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Basic Radar Plotting Plotting

Study Guide

This little guide on radar plotting is something I worked up to help a Mate on my boat get a handle on radar plotting, for an exam that he had to take. This does not teach you anything other than the basics of radar plotting (and plotting on a maneuvering board). There are plenty of texts on how radar works and how to operate it. This guide has been stripped down to the most basic information necessary to do a plot and still cover it. There are other great sources out there if you want to know more on the subject.

If all you are looking for is to plot a target and do a basic work-up, CPA, ect., this will get you there.

The basic plot

Basic Plot The basic plot we use on maneuvering boards and Radars consists of 3 course/speed vectors that form a triangle.

  1. "r → m"   is the ‘Relative’ Course/Speed Vector for the Target you are plotting
  2. "e → r"   is the Course/Speed Vector of your vessel is actually making (True)
  3. "e → m"   is the Course/Speed Vector the Target is actually making (True)

For ease of use, this example uses the following;

All directions are in True Degrees, speeds are in Knots, and the plot times are 6 minutes.

In the real world you can use whatever units work best for you, but these are pretty standard for very practical reasons.

  1. You plot a target at position "r".
    6 minutes later it is at position "m" (Fig 1). Fig.1
  2. From position "r", you plot the distance you travel in those 6 minutes, in the opposite direction you traveled. This gives you position "e" (Fig 2). Fig.2
  3. Plotting a line from "e" to "m" gives you the true course and speed of the target (Fig 3).

If you continue the line "r → m" past "m" and past the center of the board or scope, the distance from the center to the extended "r → m" line is your CPA (Closest Point of Approach) (Fig4).


To find the course needed to steer for a specified CPA

  1. Mark the Danger Zone boundary. This is the closest you want the target to get to you (Fig 5).


  2. Mark the Execution Point "x". This is the point at which you will take action, and maneuver to keep the CPA you are using (Fig 6).


  3. Draw a line from the Execution Point to the edge of the Danger Zone. This is your "r → m-new" line. This represents the the relative motion of the target after you maneuver (Fig 7).


  4. Transfer this line so it is parallel to "r → m-new", and starts at "m" going toward "r" (Fig 8).


  5. With a compass, anchor the point at "e" and swing a line from "r" to intersect transferred line "r → m-new". Where these lines intersect creates a new point "r-new" (Fig 9).


  6. "e → rą" is your new course to steer to make the desired CPA.


By adjusting your speed, you can change the length of "e → r", which changes "r → m’s" relative speed and direction.

Fig. 6

*** Remember that once "e" is set, it never is moved ***