Basic Radar Plotting Plotting
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
The basic plot we use on maneuvering boards and Radars consists of 3 course/speed vectors that form a triangle.
- "r → m" is the ‘Relative’ Course/Speed Vector for the Target you are plotting
- "e → r" is the Course/Speed Vector of your vessel is actually making (True)
- "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.
- You plot a target at position "r".
6 minutes later it is at position "m" (Fig 1).
- 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).
- 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
- Mark the Danger Zone boundary. This is the closest you want the target to get to you (Fig 5).
- 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).
- 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).
- Transfer this line so it is parallel to "r → m-new", and starts at "m" going toward "r" (Fig 8).
- 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).
- "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.
- Increasing speed results in the longer "e → rą"
- Decreasing speed results in the shorter "e → r˛"
*** Remember that once "e" is set, it never is moved ***