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» Online Classroom   »   » Public Discussion of Marine Radar   » Minimum range

   
Author Topic: Minimum range
David Burch


 - posted November 26, 2004 10:59 AM      Profile for David Burch           Edit/Delete Post 
Minimum range is discussed in detail in Radar for Mariners along with example radar images. We have since found a more quantitative definition and want to add it here. We have updated the online Glossary, but this is not in the book.

The minimum range is defined by the shortest distance at which, using a scale of 1.5 or 0.75 nm, a target having an echoing area of 10 square meters is still shown separate from the point representing the antenna position. It is mainly dependent on the pulselength, antenna height, and signal processing such as main bang suppression and digital quantization.

This is from the Furuno manual that is included on the Radar Resources CD. Next step is to figure out what "main bang suppression" and "digital quantization" mean.

From: Starpath, Seattle, WA
Karl Koch


 - posted September 22, 2005 04:47 PM      Profile for Karl Koch           Edit/Delete Post 
Main bang suppression happens when the radar sends out a pulse. The electronic method of doing this is basically unimportant, but its function is to keep the outgoing pulse from overpowering the recieving circuits. The result is a blind area around the antenna because the radar cannot recieve while it is transmitting. The size of this blind area is determined by the pulse length. Without getting into the math, the result on small boat radar is an area with a radius of about 80 feet in which no targets can possibly be detected.
David Burch


 - posted September 22, 2005 06:48 PM      Profile for David Burch           Edit/Delete Post 
Thanks Karl for adding that clarification. For those who have not seen the book Radar for Mariners, this topic is covered on page 113, wherein the pulse length limit of 164 yd per microsecond of pulse length (typically 0.1 microsec at lowest scale) is increased by some 10 to 20 yds from the effects mentioned above to yield the suggested limits, which are illustrated in actual radar images in Figure 2-9.
From: Starpath, Seattle, WA


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