SUBSIM: The Web's #1 resource for all submarine & naval simulations since 1997 
07072021, 07:52 AM  #1 
Swabbie
Join Date: Mar 2014
Location: Land o' the Hoosiers
Posts: 10
Downloads: 11
Uploads: 0

Accounting for distance and AOB?
Hey all, I have a question that I cannot seem to find an answer to.
When calculating a target's speed (Lengthmeters / Timeseconds X 1.94), I've found that the results are consistently inaccurate when compared to the speed as listed on the tutorial's map view. Sometimes it's within that golden +/ 0.2kts but most of the time it's not. So, I've been trying to figure out how to minimize these errors as much as possible. All that said, and assuming my own speed is 0kts, I was curious if any of you know a way, when calculating the target's speed, to accountfor: 1, my own distance from the target; and 2, the target's AOB? For example, if an 150m tanker, at a 90degree AOB, takes 50 seconds to cross my reticle from "tip to tip," then the formulaabove results in 5.82kts. However, at a further distance, if the same ship with the same AOB take 30 seconds, then the formula spits out 9.7kts (a rathermassive difference of 3.88kts!). Put another way, my distance from the target and its AOB willnecessarily alter the time it takes to cross my reticle. But, I have no idea how to account for either of these. I've been searching high and low but haveyet to find (let alone figure out on my own) how this can be done. Thus, any help would be greatly appreciated! (Edits for grammar and clarity.) Last edited by SubSimTrooper; 07072021 at 08:55 AM. 
07072021, 12:07 PM  #2 
Swabbie
Join Date: May 2021
Location: France
Posts: 8
Downloads: 2
Uploads: 0

Well, first of all, if you want accuracy, the conversion factor is not 1.94 but 1.944.
Distance plays virtually no role. If your speed is 0 or your bearing is 0 or 180, you don't need any correction, else you need to add or subtract own_speed * sin( bearing ). If AOB is 90, you don't need correction, else your measure will probably be slower than actual target speed, due to hull width. Correction for that is hard to accurately compute though, due to hull shape. 
07072021, 06:03 PM  #3 
Mate

Speed measurement when stationary on the surface is also subject to noise due to the Uboat slightly rocking on the waves. The stabilization of the periscope translates it into lateral motion that can easily mess up a speed read. For best results, do it while submerged at periscope depth.
For faster targets, there's another hurdle which is that it passes through the vertical post too quickly and the resulting speed calculation becomes noisy due to failure to record fractional seconds. To mitigate this, it's better to measure the speed while running parallel to the target at a known moderate speed (e.g. Half, which is normally 12.412.5 kts). If you measure the target speed as though it was stationary and then subtract your own speed from the result, you'll get a moderately reliable speed read on a fast target. I refer to this as the "differential" speed read. If all else fails, you can play with the engine telegraph on a parallel course until you match the target speed to the dot. It takes longer to do well, but it works. Lastly, Efshapo is right about hull width being a factor when measuring at angles that sufficiently deviate from the 90. In such circumstances, it's easy to underestimate the speed by a lot, up to approximately 2 kts at extreme angles. 
07072021, 06:15 PM  #4 
Swabbie
Join Date: Mar 2014
Location: Land o' the Hoosiers
Posts: 10
Downloads: 11
Uploads: 0

After some very helpful feedback and questions from Efshapo and a couple other folks on Discord, it was also suggested that I provide some screencaps to illustrate the point better.
Long and short, the time from 'tip to tip' goes down as distance from target increases. In this case, the first result was 2.09kts and the second was 2.19. While both of these are within that +/ 0.2kts for long range shooting, it seems that the error only increases with distance and various AOBs. The point is that, it seems that the further away from the target you are when speed is checked, the lessaccurate the speed measurement is, with AOB only introducing further error. Thus, is there a way to account for these variables when using the length/distanceX1.944 formula? Finally, I appreciate the additional input, gurudennis, thanks! I see that I'll need to go back at some point when I've more time and buttondown some of these variablesfurther. Last edited by SubSimTrooper; 07072021 at 06:42 PM. 
07072021, 07:02 PM  #5 
Swabbie
Join Date: May 2021
Location: France
Posts: 8
Downloads: 2
Uploads: 0

I think that's just because the farther the target is, the less pixels you see to determine when the wire is crossed. So the error should increase indeed.

Thread Tools  
Display Modes  

