3 minute rule, using imperial measures, there are many variations, but this is how I do it:
step #1- identify target, determine range and bearing;
step #2- plot range and bearing on NAV map as Mark #1;
step #3- start chronometer, wait 3 minutes;
step #4- determine new range and bearing;
step #5- plot new range and bearing on NAV map as Mark #2;
step #6- measure distance between Mark #1 and #2 (this requires a bit of a guesstimate, since the ruler only measures tenth of nautical miles)
step #7- multiply distance traveled x 20 = nautical miles per hour ( 1 knot = 1 nautical mile per hour).
The speed estimate is often 10-20% off, but it gives you a trial speed to plug into the PK. Because you are manually plotting the target's course on the map, it also allows you to doublecheck that you have the correct AOB.
ps - you can also use the Nomograph to calculate step #7 - it works very well, I dont use it because I find it clutters up the map, but that is a personal choice.
|