not in use. It is a simple matter > to take it out and unfold it and aim it at the ship. Opened, it is > slightly smaller than the Davis radar reflector but returns an echo in > the direction of the radar of 318 meters^2 at x-band and 32.6 meters^2 > at s-band, thirty three (33) times stronger than the standard radar > reflector. > > I haven't been run over yet. > > gl > George Huxtable wrote: > > >>Fatal interaction between yacht and ferry. >> >>There's quite a lot to ponder, in the highly workmanlike report of the UK's >>Marine Accident Investigation Branch, into the sinking last Summer of the >>yacht "Ouzo", off the Isle of Wight, resulting in the death of all three >>aboard her. >> >>Go to- >> >>http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm >> >>and from that download the fifty-odd page report Ouzo_.pdf at about 1.4 MB. >>Other options are a short synopsis, and a bulky technical appendix, mainly >>about photochromic spectacles and radar detection of small craft. >> >>The MAIB brief is to find out what happened, and it seems to me they have >>bent over backward to avoid apportioning blame. >> >>The second officer of the P & O ferry, "Pride of Bilbao", is due to stand >>trial for manslaughter, later this year, as a result. >> >>My reaction, and I suspect that of many other sailors, is to think "There, >>but for the grace of God, go I". And no doubt many ship's officers will, > >>from another viewpoint, think exactly the same, . > >>It surprises me that the master, though he was below at the time, appears >>not to be bearing any of the responsibilty. >> >>I won't comment further, to avoid prejudicing those that prefer to make up >>their own minds. Read it yourself. >> >>George. >> >>contact George Huxtable at george---.u-net.com >>or at +44 1865 820222 (from UK, 01865 820222) >>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. >> >> >> >> >>contact George Huxtable at george---.u-net.com >>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 -~----------~----~----~----~------~----~------~--~---