You can plot over a longer time and track, and calculate the average movement of it. Over a known time you'll have speed which it what you really need for the TDC. If your range accuracy is only good enough to lets say 2 km, but then trail it and letting it move 20 km before the next plot is made, then you still have a 90% accurate target motion plotted.
Also, if your range accuracy isn't good, consider fixing his forward movement with a periscope bearing only. Start thinking of them as start- and finish-lines. Bearings from the periscope are usually much narrower than the in-line inaccuracy of your range measurements. This works best when viewing it from its beam/side while following it. Due to the range inaccuracy of your plots the course accuracy fails a bit, but again, over longer stretches this becomes less of a problem. You just have to be lucky it doesn't make any turns and have to start over again. Plot often and make best use of your position to track it!
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