SUBSIM Radio Room Forums - View Single Post - Intercept Plotting

Pisces · 05-07-09, 10:31 AM

The method works perfect for ANY range. However, it only works if the course and speed of the contact is actually true. And likely it isn't! The course reported has a validity-fan of 11.25 degrees on either side (for example, if NE is reported it could really mean any course within 33.75 and 56.25). To easily plot these fan-edges is to draw a line of 175km along the stated course. Draw a 34km radius circle at the end. Draw 2 lines tangent to the circle. Why 34km at 175km? That distance is where the course edges start to fall outside of your hydrophone coverage. Any further and finding them is not guaranteed anymore if the course was near the edges. It is still possible finding it if it is on the inside though. But who knows that for sure?

If the actual speed of the contact is different from the reported speed (as it seems to be the rounded value, or it is stated as a speed-range) the intercept also looses chance of working. This means the intercept method in the link can only be an approximation.

Therefore very distant intercepts, based on these uncertainties are prone to fail. Especially considering it can ALSO change course in the mean time. My personal solution to this problem is to make the drawing based on worst cases, and the 175km 'contact maximum move' rule :

speed:
- upper-end of the speed-range for single units. (slow=7.5 knots, medium=11.5 knots)
- add 0.5 knots to the reported convoy speed

course:
- assume course is the 11.25 edge that has AOB nearest to 90 (at the start of the intercept).

Once you reach that edge of the course-fan and you can't hear anything (yourself) with the hydrophone you should attempt another intercept based on the other course edge. But with the expected position of that moment. Not the old location. So you need to plot where he would be now, based on how much time has passed since the icon appeared. NEVER forget to write down that time!

If you chased a single unit with a worst-case speed it may not yet be inside your hydrophone range when you are on the central-course. But you can safely assume you are ahead of him. It just requires patience to wait for him while being on the central course. Or you move to meet him along the central course (but ofcourse going in opposite direction) while doing frequent hydrophone checks.

And ofcourse, it all depends on how accurate you made the drawing in the first place. Perhaps the drawing is more accurate if you make the circles with a scale of 10 kilometers for each knot.

05-07-09, 10:31 AM	#5
Pisces Silent Hunter Join Date: Dec 2004 Location: AN9771 Posts: 4,904 Downloads: 304 Uploads: 0	The method works perfect for ANY range. However, it only works if the course and speed of the contact is actually true. And likely it isn't! The course reported has a validity-fan of 11.25 degrees on either side (for example, if NE is reported it could really mean any course within 33.75 and 56.25). To easily plot these fan-edges is to draw a line of 175km along the stated course. Draw a 34km radius circle at the end. Draw 2 lines tangent to the circle. Why 34km at 175km? That distance is where the course edges start to fall outside of your hydrophone coverage. Any further and finding them is not guaranteed anymore if the course was near the edges. It is still possible finding it if it is on the inside though. But who knows that for sure? If the actual speed of the contact is different from the reported speed (as it seems to be the rounded value, or it is stated as a speed-range) the intercept also looses chance of working. This means the intercept method in the link can only be an approximation. Therefore very distant intercepts, based on these uncertainties are prone to fail. Especially considering it can ALSO change course in the mean time. My personal solution to this problem is to make the drawing based on worst cases, and the 175km 'contact maximum move' rule : speed: - upper-end of the speed-range for single units. (slow=7.5 knots, medium=11.5 knots) - add 0.5 knots to the reported convoy speed course: - assume course is the 11.25 edge that has AOB nearest to 90 (at the start of the intercept). Once you reach that edge of the course-fan and you can't hear anything (yourself) with the hydrophone you should attempt another intercept based on the other course edge. But with the expected position of that moment. Not the old location. So you need to plot where he would be now, based on how much time has passed since the icon appeared. NEVER forget to write down that time! If you chased a single unit with a worst-case speed it may not yet be inside your hydrophone range when you are on the central-course. But you can safely assume you are ahead of him. It just requires patience to wait for him while being on the central course. Or you move to meet him along the central course (but ofcourse going in opposite direction) while doing frequent hydrophone checks. And ofcourse, it all depends on how accurate you made the drawing in the first place. Perhaps the drawing is more accurate if you make the circles with a scale of 10 kilometers for each knot. __________________ My site downloads: https://ricojansen.nl/downloads Last edited by Pisces; 05-07-09 at 11:30 AM.