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» Online Classroom   » Radar   » Public Discussion of Marine Radar   » Adjustable sweep rates

Author Topic: Adjustable sweep rates
David Burch

 - posted November 25, 2004 10:24 PM      Profile for David Burch           Edit/Delete Post 
One topic that is not covered in the Radar for Mariners is the radar control that lets users change the rate that the antenna rotates. This is not a common control on small-craft radars yet, but it may well become so in the future.

At present most units have a single antenna rotation rate of about 24 rpm, which corresponds to 60/24 = 2.5 seconds per revolution. Several units in the past and more in the future will add the option of increasing this rotation speed. Usually there is an option for normal or standard (24 rpm) or fast (48 rpm).

Like several other control options, there are pros and cons. The call for a fast option is primarily driven by the increasing presence of very fast vessels. A look around you every 2.5 seconds can be dangerously slow on a low range in the presence of vessels that travel 50 kts, or more, or if you yourself are one of these faster vessels then you would want to consider this option.

Recall that we can figure speed in minutes per mile by dividing 60 by boat speed. At 6 kts it takes 10 min per mile, at 10 kt it takes 6 min. At 20 kt it takes 3 min per mile, and so on. A vessel travelling 60 kts, is covering 1 mile in 60 seconds or one quarter of a mile in 15 seconds. And if you are both moving at 60 kts, the speed of relative motion could double that.

In short, you want to see as quickly as possible what these very fast vessels might be doing, looking every 1.25 seconds rather than every 2.5 or 3 seconds is an improvement.

The price you pay is less wave packets strike the target as it goes by and thus you have lower sensitivity to weak targets. This is discussed in Chapter 7.

An advantage might be for tuning or optimizing the picture. Generally you have to do these adjustments very slowly since the picture is redrawn only every 3 seconds. You must tweak a knob slightly, wait 3s and try again to see your results. With a fast scan option, you might be able to tune the unit more quickly... but i have not tested this idea in practice.

From: Starpath, Seattle, WA

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