Assuming, that no VHF coverage is avail. ACARS is communicating via SATCOM. To contact the satellite, the A/C has to know his position in regard to the satellite to adjsut the dish.
ACARS is linked with most of the computers , so it should be easy to track it by sending LAT/LON datas.
I believe it's been established there is no dish. They use Inmarsat as their carrier. And the antenna is omnidirectional for this mode.