 | From Ocean Navigator #102 November/December 1999 |
Figure 1: Radars with an echo trail feature produce a line on the screen that is, in effect, a relative motion line.
The radars found on most yachts have small screens and usually have raster scan, heading-up, unstabilized type displays. Unlike the radar sets found on large ships, yacht radar units lack the reflective layer that allows for plotting directly on the screen. Because of limited space and time constraints, transfer plotting is not practical. Unfortunately, experience shows that, without use, plotting skills deteriorate. To keep these skills sharp, Post-It notes and the use of echo trails (where the radar displays a “trail” of the target’s positions during some time interval), or the plotting feature on certain radar units can be used to substitute for plotting with pencils and rulers. The Post-It note method seems to be quick and easy to use. And it deals with the four common complaints voiced by many mariners about rapid radar plotting.I don’t have a reflection plotter
In exchange for a reflection plotter, the plot feature on certain small-screen radars allows the operator to view the relative track of the target at selected intervals of 15, 30, or 60 seconds, or more. A continuous track of the target with a timer that counts up in seconds can also be selected. In figure 1, a continuous echo trail has been selected and allowed to run for three minutes. This is the equivalent of a three-minute relative motion line for the target when rapid radar plotting.I don’t stay on course long enough to plot a target.
To this statement the question is asked, “Do you stay on course for three minutes?” The answer is usually “Yes.” The plot feature allows the operator to note the time the target began tracking and choose a time interval that is appropriate for the vessel, the range scale used on the radar and the speed of the vessel. In the example shown in figure 2 below, our vessel is moving at a speed of eight knots. A time interval of three minutes is selected. Using the six minute rule, a vessel moving eight miles in 60 minutes will move 0.8 miles in six minutes (1/10 the time and 1/10 the distance). In order to find the distance traveled in three minutes, the distance for six minutes is cut in half and a vessel moving 0.8 miles in six minutes will move 0.4 miles in three minutes (1/2 the time and 1/2 the distance).
The radar range scale in use is three miles. A distance of 0.4 miles is measured on the radar using the Variable Range Marker (VRM). Place the Post-It note parallel to the heading flasher and the upper left or right corner touching the 0.4nm VRM. Mark the Post-It note at the corner and at the start point of the heading flasher. This measured distance on the Post-It note is the equivalent of a three-minute segment of our vessel’s movement. It is the equivalent of the “er” vector in rapid radar plotting. Repeat the process for the other corner/side of the Post-It note. Once made, the Post-It note will work for that range scale and speed, and can be stuck to the side of the radar ready for use at any time. Other scales can be made for different speeds or ranges as needed. This process only takes a few seconds and can be done “on the spot.”I don’t have time to plot.
The echo trail feature of a radar allows the radar user to “systematically observe” the movement of vessels. The echo trails alone, however, will not give a user much more information than which targets are collision threats. The Post-It note will allow him or her to obtain more information. This includes the aspect of the target as well as the ability to obtain the approximate course and speed of the target. Assume in this example that our course is 270° at a speed of eight knotssee figure 3 below. To obtain the course and speed of the target, place the corner with the first mark on the Post-It note at the beginning of the target trail or plot echo parallel to the heading flasher. Observe the direction of a line that would connect the second mark on the Post-It note with the target. This line indicates the course of the target (indicated by the red line). The speed of the target during the three-minute time period can be compared with the distance we would travel over three minutes as indicated by the two marks on the Post-It note.
If you drew a line from the second mark to the target at the end of a three-minute interval you could determine the target’s course relative to our heading of 270°. The dashed EBL line shown above is parallel to the line drawn from the Post-It note to the target position at minute three. It has to be read in the direction from the Post-It note to the target (hence the solid line in the direction of 260). With our heading of 270° the relative bearing will read 260°. If you add 260 and 270 (530) and then subtract 360 the target’s true course is found to be 170°. The length of the line is a little shorter than the distance between marks on the Post-It note. This length could be measured at about 0.35nm in three minutes, which translates to about seven knots. This line is the equivelent of the target course and speed vector “em”in rapid radar plotting.
A second example is shown in figure 4 for a target on a reciprocal course at a speed approximately equal to ours. We hope the Post-It note method will assist you in your efforts to “systematically observe” all targets.
