as far as i know, you need to collect the counters at the BSC level (or easier, at the OMC-R), to know the average TA of all the mobiles in a base station.
If you want to know the TA of one particular MS, then you will need to make an ABIS trace with a protocol analyser and filter out the IMSI of that MS. The TA values are regularly sent on the ABIS.
Constraint : an ABIS trace can be performed only on one or two BTS at once… so you have to know in which BTS they are.