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    Re: Fatal interaction betweeu yacht and ferry.
    From: Gary LaPook
    Date: 2007 May 08, 17:52 -0700

    Gary writes:
    
     Absolutely, that comes from the standard radar equation. To double
    the detection range you need to increase the power of the echo by a
    factor of 16, holding everything else constant, and this can be
    accomplished by increasing the power of the transmitter  or increasing
    the size of the radar cross section.  Since you can't control the
    output power of the other guy's radar the only thing you can do is
    increase the size of the radar reflector. (Or you can use an active
    transponder.) So you are right, after you do the math the detection
    range increases linearly with the size of the radar reflector, double
    the size and you can be detected at double the range, which is not a
    bad thing..
    
    But what is really critical is not the distance that your boat can be
    detected but whether it can be detected at all. With sea clutter,
    unless your boat produces an echo above that of the existing sea
    clutter you will not be detected at all. An echo that is 16 times as
    strong because you are using a reflector twice as big has a very good
    chance of lifting you out of the clutter so you can be detected. Even
    just doubling the power by using a reflector 19% bigger may make all
    the difference in being detected or run down.
    
    On May 8, 4:42 pm, Paul Hirose  wrote:
    > glap...{at}PACBELL.NET wrote:
    >
    > > I don't think most sailors realize how much size matters when it comes
    > > to radar reflectors and may be tempted to "just get the slightly
    > > smaller one." Since the echo varies with the fourth power of the size
    > > a small change in size  will make a large increase in echo strength.
    > > Changing the size by 19% changes the echo by a factor of two since
    > > 1.19 is the fourth root of two.
    >
    > On the other hand, isn't it true that detection range varies with the
    > fourth root of the radar cross section? If so, the detection range of
    > the reflector alone would be proportional to the linear size of the
    > reflector.
    >
    > --
    > I block messages that contain attachments or HTML.
    
    
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