TMA - Range Rate and Target Zig

Consider this case:
I have a solution on the target accurately and in a LOS LAG, ownship Route 90 and Route 270 ° Target. At time t1 the Target zig. Calculating the Range Rate for all Bearing in order to have a history of the Range Rate, Target is the zig to the right or left, the range rate increases, how can I guess which side has zig?
If I had the accurate calculation of the Bearing Rate this problem did not exist, unfortunately Bearign Rate calculated from DW refers to the solution we estimated. : (
In TopTorp manual shows that the range rate can know which side has the zig Target, but how to get a Range error rate that is linked with the zig Target ?????

[Pisces]

Like I said, you can't. Atleast not in the subs. You yourself need to make a turn (I hope that is what you mean with 'zig') or change speed to make the bearing rate change significantly. You are blind for range rate by bearings alone. Well apart from increasing/decreasing signal strength, but that takes too long and isn't precise enough.