# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Fatal interaction between yacht and ferry.
From: Gary LaPook
Date: 2007 May 10, 04:51 -0700

```Gary wrote:

The report on radar reflectors available at
http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm
that you mentioned proves my point about the advisability of having a
radar reflector of sufficient size to point at the approaching ship.
Figure 1 shows the probability of being detected based on the radar
cross section of the radar reflector and on real world conditions
including surface clutter and multipath reflection. The theoretical
detection range predicted by the radar equation assumes perfect
conditions and was actually developed in conjunction with development
of air defense radars designed to detect incoming Russian bombers and
is also used to calculate the effectiveness of radar jammers carried
by our planes and the point of "burn through."

One of the first things to notice from this graph is that the
detection range does not follow the idealized fourth root rule. The
graph show the maximum detection of a 1 m^2 RCS target of about 9 nm
for the 60% probability case. If the fourth root rule worked in this
type of situation then the 10 m^2 RCS target should have been detected
at 16 nm at the 60% probability level instead of 10.5 NM as shown on
the graph.

The main point, however is looking at the probability of detection
when within 2 miles. It is obvious that the 10 m^2 RCS target has a
much greater chance of being detected than the 1 m^2 target, with
peaks up to about 90%. Using the one bucket reflector, 12 inches on a
side, that I described, you would be showing a 300 m^2 RCS to the
radar so the probability of detection should be about 100% since it is
15db stronger than the 10 m^2 target.
.

On May 8, 8:52 pm, Paul Hirose  wrote:
> George Huxtable wrote:
> > 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.
>
> I'm guessing that the beamwidth (radians) is on the order of 1 over the
> width of the reflector expressed in wavelengths. So at X band (3 cm
> wavelength), a reflector 30 cm aross will have a beamwidth of roughly
> 1/10 radian, or 6 degrees.
>
> > 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.
>
> In optics it is said that an image will not fall seriously short of
> perfection if the peak to valley errors on the wavefront do not exceed
> 1/4 wave. Perhaps that applies to the geometry of corner cube reflectors
> as well. That is, if all the surfaces coincide with a perfect corner
> cube within some fraction of a wavelength, then the final reflection
> will be "diffraction limited".
>
> A companion report comparing the performance of several radar reflectors
>   is now available at the MAIB:
>
> http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm
>
> --
> I block messages that contain attachments or HTML.

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