[Etienne]

Radar plotting (Or TMA, if you will) is really, really simple, and described at great lenght in just about any book about collision avoidance, navigation control, or radar. Dutton's Navigation and Pilotting, IIRC has a chapter about plotting. So does Burger's Radar Operator Handbook, Sonnenberg's Radar and Electronic Navigation, and so on.

It's freaking easy, if you can do vectors graphically. First, as you detect the target, mark your position (And, if you want to work with relative vector, trace your course).

Using the range circle, mark the target range. Then, with the protractor, mark the bearing. This is point O (Origin).

While you wait for your next point, we'll trace vector MO (Motion of ownship). Decide over what period of time you'd like to plot - 6, 8, 10, 12, 15 are all popular. Too short, and you'll end up with very small vector. Too long and, well, the target might manoeuver and screw up the plot. 8 or 12 minutes are usually good.

Vector MO has to be the distance traveled by your ship, in the direction travelled by your ship, during the plotting. Imagine that a ship maintaining formation with you will arrive at point O at the end of the plot.

*Deep breath* All good? Ok, I,ll try to rephrase MO again. Suppose you have decided to plot for 12 minutes. (1/5 of an hour, so that's why it's such a popular interval) Your vessel is travelling at 10 knots, course 090. 10 kts / 5 = 2 NM for 12 minutes, bearing 090. Starting from point O, trace a line in the reciprocial heading of your course (270, in this case) and, with the range circle, measure the distance traveled. This is point M.

Note that the game makes this an unnecessary pain in the butt, as the distances on the map are in KILOMETERS.

After half of your plotting interval, (6 minutes for a 12 minutes plotting), mark the target on the chart, using its range and bearing FROM YOUR INITIAL POSITION. This mark is just there to make sure the target is not manoeuvring.

At the end of the interval (IE, 12 minutes after the initial mark), mark the position of the target, relative to your initial position (Point T). Draw a line linking the initial mark (Point O) thru the middle mark and the final mark (Point T) - This is vector OT, Origin To cpa. Continuing this line will give you range and bearing of CPA. (And time, if you bother) The line probably won't go through all the points ; some deviation is to be expected. Use your judgement ; if the spread is too large, the target is manoeuvring. If you used four points (IE, a mark every three minutes for 12 minutes), you'll get a nice curve if the target turned.

Draw a line from point M to point T. This vector is Motion of Target. It represent the true motion of the target - Course and distance travelled in the interval. Vector OT is the relative motion of the target.

To measure AOB, place the protractor reference line on the extension of vector MT, with the angle at point T. Use the other line to connect your initial position.

That's how one does a relative plot. I've just realised that SHIII's gameplay makes a true motion plot much, much easier... If anyone's interested, I can write that up as well, but it's foolishly simple.