Relative motion problem

jmr · 05-10-09, 05:23 PM

Here's an old post on how to use the in game map as a maneuvering board. I set my map accordingly and plot the two contacts and then perform the final step of vector addition to get the target's true course and speed. My solution matches the example in that post.

However, when I replicate that same problem in Mobo I get a different result.

Here's my plot using the in game map.

And here's the same problem using Mobo. Notice the target's true course and speed are wildly different from the worked example done using the in game map.

Not being the sharpest tool in the shed, can you explain why the results differ?

XLjedi · 05-10-09, 08:34 PM

The result in MoBo looks right...

But the drawing on the in-game map doesn't look right. To get the correct result with the in-game tools you would need to...

1) Draw a 1300m radius from your sub as a scale vector to indicate speed and heading.

2) Move the 3200m radius/vector to the terminal point of you sub 1300m radius vector

3) Draw a new radius from your sub to the new terminal point of the 3200m radius after you moved it.

You should see the new radius points 188° and has a 2740m radius (heading 188° 27.4kts)

jmr · 05-10-09, 08:47 PM

The thing is, the original poster, through using the in game map as a make shift mobo, ended up with a true course/speed of 271/11.6

XLjedi · 05-10-09, 08:59 PM

Uhhh... hold on... I'll look at it again.

XLjedi · 05-10-09, 10:26 PM

OK, without having the graphics from the OP, makes it a little challenging, but I can see from your graphic what's happened.

You don't have the same scale as the OP...

The OP is using the 3:15 rule to determine speed. He's using the line tool to measure the distance between M1 and M2 and getting a length of 4 (or 4kts).

You must be using a significantly different scale to get a speed of 33kts between M1 and M2. I can tell you that for the OP to end up with a 270° heading at 11kts the speed had to be 4kts, not 33kts for the 3:15 ellapsed time. (makes more sense too, was there even a 20kt+ unit in SH3?)

...and that's why your result is so much different. Lemme see if I can put it MoBo so the 4kt result falls out.

XLjedi · 05-10-09, 10:58 PM

Ya know... I'm looking at that example again, and I don't know what to tell ya; it just looks like the OP is wrong.

Based on the bearings and distances stated, the contact moved over 3300 meters (2 miles) in just 3 minutes? There's just no way you'd get a 4kt speed (relative motion) from that...

I dunno, maybe I'm doing something wrong.

I should go get Lurker to take a look...

05-10-09, 05:23 PM	#1
jmr Commander Join Date: Sep 2004 Posts: 462 Downloads: 62 Uploads: 0	Relative motion problem Here's an old post on how to use the in game map as a maneuvering board. I set my map accordingly and plot the two contacts and then perform the final step of vector addition to get the target's true course and speed. My solution matches the example in that post. However, when I replicate that same problem in Mobo I get a different result. Here's my plot using the in game map. And here's the same problem using Mobo. Notice the target's true course and speed are wildly different from the worked example done using the in game map. Not being the sharpest tool in the shed, can you explain why the results differ?

05-10-09, 08:34 PM	#2
XLjedi Ace of the Deep Join Date: Jun 2005 Location: Palm Beach, Florida Posts: 1,243 Downloads: 53 Uploads: 8	The result in MoBo looks right... But the drawing on the in-game map doesn't look right. To get the correct result with the in-game tools you would need to... 1) Draw a 1300m radius from your sub as a scale vector to indicate speed and heading. 2) Move the 3200m radius/vector to the terminal point of you sub 1300m radius vector 3) Draw a new radius from your sub to the new terminal point of the 3200m radius after you moved it. You should see the new radius points 188° and has a 2740m radius (heading 188° 27.4kts) __________________ Last edited by XLjedi; 05-10-09 at 08:56 PM.

05-10-09, 08:59 PM	#4
XLjedi Ace of the Deep Join Date: Jun 2005 Location: Palm Beach, Florida Posts: 1,243 Downloads: 53 Uploads: 8	Uhhh... hold on... I'll look at it again. __________________

05-10-09, 10:26 PM	#5
XLjedi Ace of the Deep Join Date: Jun 2005 Location: Palm Beach, Florida Posts: 1,243 Downloads: 53 Uploads: 8	OK, without having the graphics from the OP, makes it a little challenging, but I can see from your graphic what's happened. You don't have the same scale as the OP... The OP is using the 3:15 rule to determine speed. He's using the line tool to measure the distance between M1 and M2 and getting a length of 4 (or 4kts). You must be using a significantly different scale to get a speed of 33kts between M1 and M2. I can tell you that for the OP to end up with a 270° heading at 11kts the speed had to be 4kts, not 33kts for the 3:15 ellapsed time. (makes more sense too, was there even a 20kt+ unit in SH3?) ...and that's why your result is so much different. Lemme see if I can put it MoBo so the 4kt result falls out. __________________

05-10-09, 10:58 PM	#6
XLjedi Ace of the Deep Join Date: Jun 2005 Location: Palm Beach, Florida Posts: 1,243 Downloads: 53 Uploads: 8	Ya know... I'm looking at that example again, and I don't know what to tell ya; it just looks like the OP is wrong. Based on the bearings and distances stated, the contact moved over 3300 meters (2 miles) in just 3 minutes? There's just no way you'd get a 4kt speed (relative motion) from that... I dunno, maybe I'm doing something wrong. I should go get Lurker to take a look... __________________ Last edited by XLjedi; 05-10-09 at 11:21 PM.

05-10-09, 08:47 PM	#3
jmr Commander Join Date: Sep 2004 Posts: 462 Downloads: 62 Uploads: 0	The thing is, the original poster, through using the in game map as a make shift mobo, ended up with a true course/speed of 271/11.6