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Method of Determining

Enemy Course & Speed

Using SJ Radar


Introduction

This procedure is to help New Skippers to enable them to improve their use of radar when on patrol. I should stress that it is NOT 100% accurate, sources of error will be shown, nor is it particularly good with enemy task groups or CL groups whose speed is high (20kts +) and who are zig zagging. However this procedure will, I hope, add to your enjoyment of the game.

I imagine that when a call comes from the SJ operator "Target bearing 095" most skippers will immediately alter course 95 degrees to stbd and come to flank speed to bring the target on the nose and thereafter will alter heading progressively to maintain this condition. However this generally results in steering a curved line to the point where visual contact is made.

The following procedure looks at first sight to be a mass of lines more suited to a geometry lesson than fighting the foe. I hope to show that it is relatively straightforward and once the principle is understood all will be made clear.

Requirements

1. A sheet of plain or squared paper. A4 is suitable.

2. Protractor. I use a 5ins (125mm) square one.

3. Ruler graduated in either mm or inches.

4. A soft pencil (2HB).

5. An eraser.

Optional. A 90 deg setsquare.

Principle

This procedure is based on a navigational technique known as a 'Running fix' but used here with two moving points.

In the below diagram I have shown TRUE headings and courses in BLACK and RADAR bearings and ranges in RED. Click on the image thumbnail for a larger picture.

Method of Determining Enemy Course and Speed using SJ Radar Bearings

Method

Example. At 0604hrs SJ radar reports a contact bearing 055R range 35,500yds. You are on a course 235T at 4.8kts.

1. When a contact is first reported note the following: time, bearing and range to contact; own heading and speed. (Note 1).

2. On the sheet of paper make a point 'A' and from it draw a line to represent your heading. Make a note here of the time. See diagram. (Note 2)

3. From point 'A' mark off a line bearing 055R and on it measure off the reported range. I recommend using 1 cm, or 0.5 ins per 2000yds, Mark this point 'P' and against it note the time. See diagram. (Note 3) If you have paused the game restart it now.

4. At this point there are two ways to proceed. You can either alter course toward the target or remain on the present heading and speed. I prefer the latter option. (Note 4) Whichever option is chosen allow a period of time to elapse. This should be 15, 20 or 30 minutes. (My own preference is 20 mins).

5. After the elapsed time (20 mins in this example) take a new SJ bearing and range, (In this case Brg 073R 25,310yds) again making a note of the time, 0624hrs, and your own course and speed.

6. On the paper: On the line representing your heading from point 'A' mark off the distance traveled in 20mins. In this case with 1cm = 1 kt a distance of 1.6cms. ( i.e. 1/3rd of an hour at 4.8 kts). Mark this point 'B' and note the time against it. See diagram. (Note 6)

7. From point 'B' draw a line of bearing, 073R, and on it mark off the range, 25310yds. Make this point 'Q' and note the time, 0624, against it.

8. Join points 'P' and 'Q'. You may wish to extend this line some way. More of this anon.

9. The direction of the line 'P' - 'Q' represents the Mean Line of Advance (MLA) of the target and its length the distance covered in 20 mins. If the target has maintained a steady heading then the MLA is the course. In this example a MLA of 083T and a speed of 15.9kts.

10. If so desired the procedure from 4 to 9 can be repeated, starting from point 'B'.

I mentioned extending the line 'P' - 'Q' If you do this then use your rule and 90 deg set square to draw a line from it to point 'B' it will give the course and shortest distance to intercept the target. In the example point 'R' is the intercept. From Q to R is 18,000yds, 9nm, and at 15.9 kts the target will get there in about 36 minutes. From point 'B' to 'R' is also about 9nm and at a flank speed of, say, 19kts will take your sub about 30 mins giving you a few minutes for final positioning.

Note 1. At this point I would suggest pausing the game until familiar with the procedure.

Note 2. The position of this first point on the paper is a matter of guesswork to begin with but if the target is to stbd of your heading then place it toward the left hand side if the sheet and vice versa.

Note 3. Here is the first cause of error. Because of the scale it is difficult to work to better than, say, 250yds so rounding up or down may be necessary.

Note 4. I maintain my course and speed because I feel that altering either at this point induces errors due to the time taken to reach a steady speed on the new heading, as will become apparent when plotting the second position.

Note 6. If you have chosen either 15 or 30 min. interval then the measurement will be 1.2 or 2.4 cms.

Comments

I have found that this method gives reasonable accuracy providing care is taken. I said at the beginning that this method was not much good with Task Groups and CL groups. The reason is that in the interval between taking bearings a large course change can have taken place and this combined with high speeds will make nonsense of you calculations. It becomes fairly obvious when this happens. I might add though that if you can take two or possibly three further bearings and find that the MLA is all over the place then you might guess that the average of these is the base course of the TG.

I noted in the introduction that this was not 100% accurate and there are several reasons for this.

1. Carelessness in plotting.

2. Rounding the range up or down. A good eye will allow more accurate measuring. I work to 125yds and 0.1kts.

3. The difficulty in picking the same ship each time a bearing is taken.

4. The bearing from one edge of the convoy to the other can be as much as 10 degrees.

For these last two reasons I recommend using the radar in the A scope mode. A nautical mile is slightly more than 2000yds (2026.6yds), and this too creates a small error.

Patrick Malone. RVM
April 2004
Copyright © 2004 Patrick Malone. Used with the author's permission.