i concur, the devil is in the details. im working to try and resolve a totally passive submerged prosocution of contact. it may not work in game, but i know it can be done in real life. the idea may be a pipe dream, kind of like doing celestial navigation in game. i am not sure, as i need to spend more time "in the pit" and work out some math.
by and large, it is irrelevant as to what sensor gives you the data point when you are projecting the course from last known position. there is uncertainty there, but that is why you use time as a constant. you can check, just like you would for a train or any other thing that has a schedule, for your target "to be on time" .
say the target is supposed to be at X location at 23:07 if the target is ahead of where he is supposed to be, that means he is going faster. lagging behind means he is slower. if you know the projected path, you know the location in "time", you can plot that out and watch for abnormalities. same thing for course. one out of wack reading and its a blip, but you see the next few marks leaning to port and a little slower(relative) you can bet the target has changed course.
sorry for getting long winded. am getting ready to try and chase a DD for a little role reversal
and then work on this a little more tonight. at least after i get done looking at jupiter. moon is being accommodating for once and making jupiter a little more accommodating to my eye, though killing my fuzzies.