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Jan 1, 2003
 From Ocean Navigator #118 November/December 2001
According to Rule 7b of the Colregs, vessels equipped with an operational radar set are obligated to use it properly, 'including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.' Using the radar for collision avoidance is a straightforward proposition. And in order to do this, a navigator needs to have a basic understanding of relative motion and a few simple tools.

Take the range and bearing to a target, mark the point R and note time. Take another reading six minutes later and mark it as M, again noting time. Connect these two points and draw the line past the crosshairs at the center of the plotting sheet. The resulting line is called the relative motion line.
Image Credit: Andy Howe

First, relative motion. If our vessel (at the center of the screen) is standing still, then every object showing movement on the radar screen is showing a true (actual) motion. As soon as our vessel begins to move forward, it imparts a motion on all targets on the radar display (on the most common heading-up display, our motion in effect slides everything straight down the radar screen). This results in our display showing relative motion, a combination of our vessel's actual motion and the target's actual motion.

To get the most information out of the radar, the navigator must separate the two motions from each other. The first goal is to answer the question, 'Is the target going to hit us?' Next, if needed, the navigator can come up with the target's actual course and speed. This is often called 'solving the RTM triangle' (RTM is an acronym from the words relative, true and motion). There are two ways to do this. One is to let the radar do it; radar with ARPA (Automatic Radar Plotting Aid) or MARPA (MiniARPA) installed can do this automatically but requires input from an electronic compass. Or the navigator can use a maneuvering board plotting sheet and simple nav tools.

The process for plotting and analyzing a target on a maneuvering board takes just a few minutes. Here are the steps needed to effectively plot a target and resolve its actual motion:

1. Identify the target and determine its range and bearing using the features on the radar. Note the time (only the minutes need be recorded).

2. Plot this position on the plotting sheet, marking the time. This point is labeled R.

3. Wait six minutes and repeat steps 1 and 2, marking the time. This point is labeled M. More later on why we choose an interval of six minutes.

4. Extend a line from R through M and past the center of the sheet. This line is called the direction of relative motion (DRM) line.

5. Examine the DRM line: how close it passes to the center of the sheet (where your vessel is located) yields the closest point of approach (CPA) of the target vessel. If both the target vessel and your vessel maintain the same course and speed, the target will plot right down this line. If the line passes through the center of the sheet (where your vessel is located), there will be a collision. If this is the case, or if you decide that the CPA is too close (say, within a mile), then action must be taken. If the DRM line drops straight down the chart (paralleling the motion of your vessel) the target could be a buoy or a vessel dead in the water. Check the chart!

 Image Credit: Andy Howe Using dividers, measure the distance to the target covered between min. 02 and min. 08. Take this distance and measure off six minute intervals down the DRM line. This will allow you determine CPA and TCPA.

6. Measure the number of six-minute 'intervals' (the distance from R to M) from M to the CPA point by walking the dividers down the DRM line to determine the time of CPA (TCPA).

7. To determine the target vessel's course and speed, your vessel's motion must be taken out of the plot. First, drop a line straight down the sheet from R. Using the scales on the sheet (making sure the scale matches the radar scale) make the length of this line equal to the distance your vessel travels in six minutes. Mark this point T. (To derive the length of the line, divide your speed by 10. For example, if traveling at 6.5 knots, your vessel will travel 0.65 nm in six minutes).

8. Connect T to M to complete the RTM triangle. The length of line TM is the speed of the target vessel. (Reverse the process described in step 6. For example, if the line measures 0.65 nm, the vessel speed is 6.5 knots.) If RM and RT are right on top of each other (no triangle exists), then it is likely the object is not moving.

9. To resolve the course of the target vessel, bring the line to the center of the sheet using parallel rules. On the compass rose, read off the direction this line points toward. Then, since the plot is set up in relative directions, add your vessel's course to this derived direction to get the target vessel's course, subtracting 360° if the result is greater than 360°.

 Image Credit: Andy Howe To determine the true motion of the target, construct the true motion triangle by dropping a line straight down from R based on your vessel's speed. Mark this point T. Then connect T to M. The length of this line will be the target's speed, the bearing of this TM line will be the targetUs true course.

You may need to take action to avoid collision or to 'open up' the CPA. Refer to Colregs Rule 8 for specifics. But generally, action should be made early and distinctly (small course changes are hard to detect). Turns to starboard are favored over turns to port. When in doubt, come to all stop and attempt to hail the target on the VHF.

A complete explanation of target analysis and recommended actions to avoid collision is beyond the scope of this article. Prioritizing targets for watching and plotting is helpful. A simple explanation is that targets forward of the beam on your starboard side should be first priority. These are the ones showing you a red light in a crossing situation, making you the give-way vessel. Vessels forward and to port are next, as they will show you a green light, indicating that you are the stand-on vessel. Targets aft of the beam are considered to be overtaking vessels and must give way to you.

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